Anatomy and Physiology Review Questions & Answers
1. Which of the following statements regarding the ulna is FALSE? A. The head of the ulna articulates with the radius B. The ulnar styloid process is on the anterior side of the distal end of the ulna C. The olecranon process of the ulna articulates with the olecranon fossa of the humerus D. The trochlear notch lies between the olecranon process and the coronoid process E. An interosseus membrane connects the ulna to the radius 2. What type of joint is the articulation between right rib 6 and the body of the sternum? A. Synchondrosis B. Synovial C. Symphysis D. Syndesmosis E. Suture 3. What connective tissue wrapping surrounds a muscle fascicle? A. Endomysium B. Endoneurium C. Epimysium D. Epineurium E. Perimysium F. Perineurium 4. A muscle that crosses the hip joint anteriorly and laterally will produce what movements? A. Extension and abduction B. Extension and adduction C. Flexion and abduction D. Flexion and adduction 5. What type of neuroglia is found in the CNS and helps maintain a normal extracellular environment around neurons? A. Astrocytes B. Ependymal cells C. Microglial cells D. Oligodendrocytes E. Satellite cells F. Schwann cells 7. How many total bones are in the right lower limb? 29 8. What structural type of joint is the wrist joint? condylar 9. What is the difference between a motor neuron and a motor unit? a motor unit is the motor neuron and all the nerves it activates 10. What is the primary action of coracobrachialis? What joint does it act on? adduction; glenohumeral joint 11. A 24-year-old male accidentally cut himself while shaving his beard. What structural type of neuron carried the painful impulse from his face to his brain? unipolar 12. Which of the following eye structures is most superficial? (Choroid, lens, vitreous humor, sclera, aqueous humor) sclera 13. Which specific appendicular bone contains BOTH a “spine” and a “tuberosity” ? coxa/hip 14. List all movement(s) that are normally allowable by the ankle joint. flexion/extension15. In what muscular compartment of what body region would you expect to find the muscle extensor pollicis brevis? posterior 16. A 5-year-old boy was running way way too fast in the toy section of Target and slipped and injured his left semimembranosus muscle. What movement(s) at what joint(s) will he likely have difficulty performing? extension of the hip 17. In what division(s) of the nervous system would you find ganglia: CNS only, PNS only, or both? 18. What structural type of neuron are olfactory sensory neurons?bipolar
19. Bone that is immediately medial to the lateral cuneiform 20. Label the region(s) of this bone that is(are) part of a multiaxial joint 21. Origin of masseter 22. Vastus lateralis 23. Perineurium 24. Draw an arrow indicating the direction that the pupil would move when the medial rectus contracts 25. What view of what bone is this? lateral 26. What ligament is labeled? PCL 27. What action does the labeled muscle have? What joint does it act on? pronation of forearm 28. What protein is found in the labeled structure? myosin 29. Suppose that this neuron innervates the heart. What functional type of neuron is this AND what specific nervous system division is it a part of? motor neuron; visceral motor system 30. What specific division of the nervous system innervates the labeled structure? general somatic motor a) What bone is immediately proximal to metacarpal III? Include number if necessary. capitate b) What type of tissue lines the proximal end of metacarpal III? The distal end? Proximal end – hyaline cartilage Distal end – hyaline cartilage c) What is the other name for the MCL? Full name only, no abbreviations Tibial cruciate ligament d) What structural type of joint is the femoropatellar joint? The tibiofemoral joint? Femoropatellar – plane Tibiofemoral – pivot
Joints: Structure and Function
Synarthroses (Singular Synarthrosis) Immovable joints, largely restricted to the axial skeleton
Amphiarthroses Slightly movable joints, largely restricted to the axial skeleton
Diarthroses Freely movable joints (most common in limbs)
Structural Joints Based on the material that binds the bones together and on the presence or absence of a joint cavity
Fibrous The bones are connected by fibrous tissue, namely dense regular connective tissue. No joint cavity is present. Most fibrous joints are immovable or only slightly movable. The types of fibrous joints are sutures, syndesmoses, and gomphosis. They are also known as synarthroses or immovable joints. The articulating bones are joined by fibrous connective tissue. There is little or no movement at the joint. Fibrous joints are subdivided into three types: sutures, syndesmoses, and gomphoses.
Cartilaginous the articulating bones are united by cartilage. Cartilaginous joints lack a joint cavity and are not highly movable. There are two types of cartilaginous joints: synchondroses and symphyses.
Synovial the most movable joints of the body, and all are diarthroses (freely movable). Each synovial joint contains a fluid-filled joint cavity. Most joints of the body are in this class, especially those in the limbs
Suture Fibrous Joint. Made of dense regular connective tissue with short fibers. No joint cavity and the joint is immovable. The Joint is held together with very short, interconnecting fibers and bone edges interlock. Found only in the skull. Example: lambdoid suture on the skull
Syndesmosis Fibrous Joint. The bones are connected exclusively by ligaments, bands of fibrous tissue longer than those that occur in sutures. The amount of movement allowed at a syndesmosis depends on the length of the connecting fibers. Long Fibers = Large Movement. The Joint is held together by a ligament. Fibrous tissue can vary in length but is longer than in sutures. Example: the interosseous membrane between the radius and ulna in the forearm
Gomphosis Fibrous Joint. A peg-in-socket joint. This joint allows for limited movement during mastication (chewing). Peg-in-socket fibrous joint. The periodontal ligament holds the tooth in the socket. Example: articulation of a tooth with its socket
Synovial Joint Features
Articular Cartilage The ends of the opposing bones are covered by articular cartilage composed of hyaline cartilage. These spongy cushions absorb compressive forces placed on the joint and thereby keep the bone ends from being crushed.
Joint (Articular) Cavity A feature unique to synovial joints, the joint cavity is a potential space that holds a small amount of synovial fluid.
Articular Capsule The joint cavity is enclosed by a two-layered articular capsule, or joint capsule.
Fibrous Layer outer layer of articular capsule, made of dense irregular connective tissue, continuous with periosteum layer of connecting bones, helps strengthen joints.
Synovial Membrane The inner layer, composed of loose connective tissue. This membrane covers all the internal joint surfaces not covered by cartilage. Its function is to make synovial fluid
Synovial Fluid The viscous liquid inside the joint cavity. Primarily a filtrate of blood, arising from capillaries in the synovial membrane. It helps to lubricate the joint and provide nutrients to the articular cartilage. Occurs in the joint cavity and within the articular cartilage.
Reinforcing Ligaments Some synovial joints are reinforced and strengthened by bandlike ligaments. Most often, the ligaments are capsular, thickened parts of the fibrous layer of the articular capsule. Extracapsular ligaments are located just outside the capsule and Intracapsular ligaments are internal to the capsule and are covered with a synovial membrane that separates them from the joint cavity through which they run.
Nerves and Vessels The articular capsule is highly innervated. The synovial membrane has blood vessels (that form synovial fluid) but the rest of the joint is avascular.
Articular Disc Certain synovial joints contain a disc of fibrocartilage and extend internally from the capsule and completely or partly divide the joint cavity in two. Occurs in joints whose articulating bone ends have different shapes. An articular disc fills the gaps and improves the fit, thereby distributing the load more evenly and minimizing wear and damage. These discs may also allow two different movements at the same joint.
Synovial Joint Structures
Bursae Contains synovial fluid and often are associated with the synovial joint. Reduces friction between body elements that move one another. Flattened fibrous sac lined by a synovial membrane. Bursae occur where ligaments, muscles, skin, tendons, or bones overlie each other and rub together. They help to reduce friction and provide cushioning.
Tendon Sheath Contains synovial fluid and often is associated with the synovial joint. Reduces friction between body elements that move one another. An elongated bursa that wraps around a tendon like a bun around a hot dog. Tendon sheaths occur only on tendons that are subjected to friction to protect the tendon.
Movements Allowed by Synovial Joints
Flexion A movement that decreases the angle between two bones (or body parts) within a connecting joint.
Example: flexion of the arm (bending the arm) as the angle between the humerus and the radius (or ulna) gets smaller.
Gliding The nearly flat surfaces of two bones slip across each other.
Example: Occurs at the joints between the carpals and tarsals and between the flat articular processes of the vertebrae.
Extension Increasing the angle between bones, usually in the sagittal plane
Example: Straightening the fingers after making a fist
Hyperextension Bending a joint back beyond its normal range of motion.
Example: Individuals who have loose ligaments that allow a greater range of motion can be capable of hyperextending the joints.
Abduction Moving a limb away from the body midline in the frontal plane.
Example: Raising the arm or thigh laterally
Adduction Moving a limb toward the body midline in the frontal plane.
Example: Moving digits toward the midline of the hand or foot
Circumduction Moving a limb or finger so that it describes a cone in space. This is a complex movement that combines flexion, abduction, extension, and adduction in succession.
Example: The movement of the arm at the shoulder joint in a circular motion
Rotation Turning a bone around the longitudinal axis. This motion occurs along the transverse plane. The only movement allowed between the first two cervical vertebrae.
Example: The entire vertebral column also rotates, twisting the whole trunk to the right or left.
Medial Rotation also known as internal rotation is a rotational movement of a bone or a body part towards the midline of the body. Medial rotation Rotating toward the median plane
Example: Medial rotation of the arm at the shoulder joint found in the swinging motion after hitting the ball in tennis or golf.
Lateral Rotation Rotating away from the median plane
Example: When the shoulder joint rotates laterally as you move your arm away from your body
Elevation Lifting a body part superiorly
Example: Elevation of a mandible during chewing
Depression Moving a body part inferiorly.
Example: Depression of a mandible during chewing
Protraction Moving a body part in the anterior direction.
Example: When you move your mandible (lower jaw) forward to stick out your chin.
Retraction Moving a body part in the posterior direction.
Example: When the scapulae (shoulder blades) move closer to the midline of the back, as when pulling the shoulders back.
Supination Rotating the forearm so the palm faces anteriorly.
Example: Occurs when the forearm, specifically the radius, rotates laterally so that the palm faces anteriorly.
Pronation Rotating the forearm so the palm faces posteriorly. Pronation brings the radius across the ulna so that the two bones form an X.
Example: Occurs when the radius rotates medially so that the palm faces posteriorly
Opposition Moving the thumb to touch the tips of the other fingers. In the palm, the saddle joint between metacarpal I and the trapezium allows a movement called opposition of the thumb.
Example: you move your thumb across the palm enabling it to touch the tips of the other fingers on the same hand
Inversion Turning the sole of the foot medially.
Example: To invert the foot, turn the sole medially
Eversion Turning the sole of the foot laterally.
Example: To evert the foot, turn the sole laterally
Dorsiflexion Lifting the foot so its superior surface approaches the shin.
Example: Standing with your heels
Plantar Flexion a depression movement of the feet and elevation of the heel.
Example: tip-toeing motion of the feet in ballerina when they dance.
Classification of Synovial Joints by Movement
Nonaxial Adjoining bones move but not in any specific axis.
Example: plane joint of intercarpal.
Uniaxial Movement occurs around a single axis.
Example: the elbow joint
Biaxial Movement can occur around two axes; thus, the joint enables motion along both the frontal and sagittal planes.
Example: the wrist joint, flexion-extension and abduction-adduction.
Multiaxial Movement can occur around all three axes and along all three body planes: frontal, sagittal, and transverse.
Example: The shoulder joint, flexion/extension, abduction/adduction, and medial/lateral rotation.
Types of Synovial Joints
Plane Flat articular surface allowing non-axial movement such as gliding.
Example: intercarpal & intertarsal joints, joints between vertebral articular surfaces.
Hinge The convex surface of one bone fits into the concave surface of another bone, allowing for angular motion along one plane.
Example: Elbow joints, interphalangeal joints
Pivot A rounded or pointed surface of one bone articulates with a ring formed by another bone and a ligament, allowing for rotational movement around a central axis.
Example: Proximal radioulnar joints, atlantoaxial joint
Condylar The oval-shaped condyle of one bone fits into the elliptical-shaped cavity of another bone, allowing for angular motion along two planes.
Example: Metacarpophalangeal (knuckle) joints, wrist joints
Saddle The articular surfaces of both bones have a saddle shape, allowing for angular motion along two planes with increased range of motion compared to a condyloid joint.
Example: Carpometacarpal joints of the thumbs
Ball-and-Socket The spherical head of one bone articulates with the cup-like depression of another bone, allowing for motion along multiple axes.
Example: Shoulder joints and hip joints
Factors Influencing Joint Stability
Shapes of the Articular Surfaces The articular surfaces of the bones in a joint fit together in a complementary manner. Although their shapes determine what kinds of movement are possible at the joint, articular surfaces seldom play a major role in joint stability: Most joint sockets are just too shallow. Still, some joint surfaces have deep sockets or grooves that do provide stability. This helps to stabilize the joint and prevent excessive movement.
Number and Position of Stabilizing Ligaments The capsules and ligaments of synovial joints help hold the bones together and prevent excessive or undesirable motions. Ligaments located on the medial or inferior side of a joint resist excessive abduction; lateral and superiorly located ligaments resist adduction. Anterior ligaments resist excessive extension and lateral rotation; posterior ligaments resist excessive flexion and medial rotation. The more ligaments a joint has, the stronger it is. Once stretched, ligaments stay stretched but they stretch that much.
Muscle Tone A constant, low level of contractile force generated by a muscle even when it is not causing movement. Muscle tone helps stabilize joints by keeping tension on the muscle tendons that cross over joints just external to the joint capsule. In this manner the muscle functions like a ligament holding the adjoining bone surfaces together. The muscles that surround a joint help to support and stabilize it. When the muscles are contracted, they create tension that helps to hold the joint in place. This is known as muscle tone.
Specific Joints and Their Characteristics
Temporomandibular Joint (TMJ) Bones:
– condylar process of the mandible and inferior surface of temporal bone
Structural Type:
– Synovial, modified hinge joint.
– Contains articular disc.
Functional Type:
– Diathrotic, gliding and uniaxial
Sternoclavicular Joint Bones:
– Sternum and clavicle
Structural Type:
– Synovial; shallow saddle (contains articular disc)
Functional Type:
– Diarthrotic; multiaxial (allows clavicle to move in all axes)
Shoulder (Glenohumeral) Joint Bones:
– Scapula and humerus
Structural Type:
– Synovial; balland-socket
Functional Type:
– Diarthrotic; multiaxial; flexion, extension, abduction, adduction, circumduction, rotation of humerus
Elbow Joint Bones:
– Ulna (and radius) with humerus
Structural Type:
– Synovial; hinge
Functional Type:
– Diarthrotic; uniaxial; flexion, extension of forearm
Wrist Joint Bones:
– Radius and proximal carpals
Structural Type:
– Synovial; condylar
Functional Type:
– Diarthrotic; biaxial; flexion, extension, abduction, adduction, circumduction of hand
Hip Joint Bones:
– Hip bone and femur
Structural Type:
– Synovial; ball and socket
Functional Type:
– Diarthrotic; multiaxial; flexion, extension, abduction, adduction, rotation, circumduction of femur
Knee (Tibiofemoral) Joint Bones:
– Femur and tibia
Structural Type:
– Synovial; modified hinge, structurally bicondylar (contains articular discs)
Functional Type:
– Diarthrotic; biaxial; flexion, extension of leg, some rotation allowed
Knee (Femoropatellar) Joint Bones:
– Femur and patella
Structural Type:
– Synovial; plane
Functional Type:
– Diarthrotic; gliding of patella
Ankle Joint Bones:
– Tibia and fibula with the talus
Structural Type:
– Synovial; hinge
Functional Type:
– Diarthrotic; uniaxial; dorsiflexion and plantar flexion of foot
Detailed Look at the Shoulder and Knee Joints
Shoulder (Glenohumeral) Joint Description In the shoulder joint, stability has been sacrificed to provide the most freely moving joint of the body. This ball-and-socket joint is formed by the head of the humerus and the shallow glenoid cavity of the scapula.
Glenoid Labrum A rim of fibrocartilage that slightly deepened the glenoid cavity. A shallow cavity contributes little to joint stability
Coracohumeral Ligament The superior part of the capsule that is the only strong thickening of it. Helps support the weight of the upper limb. Reinforces the upper part of the joint capsule.
Glenohumeral Ligament The anterior part of the capsule thickens slightly into three rather weak. A group of three ligaments that also help stabilize the joint.
Rotator Cuff and Knee Joint Components
Rotator Cuff Group of four tendons and the associated muscles which encircle the shoulder joint and merge with the joint capsule. The rotator cuff muscles include the subscapularis, supraspinatus, infraspinatus, and teres minor. Moving the arm vigorously can severely stretch or tear the rotator cuff. Attaches to the scapula and surrounds the head of the humerus. These muscles and tendons work together to provide stability to the shoulder joint, as well as to lift and rotate the arm.
Knee Joint The knee joint, the largest and most complex joint in the body, primarily acts as a hinge. However, it also permits some medial and lateral rotation when in the flexed position and during the act of leg extension. Structurally, it is compound and bicondyloid, because both the femur and tibia have two condylar surfaces. In this joint, the wheel-shaped condyles of the femur roll along the almost-flat condyles of the tibia like tires on a road. Sharing the knee cavity is an articulation between the patella and the inferior end of the femur; this femoropatellar joint is a plane joint that allows the patella to glide across the distal femur as the knee bends.
Subcutaneous Prepatellar Bursa A small fluid-filled sac located in front of the patella. The bursa helps to reduce friction between the skin and the patella during movements.
Lateral and Medial Menisci Crescent-shaped pieces of fibrocartilage located on the tibial plateau. The menisci help to distribute the forces across the knee joint and provide stability to the joint. The medial meniscus is larger and more C-shaped, while the lateral meniscus is more circular. The menisci help to stabilize the joint by guiding the condyles during flexion, extension, and rotation movements and preventing side-to-side rocking of the femur on the tibia.
Patellar Ligament A continuation of the tendon of the main muscles on the anterior thigh, the quadriceps femoris. The patella ligament connects the patella to the tibia and helps to stabilize the joint during movements.
Medial and Lateral Patellar Retinaculum Bands of connective tissue that anchor the patella (kneecap) to the femur and tibia. The retinaculum on the medial side of the knee is thicker and stronger than the one on the lateral side. Together, they form a sort of “track” for the patella to slide up and down during movements of the knee joint. This helps to ensure proper tracking and stability of the patella during activities such as walking, running, and jumping.
Knee Ligaments
Fibular Collateral Ligament Extracapsular ligaments located on the lateral side of the joint capsule of the knee, prevents the knee joint from sliding off to the outer side of the body axis. Halts leg extension, prevent hyperextension, and prevents the leg from moving laterally and medially at the knee.
Tibial Collateral Ligament Extracapsular ligaments located on the medial side of the joint capsule of the knee. The tibial collateral ligament runs from the medial epicondyle of the femur to the medial condyle of the tibia. Halts leg extension, prevent hyperextension, and prevents the leg from moving laterally and medially at the knee.
Oblique Popliteal Ligament Crosses the posterior aspect of the capsule. It is a part of the tendon of the semimembranosus muscle that fuses with the joint capsule and helps stabilize the joint. Provide additional stability to the posterior aspect of the knee joint
Arcuate Popliteal Ligament Arcs superiorly from the head of the fibula over the popliteus muscle to the posterior aspect of the joint capsule. Provide additional stability to the posterior aspect of the knee joint
Anterior Cruciate Ligament Attaches to the anterior part of the tibia, in the intercondylar area. From there, it passes posteriorly to attach to the femur on the medial side of the lateral condyle. The ACL prevents anterior displacement of the tibia on the femur and provides rotational stability to the joint.
Posterior Cruciate Ligament Arises from the posterior intercondylar area of the tibia and passes anteriorly to attach to the femur on the lateral side of the medial condyle. The PCL prevents posterior displacement of the tibia on the femur and provides stability during flexion of the knee joint.
Muscle Tissue Characteristics
Contractility Muscle tissue contracts forcefully. Muscle cells contain myofilaments, specific types of microfilaments that are responsible for the shortening of muscle cells. There are two kinds of myofilaments, one containing the protein actin and the other containing the protein myosin. These two proteins generate contractile force in every cell in the body. This contractile property is most highly developed in muscle cells.
Excitability Nerve signals or other stimuli excite muscle cells, causing electrical impulses to travel along the cells’ plasma membrane. These impulses initiate contraction in muscle cells.
Muscle Tissue Properties
Extensibility Muscle tissue can be stretched. Contraction of one skeletal muscle will stretch an opposing muscle. The muscular wall of a hollow organ is stretched by the substances contained within that organ—a bolus of food in the digestive tract or urine in the urinary bladder, for example.
Elasticity After being stretched, muscle tissue recoils passively and resumes its resting length.
Functions of Muscle Tissue
Four Functions of Muscle Tissue Produce Movement
– contraction of multiple muscles in the legs helps you raise the feet and thrust the hip to move the body forward.
Open and Close Body Passageways
– Muscle fibers in the iris of the eye constrict and relax to change pupil diameter
Maintain Posture and Stabilize Joints
– Muscle tone, the constant low-level contraction of muscles, helps stabilize and strengthen many synovial joints.
Generate Heat
– Excess heat generated during exercise stimulates sweating to cool us down
Types of Muscle Tissue
Two Characteristics of Different Muscle Types (1) the presence or absence of light and dark stripes, called striations, in the muscle cells
(2) whether control of contraction is voluntary or involuntary
Skeletal Muscle The muscle cells of skeletal muscle tissue are striated muscle. Discrete organs that attach to and move the skeleton. The elongated, cylindrical skeletal muscle cells are called muscle fibers. Skeletal muscle is innervated by the voluntary division of the nervous system and is subject to conscious control; you can control this muscle tissue at will.
Cardiac Muscle Cardiac muscle tissue occurs only in the wall of the heart. The muscle cells of cardiac muscle are striated muscle, but its contraction is involuntary. Cardiac muscle can contract with no nervous stimulation. The involuntary division of the nervous system regulates contraction of cardiac muscle tissue; we have no direct, conscious control over how fast our heart beats. visceral muscle, a term reflecting the fact that both occur in the visceral organs and are innervated by the involuntary division of the nervous system.
Smooth Muscle and Muscle Tissue Components
Smooth Muscle Most smooth muscle tissue in the body is found in the walls of hollow internal organs other than the heart, such as the stomach, urinary bladder, blood vessels, and respiratory tubes. The muscle cells of smooth muscle lack striations. Like skeletal muscle, these cells are elongated and referred to as muscle fibers. As with cardiac muscle, the involuntary division of the nervous system innervates smooth muscle. visceral muscle, a term reflecting the fact that both occur in the visceral organs and are innervated by the involuntary division of the nervous system.
Tissues That Make Up Skeletal Muscles Skeletal muscle tissue, connective tissue, blood vessels, and nerves.
Connective Tissue Sheaths
Connective Tissue Sheaths of Skeletal Muscles These fibrous connective tissues bind muscle fibers together and hold them in parallel alignment so they can work together to produce force. These sheaths are continuous with each other: The endomysium merges with the perimysium, which in turn is continuous with the epimysium.
Epimysium Outer layer of dense, irregular connective tissue
Surrounds the whole skeletal muscle.
Sometimes the epimysium blends with the deep fascia that lies between neighboring muscles
Perimysium A layer of fibrous connective tissue surrounding each fascicle
Fasicles are groups of separated muscle fibers that resemble a bundle of sticks
Endomysium Within a fascicle, each muscle fiber is surrounded by a fine sheath of loose connective tissue consisting mostly of reticular fibers
Tendon All three sheaths (epimysium, perimysium, and endomysium) converge to form a tendon, the connective tissue structure that joins skeletal muscles to bones. When muscle fibers contract, they pull on the surrounding endomysium. Because of the continuity between sheaths, this pull is then exerted on the perimysium, epimysium, and tendon, a sequence that transmits the force of contraction to the bone being moved. The sheaths also provide a muscle with much of its natural elasticity and carry the blood vessels and nerves that serve the muscle fibers.
Muscle Attachments: Origins and Insertions
How does a muscle attach to a bone? A muscle attaches to a bone through tendons. The tendons attach muscle to bone by fusing the collagen fibers of the tendon with the periosteum, the tough outer layer of the bone. A muscle attachment is the location on a bone where a muscle connects to the bone. Each skeletal muscle extends from one bone to another, crossing at least one movable joint. When the muscle contracts, it causes one of the bones to move while the other bone usually remains fixed.
What is the difference between an origin and an insertion? The attachment of the muscle on the less movable bone is called the origin of the muscle, whereas the attachment on the more movable bone is called the muscle’s insertion. Thus, when the muscle contracts, its insertion is pulled toward its origin. In the muscles of the limbs, the origin is by convention the more proximal attachment of the muscle, and the insertion is the more distal attachment. The functions of the origin and the insertion may switch, depending on body position and the movement produced when the muscle contracts.
Indirect vs Direct Attachment Muscles attach to their origins and insertions via strong fibrous connective tissues that extend into the fibrous periosteum of the bone. In direct, or fleshy, attachments, the attaching strands of connective tissue are so short that the muscle fascicles themselves appear to attach directly to the bone. In indirect attachments, the connective tissue extends well beyond the end of the muscle fibers to form either a cordlike tendon or a flat sheet called an aponeurosis.
Indirect attachments are more common than direct attachments, and most muscles have tendons. Raised bone markings are often present where tendons meet bones. These markings include tubercles, trochanters, and crests. Although most tendons and aponeuroses attach to bones, a few attach to skin, to cartilage, or to a raphe.
Aponeurosis A cordlike tendon or a flat sheet.
Raphe A seam of fibrous tissue.
Skeletal Muscle Fiber Structure
Skeletal Muscle Fiber Striated appearance: Skeletal muscle fibers have a banded or striped appearance due to the arrangement of contractile proteins called actin and myosin within the sarcomeres of the muscle fiber.
Multiple nuclei: Unlike most cells in the body, skeletal muscle fibers are multinucleated, meaning they contain multiple nuclei within a single cell. Sarcoplasmic reticulum: Skeletal muscle fibers have a specialized endoplasmic reticulum called the sarcoplasmic reticulum, which stores and releases calcium ions required for muscle contraction.
Myofibrils Internal structure of long, rod-shaped organelles. Unbranched cylinders that are present in large numbers, making up more than 80% of the sarcoplasm. They are specialized contractile organelles unique to muscle tissue. Myofibrils contain myofilaments. The myofibrils in a fiber are separated from one another by other components of the sarcoplasm. Among those components are mitochondria and glycosomes, both of which supply energy for muscle contraction.
Sarcomere Repeating segments that makeup a myofibril (many myofibrils in a muscle cell myofiber). Acts as a basic unit of contraction in skeletal muscle. Bounded by Z disks.
Z disc (Z line) The boundaries at the two ends of each sarcomere.
Thin Filament Attached to each Z disc and extending toward the center of the sarcomere are many fine myofilaments. The thin filaments are composed primarily of the contractile protein actin and two regulatory proteins, troponin and tropomyosin.
Troponin Globular protein with three binding sites: one for actin, one for tropomyosin, and one for calcium. Troponin attaches the tropomyosin strand to the actin molecule.
Tropomyosin Forms a thin strand that spirals around the actin molecule.
Thick Filament In the center of the sarcomere and overlapping the inner ends of the thin filaments is a cylindrical bundle of thick (myosin) filaments. Thick filaments consist largely of myosin molecules. They also contain ATPase enzymes that split ATP (energy-storing molecules) to release the energy required for muscle contraction. Both ends of a thick filament are studded with knobs called myosin heads.
A Band The dark bands are created by the full length of the thick filaments in the sarcomeres, along with the inner ends of the thin filaments, which overlap the thick filaments
H Zone The central part of an A band, where no thin filaments reach
I Band The two regions on either side of the A band. Contains only thin filaments. Each I band is part of two adjacent sarcomeres and has a Z disc running through its center. The I band shrinks as the muscle contracts.
M Line In the center of the H zone that contains tiny rods that hold the thick filaments together.
Titin Springlike molecule in sarcomeres that resists overstretching. The titin molecules in a sarcomere are found in the elastic filaments. They extend from the Z disc to the thick filament and run within the thick filament to attach to the M line. It holds the thick filaments in place in the sarcomere, thereby maintaining the organization of the A band and it unfolds when the muscle is stretched and then refolds when the stretching force is released, thereby contributing to muscle elasticity. Titin does not resist stretching in the ordinary range of extension, but it becomes stiffer the more it uncoils; therefore, it strongly resists excessive stretching that tries to pull the sarcomere apart.
Sarcolemma The plasma membrane of muscle cells.
Synaptic Cleft The terminal boutons are separated from the sarcolemma of the muscle fiber by a space called the synaptic cleft
At the synapse, the plasma membranes of the two neurons are separated by a synaptic cleft
Axon Terminal / Terminal Bouton The nerve part of the neuromuscular junction or a motor end plate is a cluster of enlargements at the end of the axonal process that stores chemical messenger molecules, neurotransmitters.
Sarcoplasmic Reticulum An elaborate smooth endoplasmic reticulum whose interconnecting tubules surround each myofibril like the sleeve of a loosely crocheted sweater surrounds your arm. Most SR tubules run longitudinally along the myofibril. Store large quantities of calcium ions (Ca 2+). These ions are released when the muscle is stimulated to contract.
T Tubule – (Transverse Tubules) Deep invaginations of the sarcolemma that run between each pair of terminal cisterns
Terminal Cistern Sarcoplasmic reticulum (SR) form larger, perpendicular cross channels over the junction between each A band in a myofibril and its adjacent I bands (A-I junctions). Store large quantities of calcium ions (Ca 2+). These ions are released when the muscle is stimulated to contract.
Triad The complex of the T tubule flanked by two terminal cisterns at the A-I junction
Concentric Contraction Concentric contraction is the more familiar type, in which the muscle shortens and does work—picking up a book or kicking a ball. When contracting concentrically, a muscle generates force while shortening.
Eccentric Contraction Eccentric contraction occurs when a muscle generates force as it lengthens. When contracting eccentrically, a muscle generates force while lengthening. This type of contraction is essential for controlled movement and resistance to gravity. Eccentric contraction occurs in many movements that resist gravity: going down stairs, running downhill, landing from a jump. Whenever muscles are acting as a brake, they are contracting eccentrically.
Sliding Filament Theory of Muscle Contraction Concentric contraction of skeletal muscle is explained by the sliding filament mechanism. The sliding filament mechanism is initiated by the release of calcium ions from the sarcoplasmic reticulum and the binding of those ions to the troponin molecule on the thin filament. This results in a change of shape of the troponin, which moves the tropomyosin molecule and exposes the binding sites on the actin filament for the myosin heads. Contraction results as the myosin heads of the thick filaments attach to the thin filaments at both ends of the sarcomere and pull the thin filaments toward the center of the sarcomere by pivoting inward. After a myosin head pivots at its “hinge,” it lets go, returns to its original position, binds to the thin filament farther along its length, and pivots again. This ratchet-like cycle is repeated many times during a single contraction. ATP powers this process. It should be emphasized that the thick and thin filaments themselves do not shorten: The thin filament merely slides over the thick filament.
Motor Neuron The release of calcium ions from the sarcoplasmic reticulum and the subsequent contraction of skeletal muscle is initiated by nervous stimulation. The nerve cells that innervate muscle fibers are called motor neurons. A neuron has cell processes that extend from the cell body: Dendrites are receptive regions of the neuron; an axon is a long, singular cell process that initiates and transmits nerve impulses
Neuromuscular Junction (Motor End Plate) Each muscle fiber in a skeletal muscle is served by a nerve ending, which signals the fiber to contract. The point at which the nerve ending and fiber meet is called a neuromuscular junction or a motor end plate. The nerve part of the junction is a cluster of enlargements at the end of the axonal process that stores chemical messenger molecules, neurotransmitters. The neuromuscular junction has several unique features. Each terminal bouton lies in a trough-like depression of the sarcolemma, which in turn has its own invaginations. The invaginations of the sarcolemma are covered with a basal lamina. This basal lamina contains the enzyme acetylcholinesterase, which breaks down acetylcholine immediately after the neurotransmitter signals a single contraction. This ensures that each nerve impulse to the muscle fiber produces just one twitch of the fiber, preventing any undesirable additional twitches that would result if acetylcholine were to linger in the synaptic cleft.
Motor Unit The axon of a motor neuron branches to innervate a number of fibers in a skeletal muscle. A motor neuron and all the muscle fibers it innervates are called a motor unit. When a motor neuron fires, all the skeletal muscle fibers in the motor unit contract together. A motor unit can have as many as 4 to several hundred muscle fibers. Muscles that require very fine control (such as the muscles moving the fingers and eyes) have few muscle fibers per motor unit, whereas bulky, weight-bearing muscles, whose movements are less precise (such as the hip muscles), have many muscle fibers per motor unit. The muscle fibers of a single motor unit are not clustered together but rather are spread throughout the muscle. As a result, stimulation of a single motor unit causes a weak contraction of the entire muscle. The addition of motor units to accomplish a movement is called recruitment. If a small force is required, a small number of motor units are stimulated. As more force is needed, additional motor units are recruited.
A motor unit consists of a single motor neuron and all of the muscle fibers it innervates. When a motor neuron fires an action potential, all of the muscle fibers in its motor unit contract simultaneously.
Prime Mover / Agonist A muscle that has the major responsibility for producing a specific movement of that motion. Sometimes, two muscles contribute so heavily to the same movement that both are called agonists.
Antagonist Muscles that oppose or reverse a particular movement act. When a prime mover is active, it is possible for its antagonists to be stretched or remain relaxed. Usually, however, the antagonists contract slightly during the movement to keep the movement from overshooting its mark or to slow it near its completion. Antagonists can also be prime movers in their own right; that is, an antagonist for one movement can serve as an agonist for the opposite movement.
Synergist Most movements also involve one or more muscles. Synergists help the prime movers, either by adding a little extra force to the movement being carried out or by reducing undesirable extra movements that the prime mover may produce. Some prime movers cross several joints and can cause movements at all of them, but synergists act to cancel some of these movements. Synergists prevent the particular movement that is inappropriate at a given time
Fixator A synergist that hold a bone firmly in place so that a prime mover has a stable base on which to move a body part. Muscles that maintain posture and stabilize joints also act as fixators.
Upper Limb Compartments Anterior
– biceps brachii, brachialis, coracobrachialis
– Perform flexion of the arm.
Posterior
– the triceps brachii, the digital and carpal extensors, wrist, digital flexors, and pronators
– extends the elbow and the wrist
Lower Limp Compartments Posterior
– the hamstring group
– extend the thigh at the hip and flex the leg at the knee
Anterior
– iliopsoas and the quadreceps femoris group
– flex the thigh at the hip and extend the leg at the knee
Medial
– the adductor group
– adduct the thigh
7 Criteria for Naming Skeletal Muscle Direction of muscle fibers or fascicles
Example: rectus abdominis. (rectus = straight)
Location
Example: brachialis muscle (brachium = arm)
Shape
Example: deltoid is triangular (the Greek letter delta is written ∆)
Relative Size
Example: gluteus minimus muscles of the buttocks ; minimus (smallest)
Location of Attachments
Example: brachioradialis muscle in the forearm originates on the bone of the brachium, the humerus, and inserts on the radius
Number of Origins
Example: biceps brachii has two origins
Action
Example: adductor longus on the medial thigh adducts the thigh at the hip
Masseter Closes jaw; elevates mandible
Temporalis Closes jaw; elevates and retracts mandible; maintains position of the mandible at rest; deep anterior part may help protract mandible
Buccinator Compresses the cheek; helps keep food between grinding surfaces of teeth during chewing
Medial Pterygoid Acts with lateral pterygoid muscle to protract mandible and to produce side-to-side (grinding) movements; synergist of temporalis and masseter muscles in elevation of the mandible
Lateral Pterygoid Protracts mandible and produces side-to-side grinding movements of the lower teeth
Digastric Has two bellies linked at hyoid bone. Opens mouth and depresses mandible. Swallowing muscle. Elevates hyoid bone and holds it steady during speech and swallowing.
Pectoralis Minor Protracts and rotates scapula downward OR draws ribcage superiorly (depending on which is held fixed). Anterior muscle deep to pectoralis major.
Serratus Anterior Rotates scapula upward; prime mover to protract and hold scapula against chest wall; important role in abduction and raising of arm and in horizontal arm movements (pushing, punching); called “boxer’s muscle”
Subclavius Helps stabilize and depress pectoral girdle
Trapezius Stabilizes, elevates, retracts, and rotates scapula; middle fibers retract (adduct) scapula; superior fibers elevate scapula (as in shrugging shoulders) or can help extend head when scapula is fixed; inferior fibers depress scapula (and shoulder)
Levator Scapulae Elevates and adducts scapula in synergy with superior fibers of trapezius; rotate scapulae downward; when scapula is fixed, flexes neck to same side
Rhomboids Stabilize scapula; act together (and with middle trapezius fibers) to retract scapula, thus “squaring shoulders”; rotate scapulae downward (as when arm is lowered against resistance; e.g., paddling a canoe)
Pectoralis Major Origin:
– sternal end of clavicle, sternum, cartilage of ribs 1-6 (or 7), and aponeurosis of external oblique muscle
Insertion:
– fibers converge to insert by a short tendon into greater tubercle of humerus
Action:
– Prime mover of arm flexion; rotates arm medially; adducts arm against resistance; with scapula (and arm) fixed, pulls rib cage upward, and thus can help in climbing, throwing, pushing, and in forced inspiration
Deltoid Origin:
– Anterior surface of clavicle and spine of scapula
Insertion:
– Anterior surface of clavicle and spine of scapula
Action:
– Abduction of the arm at glenohumeral (GH) joint. Prime mover of arm abduction when all of the muscle contracts. Anterior contraction causes flexion and medial rotation of arm, posterior contraction causes extension and lateral rotation of arm.
Latissimus Dorsi Origin:
– indirect attachment via thoracolumbar fascia into spines of lower six thoracic vertebrae, lumbar vertebrae, lower 3 to 4 ribs, and iliac crest
Insertion:
– spirals around teres major to insert in floor of intertubercular sulcus of humerus
Action:
– Prime mover of arm extension; adduction and medial rotation of arm; it plays an important role in bringing the arm down in a power stroke, as in striking a blow, hammering, and swimming; with arms reaching overhead, it pulls the rest of the body upward and forward, as in chin-ups
Teres Major Origin:
– posterior surface of scapula at inferior angle
Insertion:
– crest of lesser tubercle on anterior humerus; insertion tendon fused with that of latissimus dorsi
Action:
– Extends, medially rotates, and adducts the arm; synergist of latissimus dorsi
Subscapularis Origin:
– subscapular fossa of scapula
Insertion:
– lesser tubercle of humerus
Action:
– Medically rotates arm; assisted by pectoralis major; helps to hold head of humerus in glenoid cavity, stabilizing shoulder joint
Supraspinatus Origin:
– supraspinous fossa of scapula
Insertion:
– superior part of greater tubercle of humerus
Action:
– Initiates abduction of arm; stabilizes shoulder joint; helps to prevent downward dislocation of humerus, as when carrying a heavy suitcase
Infraspinatus Origin:
– infraspinous fossa of scapula
Insertion:
– greater tubercle of humerus posterior to insertion of supraspinatus
Action:
– Laterally rotates arm; helps to hold head of humerus in glenoid cavity, stabilizing the shoulder joint
Teres Minor Origin:
– lateral border of dorsal scapular surface
Insertion:
– greater tubercle of humerus inferior to infraspinatus insertion
Action:
– Same actions as infraspinatus muscle
Coracobrachialis Origin:
– coracoid process of scapula
Insertion:
– medial surface of humerus shaft
Action:
– Flexion and adduction of arm; synergist of pectoralis major
Biceps Brachii Origin:
– short head: coracoid process; long head: supraglenoid tubercle and lip of glenoid cavity; tendon of long head runs within capsule of shoulder joint and descends into intertubercular sulcus of humerus
Insertion:
– by common tendon into radial tuberosity
Action:
– Flexes and supinates forearm; these actions usually occur at same time (e.g., when you open a bottle of wine it turns the corkscrew and pulls the cork); weak flexor of arm at shoulder
Triceps Brachii Powerful forearm extensor (prime mover, particularly medial head); antagonist of forearm flexors; long and lateral heads mainly active in extension against resistance; long head tendon may help stabilize shoulder joint and assist in arm adductio
Anconeus Abducts ulna during forearm pronation; synergist of triceps brachii in elbow extension
Brachialis Flexes forearm (lifts ulna as biceps lifts the radius)
Brachioradialis Synergist in forearm flexion; acts to best advantage when forearm is partially flexed and semipronated, as when carrying luggage; stabilizes the elbow during rapid flexion and extension
Sartorius Flexes, abducts, and laterally rotates thigh; flexes leg (weak) as in a soccer kick; helps produce the cross-legged position
Quadriceps Femoris The quadriceps is a powerful leg extensor used in climbing, jumping, running, and rising from seated position.
Rectus Femoris Extends leg and flexes thigh
Vastus Lateralis Extends leg and stabilizes knee
Vastus Medialis Extends leg; inferior fibers stabilize patella
Vastus Intermedius Extends leg
Hamstrings prime movers of thigh extension and leg flexion.
Biceps Femoris Extends thigh and flexes leg; laterally rotates leg when knee is semiflexed
Semitendinosus Extends thigh and flexes leg; medially rotates leg
Semimembranosus Extends thigh and flexes leg; medially rotates leg
Popliteus Flexes and rotates leg medially to unlock knee from full extension when flexion begins; with tibia fixed, rotates thigh laterally
Abdominal Muscles Flexion: bending the trunk forward Lateral flexion: bending the trunk to one side Rotation: twisting the trunk to one side or the other Compression: decreasing the volume of the abdominal cavity, such as during forced expiration or defecation.
External Oblique Flex vertebral column and compress abdominal wall when pair contracts simultaneously; acting individually, aid muscles of back in trunk rotation and lateral flexion; used in oblique curls
Internal Oblique As for external oblique
Transversus Abdominis Compresses abdominal contents
Rectus Abdominis Flex and rotate lumbar region of vertebral column; fix and depress ribs, stabilize pelvis during walking, increase intraabdominal pressure; used in sit-ups/curls
Psoas Major prime mover in thigh flexion and in flexing trunk (as when bowing); also causes lateral flexion of vertebral column; important postural muscle
Functions of the Nervous System It uses its millions of sensory receptors to monitor changes occurring both inside and outside the body. Each of these changes is called a stimulus, and the gathered information is called sensory input.
It processes and interprets the sensory input and makes decisions about what should be done at each moment—a process called integration.
It dictates a response by activating the effector organs, our muscles or glands; the response is called motor output.
Central Nervous System (CNS) The central nervous system (CNS) consists of the brain and the spinal cord, which occupy the cranium and the vertebral canal, respectively. The CNS is the integrating and command center of the nervous system: It receives incoming sensory signals, interprets these signals, and dictates motor responses based on past experiences, reflexes, and current conditions.
Peripheral Nervous System (PNS) The peripheral nervous system (PNS), the part of the nervous system outside the CNS, consists mainly of the nerves that extend from the brain and spinal cord. Cranial nerves carry signals to and from the brain, whereas spinal nerves carry signals to and from the spinal cord. These peripheral nerves serve as communication lines that link all regions of the body to the central nervous system. Also included in the PNS are ganglia, areas where the cell bodies of neurons are clustered.
Afferent Signals In the sensory, or afferent, division, signals are picked up by sensory receptors located throughout the body and carried by nerve fibers of the PNS into the CNS.
Efferent Signals In the motor, or efferent, division, signals are carried away from the CNS by nerve fibers of the PNS to innervate the muscles and glands, causing these organs either to contract or to secrete
Somatic Sensory General somatic senses include Touch, pain, pressure, vibration, temperature, and proprioception from the skin, body wall, and limbs. Special somatic senses include hearing, equilibrium, and vision
Visceral Sensory General visceral senses are those felt by internal organ systems like digestive and reproductive organs. The general visceral senses include stretch, pain and temperature, hunger, and nausea. Special visceral senses include taste and smell.
Somatic Motor Motor innervation to skeletal muscles
Visceral Motor (Autonomic) Motor innervation to smooth muscle, cardiac muscle, and glands
Functional Characteristics of Neurons Neurons are highly specialized cells that conduct electrical signals from one part of the body to another that are transmitted along the plasma membrane (neurilemma) in the form of nerve impulses / action potentials.
Neurons have extreme longevity.
Neurons do not divide.
– neural stem cells have been identified in certain areas of the CNS.
Neurons have an exceptionally high metabolic rate, requiring continuous and abundant supplies of oxygen and glucose.
– Neurons cannot survive for more than a few minutes without oxygen.
Cell Body Also called soma. Cell bodies of different neurons vary widely in size and all consist of a single nucleus surrounded by cytoplasm. In all but the smallest neurons, the nucleus is spherical and clear and contains a dark nucleolus near its center. The cell body is the focal point for the outgrowth of the neuron processes during embryonic development. In most neurons, the plasma membrane of the cell body acts as a receptive surface that receives signals from other neurons.
Chromatophilic Substance clusters of ribosomes or rough endoplasmic reticulum that stains dark. (used to find cell bodies in histology)
Neurofibrils Bundles of intermediate filaments (neurofilaments) that run in a network between the chromatophilic substance. Like all other intermediate filaments, neurofilaments keep the cell from being pulled apart when it is subjected to tensile forces.
Ganglia (Ganglion) Clusters of cell bodies that lie along the nerves in the PNS
Neural Process Armlike processes extend from the cell bodies of all neurons. These processes are of two types – dendrites and axons, which differ from each other both in structure and in functional properties.
Dendrite Most neurons have numerous dendrites, processes that branch from the cell body like the limbs on a tree. Virtually all organelles that occur in the cell body also occur in dendrites, and chromatophilic substance extends into the basal part of each dendrite. Dendrites function as receptive sites, providing an enlarged surface area for receiving signals from other neurons. By definition, dendrites conduct electrical signals toward the cell body.
Axon A neuron has only one axon, which arises from the axon hillock. Axons are thin processes of uniform diameter throughout their length. By definition, axons are impulse generators and conductors that transmit nerve impulses away from their cell body.
Axon Hillock A cone-shaped region of the cell body
Nerve Fiber Any long axon
Axon Collateral Side branches that occur along their length that extend from the axon at more or less right angles. These collaterals provide modulation and regulation of the cell firing pattern and represent a feedback system for neuronal activity.
Terminal Bouton (Axon Terminal) The end of the axon where it forms a synapse with another neuron or an effector cell. Mitochondria are abundant in the terminal bouton because the secretion of neurotransmitters requires a great deal of energy.
Synapse The site at which neurons communicate. Most synapses in the nervous system transmit information through chemical messengers. Because signals pass across most synapses in one direction only, synapses determine the direction of information flow through the nervous system.
Specialized cell junction between two neurons where the presynaptic and postsynaptic neurons communicate
Presynaptic Neuron The neuron that conducts signals toward a synapse
Postsynaptic Neuron The neuron that transmits signals away from the synapse
Axodendritic Synapses Synapse between the terminal boutons of one neuron and the dendrites of another neuron (more common than axosomatic synapses)
Synaptic Vesicle On the presynaptic side, the terminal bouton contains synaptic vesicles. These are membrane-bound sacs filled with neurotransmitters, the molecules that transmit signals across the synapse.
Multipolar Neurons Most neurons are multipolar. More than two processes. Usually, multipolar neurons have numerous dendrites and a single axon. However, some small multipolar neurons have no axons and rely strictly on their dendrites for conducting signals.
Bipolar Neurons Rare. Two processes extending from the cell body: a fused dendrite and the axon. Found in some sensory organs like the nose, eye, and ear.
Unipolar Neurons A short, single process that emerges from the cell body and divides like an inverted T into two long branches. One process extends from the cell body and forms central and peripheral processes, which together comprise an axon. Most start out as bipolar. Unipolar neurons are found in the sensory ganglia in the PNS, where they function as sensory neurons. The short, single process near the neuron cell body divides into two longer branches
Sensory Neurons Sensory neurons, or afferent neurons, make up the sensory division of the PNS. They transmit impulses toward the CNS from sensory receptors in the PNS. Most sensory neurons are pseudounipolar, and their cell bodies are in ganglia outside the CNS. The peripheral process extends from a sensory receptor; the central process terminates in the CNS. These two processes function as one, carrying impulses directly from the peripheral receptors to the CNS. Some sensory neurons are bipolar in structure. These neurons are restricted to some of the special sense organs.
Motor Neurons Motor neurons or efferent neurons, make up the motor division of the PNS. These neurons carry impulses away from the CNS to effector organs (muscles and glands). Motor neurons are multipolar, and their cell bodies are located in the CNS (except for some neurons of the autonomic nervous system). Motor neurons form junctions with effector cells, stimulating muscles to contract or glands to secrete.
Interneurons Interneurons lie between motor and sensory neurons. These multipolar neurons are confined entirely to the CNS. Interneurons link together into chains that form complex neuronal pathways. The fact that interneurons make up 99.98% of the neurons of the body reflects the vast amount of information processed in the human CNS. These multipolar neurons show great diversity in size and in the branching patterns of their processes
Astrocytes Structure:
– Star-shaped
Function:
– Bulb ends surround neurons or capillaries.
– Regulate neurotransmitters and blood flow.
– Produce signaling molecules.
Location:
– CNS
Microglial Cells Structure:
– elongated cell bodies and cell processes with many pointed projections
Function:
– Phagocytes derived from blood cells, migrate to CNS. Destroy pathogens and dead neurons.
Location:
– CNS
Ependymal Cells Structure:
– Form a simple epithelium
Function:
– Provide a fairly permeable layer between the cerebrospinal fluid that fills this cavity and the tissue fluid that bathes the cells of the CNS. Ependymal cells bear cilia that help circulate the cerebrospinal fluid.
Location:
– CNS
Oligodendrocytes Structure:
– Line up in small groups, wrap around axons
Function:
– Producing insulating coverings called myelin sheaths.
Location:
– CNS
Satellite Cells Structure:
– Surround cell bodies within ganglia
Function:
– Similar to astrocytes in CNS
Location:
– PNS
Schwann Cells Structure:
– Surround all axons in the PNS
Function:
– Sheaths in myelinated axons, surround multiple neurons in nonmyelinated axons.
Location:
– PNS
Myelin Sheaths Myelin sheaths are produced by oligodendrocytes in the CNS and Schwann cells in the PNS. These sheaths are segmented structures that are composed of the lipoprotein myelin and surround the thicker axons of the body. Each segment of myelin consists of the plasma membrane of a glial cell rolled in concentric layers around the axon. Myelin sheaths form an insulating layer that prevents the leakage of electrical current from the axon, increases the speed of impulse conduction along the axon, and makes impulse propagation more energy-efficient.
In both the central nervous system (CNS) and the peripheral nervous system (PNS), myelin sheaths are formed by glial cells (oligodendrocytes in the CNS and Schwann cells in the PNS).
The myelin sheaths in the PNS are formed by Schwann cells. Each Schwann cell can form myelin sheaths around only one axon. The myelin sheaths produced by Schwann cells are typically longer than those produced by oligodendrocytes. In addition, in the PNS, there are usually no nodes of Ranvier between adjacent Schwann cells, so action potentials have to travel the entire length of each myelinated segment.
In the CNS, each oligodendrocyte can form myelin sheaths around several different axons. The myelin sheaths produced by oligodendrocytes are typically shorter than those produced by Schwann cells, and they have a slightly different composition. In addition, in the CNS, there are usually gaps in the myelin sheath called nodes of Ranvier, which allow for saltatory conduction of nerve impulses. In contrast to Schwann cells, each oligodendrocyte has multiple processes that coil around several different axons. Myelin sheath gaps are present, although they are more widely spaced than those in the PNS. As in the PNS, the thinnest axons in the CNS are nonmyelinated. These nonmyelinated axons are covered by the
Myelinated vs Non-Myelinated Axons in PNS Myelinated axons are covered by a myelin sheath, which is produced by Schwann cells, while non-myelinated axons are not covered by a myelin sheath. Only thick, rapidly conducting axons are sheathed with myelin. In contrast, thin, slowly conducting axons lack a myelin sheath and are called nonmyelinated axons.
Gray Matter The gray matter is a gray-colored zone that surrounds the hollow central cavity of the CNS. In the spinal cord, it is a butterfly-shaped region in which the dorsal half contains cell bodies of interneurons and the ventral half contains cell bodies of motor neurons. Thus, gray matter is the site where neuron cell bodies are clustered. More specifically, the gray matter of the CNS is a mixture of neuron cell bodies; dendrites; short, nonmyelinated neurons; and neuroglia. Synapses occur in gray matter.
Gray matter contains short nonmyelinated neurons and neuron cell bodies. There are areas of grey matter next to the ventricles, but also in sheets near the surface of the cerebellum and cerebrum. These sheets of gray matter at the surface are cortexes. Other gray matter is in clusters of neuron cell bodies called nuclei. Gray matter is most important for processing.
In two regions of the brain (the cerebrum and cerebellum), there is an additional layer of gray matter located superficially, the cortex.
White Matter External to the gray matter is white matter, which contains no neuron cell bodies but millions of axons and neuroglia. Its white color comes from the myelin sheaths around many of the axons. Most of these axons either ascend from the spinal cord to the brain or descend from the brain to the spinal cord, allowing these two regions of the CNS to communicate with each other. Thus, white matter consists of axons running between different parts of the CNS. Within the white matter, axons traveling to similar destinations form axon bundles called tracts.
White matter is myelinated and nonmyelinated axons. Grouped axons are arranged in tracts and travel to different parts of the CNS for processing across distant areas of the CNS.
Throughout the CNS white matter is external to gray matter, which surrounds the hollow central cavity
Tract Found within CNS. A tract refers to a bundle of nerve fibers (axons) that are located together and share a common origin, destination, and function. Tracts are usually found within the brain or spinal cord and can be either ascending (sensory) or descending (motor).
Nerve Found in the PNS
Endoneurium A delicate layer of loose connective tissue covering the Schwann cells.
Nerve Fascicle Groups of axons are bound into bundles
Perineurium Connective tissue that wraps around nerve fascicles
Epineurium A tough fibrous sheath that surrounds the whole nerve.
Rostral The higher or more anterior regions of the brain are said to lie rostrally
Caudal The inferior or more posterior parts of the CNS are said to lie caudally
Three Functions of the Brain The brain controls heart rate, respiratory rate, and blood pressure—and maintains the internal environment through control of the autonomic nervous system and the endocrine system.
Through the cranial nerves that attach to it, the brain is involved in peripheral innervation—innervation of the head, the neck, and the thoracic and abdominal viscera.
Most notably, the brain performs high-level tasks— those associated with intelligence, consciousness, memory, sensory-motor integration, emotion, behavior, and socialization.
Ventricles of the Brain The ventricles are expansions of the brain’s central cavity, filled with cerebrospinal fluid and lined by ependymal cells. They are continuous with one another and with the central canal of the spinal cord.
Lateral Ventricles In the cerebral hemispheres, curved during development. Close at anterior end, far apart at posterior end. Most superior of the ventricles.
Third Ventricle Lies in the diencephalon. Anteriorly, it connects to each lateral ventricle through an interventricular foramen.
Cerebral Aqueduct In the midbrain, the thin tubelike central cavity connects the third and fourth ventricles.
Fourth Ventricle Lies in the brain stem, dorsal to the pons and the superior half of the medulla oblongata. Three openings occur in the walls of the fourth ventricle: the paired lateral apertures and the median aperture in its roof. These holes connect the ventricles with the subarachnoid space, which surrounds the whole CNS. This connection allows cerebrospinal fluid to fill both the ventricles and the subarachnoid space. The fourth ventricle connects caudally to the central canal of the inferior medulla and spinal cord.
Brain Stem The brain stem is composed of outer white matter surrounding an inner region of gray matter. Brain nuclei of gray matter are also embedded within the white matter of the brain stem. The three regions of the brain stem are discussed in reference to the general functions of this portion of the brain.
Medulla Oblongata Location: Most caudal part of the brain stem, continuous with the spinal cord and level with the foramen magnum of the skull.
Nearby ventricle: 4th ventricle
Cranial Nerves:
vestibulocochlear nerve (cranial nerve VIII)
glossopharyngeal nerve (cranial nerve IX)
vagus nerve (cranial nerve X)
hypoglossal nerve (cranial nerve XII)
Pyramidal Tracts Large fiber tracts that originate from pyramid shaped neurons in the cerebrum and descend through the brain stem and spinal cord carrying voluntary motor output to the spinal cord.
Decussation of the Pyramids In the caudal part of the medulla, 70-90% of these pyramidal fibers cross over to the opposite side of the brain at a point called the decussation of the pyramids
Relay Nuclei This brain nucleus is a relay station for sensory information traveling to the cerebellum, especially for proprioceptive information ascending from the spinal cord. Relay nuclei such as this process and edit information before sending it along.
Pons Functions: Contains the pyramidal motor tracts, and contains nuclei that control signals from the cerebral cortex to the cerebellum to coordinate voluntary movements.
Location: Forms a bridge between the midbrain and the medulla oblongata. Ventral to the fourth ventricle.
Cranial Nerves:
vestibulocochlear nerve (cranial nerve VIII)
glossopharyngeal nerve (cranial nerve IX)
vagus nerve (cranial nerve X)
hypoglossal nerve (cranial nerve XII)
Midbrain Functions: The midbrain acts as a relay center for sensory and motor information passing between the cerebrum, cerebellum, and spinal cord. The midbrain contains the superior colliculi, which are involved in visual processing, and the inferior colliculi, which are involved in auditory processing. The midbrain contains nuclei that help to control movement, such as the substantia nigra, which produces dopamine and is involved in the control of voluntary movement. The midbrain contains nuclei that are involved in regulating arousal and consciousness, such as the reticular activating system (RAS).
Location: The most rostral of the three regions of the brain stem, the midbrain, lies between the diencephalon and the pons
Tectum The central cavity of the midbrain is the cerebral aqueduct, which divides the midbrain into a tectum dorsally
Plays an important role in orienting the body in response to sensory stimuli
Sensory stimuli + motor neurons
Cerebral Peduncles The central cavity of the midbrain is the cerebral aqueduct, which divides the midbrain into a paired cerebral peduncles ventrally
Substantia Nigra The bandlike substantia nigra, whose neuronal cell bodies contain dark melanin pigment, is deep to the pyramidal tracts in the cerebral peduncle. This brain nucleus is functionally linked to the deep gray matter of the cerebrum, the basal nuclei (ganglia), and is involved in controlling voluntary movement. Degeneration of the neurons in the substantia nigra is the cause of Parkinson’s disease.
Corpora Quadrigemina Integrates auditory and visual reflexes, makes up the tectum.
Divided into 4 nuclei:
2 x superior colliculi = visual reflex
2 x inferior colliculi = auditory reflex
Cerebellum Location – The cauliflower-like cerebellum, the second of the brain’s major parts as we move caudally to rostrally, makes up 11% of the mass of the brain. The cerebellum is located dorsal to the pons and medulla oblongata, from which it is separated by the fourth ventricle
Major Function – smooths/coordinates body movements that are directed by other brain regions. Maintains posture/equilibrium.
Regions: Rostral to Caudal
– Cerebellar cortex (gray matter)
– Arbor vitae (white matter)
– Deep cerebellar nuclei (deep gray matter)
Cerebellar Hemispheres The cerebellum consists of two expanded cerebellar hemispheres connected medially by the wormlike vermis
Vermis Wormlike, medially connects the cerebellar hemispheres
Folia The surface of the cerebellum is folded into many platelike ridges called folia, which are separated by deep grooves called fissures.
Fissures Deep grooves that separate the folia
Arbor Vitae Internal white matter; the axons that carry information to and from the cerebellum
Information Processing Cerebellum The cerebellum receives information from the cerebrum on the movements being planned. The area of the cerebrum that initiates voluntary movements, the motor cortex of the cerebrum, passes information from the cerebral cortex through the pontine nuclei in the pons to the lateral portion of the anterior and posterior lobes of the cerebellum.
The cerebellum compares these planned movements with current body position and movements. Information on equilibrium and head position is relayed from receptors in the inner ear through the vestibular nuclei in the medulla oblongata to the flocculonodular lobe. Information on the current movements of the limbs, neck, and trunk travels from the proprioceptors in muscles, tendons, and joints up the spinal cord to the vermis and medial portions of the anterior and posterior lobes.
The cerebellum sends instructions back to the cerebral cortex on how to resolve any differences between the intended movements and current position. Using this feedback from the cerebellum, the motor cortex of the cerebrum continuously readjusts the motor commands it sends to the spinal cord, fine-tuning movements so that they are well coordinated.
Cerebellar Peduncles The superior, middle, and inferior cerebellar peduncles are thick fiber tracts that connect the cerebellum to the brain stem. These fiber tracts carry the information that travels from and to the cerebellum.
The superior cerebellar peduncles connect the cerebellum to the midbrain, carrying primarily efferent instructions from the cerebellum toward the cerebral cortex.
The middle cerebellar peduncles connect the pons to the cerebellum. This afferent pathway carries information from the cerebral cortex and the pontine nuclei into the cerebellum.
The inferior cerebellar peduncles arise from the medulla and carry primarily afferent fibers from the vestibular nuclei (equilibrium) and from the spinal cord (proprioception) into the cerebellum.
Diencephalon thalamus, hypothalamus, epithalamus
Thalamus The egg-shaped thalamus is a paired structure that makes up 80% of the diencephalon and forms the superolateral walls of the third ventricle.
Location: Located on each side of the third ventricle, superior to the midbrain.
Function: It is a major relay center for sensory impulses ascending from other parts of the nervous system to the cerebral cortex. It receives almost all sensory information (except for olfaction) and filters and relays this information to appropriate areas of the cerebral cortex for further processing. Additionally, the thalamus plays a role in regulating consciousness, sleep, and alertness.
Gateway to Cerebral Cortex Every part of the brain that communicates with the cerebral cortex must relay its signals through a nucleus of the thalamus. The thalamus can therefore be thought of as the “gateway” to the cerebral cortex.
Hypothalamus The inferior portion of the diencephalon. It forms the inferolateral walls of the third ventricle.
Location: On the underside of the brain, the hypothalamus lies between the optic chiasma (point of crossover of cranial nerves II, the optic nerves) and the posterior border of the mammillary bodies, rounded bumps that bulge from the hypothalamic floor (mammillary = “little breast”).
Functions:
controls ANS
regulates body temperature
regulation of hunger/thirst
Regulation of sleep/wake cycles
control of endocrine system
control of emotional response
control of motivational behavior
formation of memory
Epithalamus The third and most dorsal part of the diencephalon, forms part of the roof of the third ventricle
Location: Superior and posterior to the thalamus.
Function: Its main function is to regulate the sleep-wake cycle and other circadian rhythms by producing and releasing the hormone melatonin from the pineal gland. Additionally, the epithalamus is involved in the regulation of mood and the perception of pain.
Pineal Gland This gland, which derives from ependymal glial cells, is a hormone-secreting organ, secretes melatonin. Signals the body to prepare for the nighttime stage of the sleep-wake cycle.
Transverse Fissure separates the cerebellum from cerebral cortex
Longitudinal Fissure separates the left and right cerebral hemispheres
Cerebrum The cerebrum is composed of a superficial cerebral cortex of gray matter, the cerebral white matter internal to it, and the deep gray matter of the cerebrum within the white matter.
There are many shallow grooves on the surface of the cerebral hemispheres called sulci. Between the sulci are twisted ridges of brain tissue called gyri. The more prominent gyri and sulci are similar in all people and are important anatomical landmarks. Some of the deeper sulci divide each cerebral hemisphere into five major lobes: the frontal, parietal, occipital, and temporal lobes, and the insula
Frontal Lobe Located deep to the frontal bone and fills the anterior cranial fossa. It extends posteriorly to the central sulcus.
Functions: Voluntary movement (primary motor cortex) Planning movement (premotor cortex) Eye movement (frontal eye field) Speech production (Broca’s area) Executive cognitive functions (anterior association area) Emotional response (limbic association area)
Parietal Lobe Deep to the parietal bones, extends posteriorly from the central sulcus to the parieto-occipital sulcus.
Functions: General somatic sensation (somatosensory cortex and association area) Spatial awareness of objects, sounds, body parts (posterior association area) Understanding speech (Wernicke’s area)
Occipital Lobe Lies deep to the occipital bone and forms the most posterior portion of the cerebrum. It is separated from the parietal lobe by the parieto-occipital sulcus on the medial surface of the hemiphere.
Functions: Vision (visual cortex and association areas)
Temporal Lobe On the lateral side of the hemisphere, lies in the middle cranial fossa deep to the temporal bone. It is separated from the overlying parietal and frontal lobes by the deep lateral sulcus.
Functions: Hearing (auditory cortex and association area) Smell (olfactory cortex) Object identification (posterior association area) Emotional response, memory (limbic association area)
Insula (Insular Lobe) Lobe not visible on the surface. It is deep to lateral sulcus. gustatory cortex (taste) It contains the visceral sensory cortex for taste and general visceral sensations.
Precentral Gyrus Contains the primary motor cortex, lies just anterior to the central sulcus.
Postcentral Gyrus Posterior to the central sulcus, contains the primary somatosensory cortex
Central Sulcus Separates frontal lobe from parietal lobe. Just posterior to the precentral gyrus.
Parieto-Occipital Sulcus Deep groove in the brain that separates the parietal and occipital lobes of the cerebrum. The parieto-occipital sulcus serves as an important landmark in the brain, helping to define the boundaries between different regions of the cortex.
Lateral Sulcus Forms its inferior boundary.
Fissure vs Sulcus In the context of the brain, a fissure is a deep groove or indentation that separates major regions of the brain, while a sulcus is a shallower groove or indentation that separates smaller regions within those major regions. Essentially, a fissure is a larger and deeper version of a sulcus. The terms are often used interchangeably, but fissures are generally larger and more prominent, while sulci are smaller and more numerous.
Corpus Callosum The largest commissure that is a broad band that lies superior to the lateral ventricles, deep within the longitudinal fissure.
The corpus callosum is a large white matter tract that connects the two hemispheres of the cerebrum. It is located in the longitudinal fissure, which separates the left and right cerebral hemispheres. The corpus callosum is the main means of communication between the two hemispheres, allowing them to exchange information and coordinate their activities. It is composed of millions of axons that originate from various regions of the cortex and terminate in corresponding regions of the contralateral hemisphere.
Sensory Areas Allow conscious awareness of sensation
Association Areas areas of the CNS that integrate information from different sources to make informed decisions
Motor Areas The regions of the cortex that plan and initiate voluntary motor functions
Contralateral Projection The primary somatosensory cortex exhibits contralateral projection from the sensory receptors to the sensory cortex. This means that the right cerebral hemisphere receives its sensory input from the left side of the body, and the left cerebral hemisphere receives its sensory input from the right side of the body.
Limbic System Emotional Brain
Location: Located in the medial aspect of each cerebral hemisphere and the diencephalon
Function: regulates how we react to emotions, pulls memories from strong emotion. The limbic system crosses over into the olfactory cortex hence why smells can trigger a memory. Responsible for the emotional impact actions, behavior, and situations have on us (our “feelings”); it directs our response to these emotions; and it functions in creating, storing, and retrieving memories, particularly those that elict strong emotions.
Cingulate Gyrus Part of the cerebral cortex located superior to the corpus callosum.
Hippocampal Formation It is linked to all other regions of the cerebral cortex. Through its connections to the sensory regions, the cingulate gyrus mediates the emotional response to these stimuli, such as experiencing painful stimuli as unpleasant. Its connections with the hypothalamus and prefrontal cortex function to generate and control visceral and behavorial responses to emotions.
Amygdaloid Body Subcortical gray matter that contains the key brain nuclei for processing fear and stimulating the appropriate sympathetic response to fear. Processes and stores memory based on fear.
Reticular Formation Groups of brain nuclei that form columns down the length of the brain stem. Some of these nuclei work with the motor neurons of the cranial nerves to coordinate their actions, including heart rate, blood pressure, and breathing.
Location: Runs through the central core of the brain stem
Function: The reticular formation is a complex network of nuclei and nerve fibers located throughout the brainstem. It is involved in regulating consciousness and alertness, as well as controlling many essential autonomic functions such as respiration, cardiovascular regulation, and gastrointestinal motility. The reticular formation also plays a role in the modulation of pain and in the processing of sensory information, including hearing and vision. Additionally, it is involved in regulating the sleep-wake cycle and in controlling motor reflexes.
Protection of the Brain Meninges
CSF (cerebrospinal fluid)
Blood brain barrier
Three Meninges of Brain The subarachnoid space is the space between the arachnoid and pia mater, which is filled with cerebrospinal fluid (CSF) that helps to cushion and protect the brain and spinal cord. The CSF also helps to transport nutrients, remove waste, and regulate pressure in the brain. The subarachnoid space also contains blood vessels that supply oxygen and nutrients to the brain.
Dura Mater The strongest of the meninges. Where it surrounds the brain, the dura mater is a two-layered sheet of dense fibrous connective tissue. The dura mater is the tough, outermost layer that provides protection and support to the brain.
Arachnoid Mater The middle layer and is web-like in appearance, providing cushioning for the brain. The arachnoid mater lies just deep to the dura mater.
Pia Mater A layer of delicate connective tissue richly vascularized with fine blood vessels. Unlike the other meninges, it clings tightly to the brain surface, following every convolution. As its arteries enter the brain tissue, they carry ragged sheaths of pia mater internally for short distances.
Blood Brain Barrier The blood-brain barrier is a highly selective semipermeable border that separates the circulating blood from the brain extracellular fluid in the central nervous system. It is formed by tight junctions between the endothelial cells that line the capillaries of the brain.
Protects the neurons of the CNS internally from harmful substances carried in the blood, such as toxic chemicals or microorganisms. All nutrients (including oxygen) and ions needed by the neurons pass through, some by special transport mechanisms in the plasma membranes of the capillary epithelial cells. Furthermore, because the barrier is ineffective against fat-soluble molecules, which easily diffuse through all cell membranes, the barrier allows alcohol, nicotine, and anesthetic agents to reach brain neurons.
Cerebral Spinal Fluid (CSF) Cerebrospinal fluid (CSF) is a watery broth that fills the subarachnoid space and the central hollow cavities of the brain and spinal cord. It aids in protecting and nourishing the neural tissue.
Protection: CSF acts as a cushion for the brain and spinal cord, protecting them from mechanical shock or injury. The layer of CSF surrounding the central nervous system resists compressive forces and cushions the brain and spinal cord from blows and jolts.
Buoyancy: The brain is suspended in CSF, which helps to reduce its effective weight and prevents it from being crushed under its own weight.
Nourishing: CSF helps to nourish the brain, to remove wastes produced by neurons, and to carry chemical signals such as hormones between different parts of the central nervous system. Although similar in composition to the blood plasma from which it arises, CSF contains more sodium and chloride ions and less protein.
CSF continuously forms from blood plasma via filtration
Choroid plexus: A capillary rich membrane on the roof of the brain that forms the CSF; technically it consists of pia mater and ependymal cells
Spinal Nerves Thirty-one pairs of spinal nerves (PNS structures) attach to the spinal cord through dorsal and ventral nerve roots
The spinal nerves are named based on the vertebral locations in which they lie.
The 31 spinal nerves are divided into cervical (8), thoracic (12), lumbar (5), sacral (5), and coccygeal (1) groups.
Spinal Cord Because the spinal cord does not extend to the end of the spinal column, the spinal cord segments are located superior to where their corresponding spinal nerves emerge through the intervertebral foramina.
For example, spinal cord segment T5 is located at the level of vertebra T4. This discrepancy is most pronounced in the lumbar and sacral regions of the cord: The lumbar cord segment L1 is located at vertebra T11, and the sacral cord segment S1 is at vertebra L1.
The spinal cord runs superiorly from the foramen magnum to the level of the first or second lumbar vertebra. It runs inferiorly as a tapering structure called the conus medullaris, which ends at the level of the intervertebral disc between the first and second lumbar vertebrae in adults. Below the conus medullaris, the spinal cord continues as a bundle of nerve roots called the cauda equina.
Vertebral Canal The spinal cord runs through the vertebral canal of the vertebral column. The vertebral canal is formed from the successive vertebral foramina of the articulated vertebrae.
Conus Medullaris At its inferior end, the spinal cord tapers into the conus medullaris
Filum Terminale A long filament of connective tissue extends from the conus medullaris and attaches to the coccyx inferiorly, anchoring the spinal cord in place so that it is not jostled by body movements
Cervical Enlargement In the cervical and lumbar regions of the spinal cord, where the nerves to the upper and lower limbs arise, the spinal cord shows obvious enlargements. Responsible for innervating the upper limbs.
Lumbar Enlargement In the cervical and lumbar regions of the spinal cord, where the nerves to the upper and lower limbs arise, the spinal cord shows obvious enlargements. Responsible for innervating the lower limbs.
Cauda Equina Collection of nerve roots at the inferior end of the vertebral canal
Ascending Most of the ascending fibers in the spinal cord carry sensory information from the sensory neurons of the body up to the brain.
Descending Most descending fibers carry motor instructions from the brain to the spinal cord, to stimulate contraction of the body’s muscles and secretion from its glands.
Commissural Commissural fibers are white-matter fibers that carry information from one side of the spinal cord to the other.
Dorsal/Posterior Funiculus Located between the dorsal horns of the gray matter and the posterior median sulcus of the spinal cord. It contains ascending tracts that carry sensory information, such as proprioception, touch, and vibration, to the brain.
Ventral/Anterior Funiculus Located between the ventral horns of the gray matter and the anterior median fissure of the spinal cord. It contains descending tracts that carry motor information, such as voluntary movement and reflexes, from the brain to the spinal cord.
Lateral Funiculus Located on each lateral side of the spinal cord, between the anterior and posterior funiculi. It contains both ascending and descending tracts that carry sensory and motor information, respectively.
Dorsal Horns Found in the dorsal part of the spinal cord. Contains only interneurons, receives sensory information from the sensory neurons that have their cell bodies in the dorsal root ganglion.
Ventral Horns Located in the anterior part of the gray matter and contain motor neurons that send signals out to muscles and glands through efferent fibers exiting the spinal cord through the ventral roots of spinal nerves.
Lateral Horns In the thoracic and superior lumbar segments of the spinal cord, small lateral gray matter columns are present. They contain autonomic motor neurons that regulate visceral activities such as the activity of the heart, smooth muscle, and glands.
Dorsal Root Ganglion A cluster of cell bodies of sensory neurons that are located outside the spinal cord and are attached to the dorsal root of the spinal nerve. These neurons receive sensory information from the periphery and transmit it to the dorsal horns of the gray matter.
The dorsal root contains the axonal processes of sensory neurons arising from cell bodies in the dorsal root ganglion.
Dorsal Roots These are the sensory input fibers that enter the spinal cord through the dorsal surface and synapse with the neurons in the dorsal horns of the gray matter.
receives
Ventral Roots These are the motor output fibers that originate from the ventral horn of the gray matter and exit the spinal cord through the ventral surface to innervate the muscles and glands
sends out
Spinal Dural Sheath The tough dura mater does not attach to the surrounding bone and corresponds only to the meningeal layer of the brain’s dura mater. A tough membrane that covers the spinal cord and provides an extra layer of protection. It is continuous with the meninges that cover the brain.
Epidural Space Space filled with fat (cushion) and many veins. Continuous with corresponding space under the dura mater within the brain. It provides additional cushioning and support for the spinal cord.
Denticulate Ligaments Extensions of the pia mater that anchor the spinal cord to the dural sheath, helping to keep it stable and centered within the spinal canal. They run along the length of the spinal cord and prevent excessive movement of the cord within the canal.
Sensory Receptors Picks up stimuli (environmental changes) from inside and outside the body and then initiate impulses in sensory axons, which carry the impulse to the CNS.
Nerves (including the three types) Bundles of peripheral axons. Most nerves contain both sensory and motor axons and are called mixed nerves. Certain cranial nerves (the nerves attached to the brain) contain only sensory axons and thus are purely sensory in function; certain others contain primarily motor axons and are motor in function.
Ganglia Collections of neuron cell bodies outside the central nervous system
a cluster of neuronal cell bodies in the PNS
Sympathetic Ganglia is near the spinal cord, Parasympathetic Ganglia is near the organs
Motor Endings Terminal boutons of motor neurons that innervate the effector organs, muscles, and glands.
Cranial Nerves 12 pairs of cranial nerves that attach to the brain and pass through various foramina in the skull.
These nerves are numbered from I through XII in a rostral to caudal direction. The first two pairs attach to the forebrain, the rest to the brain stem. Except for the vagus nerve (X), which extends into the abdomen, the cranial nerves innervate only head and neck structures.
The cell bodies of the sensory neurons lie either in receptor organs (e.g., the nose for smell, or the eye for vision) or within cranial sensory ganglia, which lie along some cranial nerves (V, VII-X) just external to the brain.
Primary Sensory (I, II, VIII) contain special sensory fibers for smell (I), vision (II), hearing and equilibrium (VIII).
Primary Motor (III, IV, VI, XI, XII) contain somatic motor fibers to skeletal muscles of the eye, neck, and tongue
Motor and Sensory / Mixed (V, VII, IX, X) These mixed nerves supply sensory innervation to the face (through general somatic sensory fibers) and to the mouth and viscera (general visceral sensory), including the taste buds for the sense of taste (special visceral sensory). These nerves also innervate pharyngeal arch muscles (somatic motor), such as the chewing muscles (V) and the muscles of facial expression (VII).
Olfactory (I) Sensory Type: Visceral
Sensory Function:
Special visceral sensory, sense of smell
Optic (II) Sensory Type: Somatic
Sensory Function:
Special somatic sensory, vision.
Oculomotor (III) Motor Type: Somatic & Visceral
Motor Function:
SM: Multiple muscles that move the eyeball, elevate the eyelid
VM: Constrict pupil & lens shape
Origin: midbrain
Trochlear (IV) Motor Type: Somatic
Motor Function:
Innervate the superior oblique muscle. This muscle passes through a ligamentous pulley at the roof of the orbit, the trochlea, from which its name is derived. Afferent proprioceptor fibers return from the superior oblique.
Origin: midbrain
Trigeminal (V) Sensory Type: Somatic
Sensory Function:
sensation from face and teeth
Motor Type: Somatic
Motor Function:
muscles of mastication
Origin: pons
Abducens (VI) Motor Type: Somatic
Motor Function:
Innervate the lateral rectus muscle. This muscle abducts the eye. Afferent proprioceptor fibers return from the lateral rectus.
Origin: pons
Facial (VII) Sensory Type: Somatic & Visceral
Sensory Function:
VS: taste buds on anterior two-thirds of tongue.
SS: Small patch of skin on the ear.
Motor Type: Somatic & Visceral
Motor Function:
SM: Innervates the posterior belly of digastric + facial muscles
VM: Innervate the lacrimal (tear) glands, nasal and palatine glands, and the submandibular and sublingual salivary glands.
Origin: pons
Vestibulocochlear (VIII) Sensory Type: Somatic
Sensory Function:
Vestibular branch: Special somatic sensory, equilibrium.
Cochlear branch: Special somatic sensory, hearing.
Small motor component adjusts the sensitivity of the sensory receptors
Glossopharyngeal (IX) Sensory Type: Somatic & Visceral
Sensory Function:
SS: skin of ear
VS: taste, CO2 and baroreceptors
Motor Type: Somatic & Visceral
Motor Function:
parotid salivary gland
Origin: medulla
Vagus (X) Sensory Type: Somatic & Visceral
Sensory Function:
VS: thoracic and abdominal viscera, mucosa of larynx and pharynx, carotid sinus (baroreceptor for blood pressure), and carotid and aortic bodies (chemoreceptors for respiration). Taste buds on the epiglottis.
SS: Small area of skin on external ear.
Motor Type: Somatic & Visceral
Motor Function:
SM: Innervates skeletal muscles of the pharynx and larynx involved in swallowing and vocalization. Afferent proprioceptor fibers return from the muscles of the larynx and pharynx.
VM: Innervates the heart, lungs, and abdominal viscera through the transverse colon. Regulates heart rate, breathing, and digestive system activity
Origin: medulla
Accessory (XI) Motor Type: Somatic
Motor Function:
Innervate the trapezius and sternocleidomastoid muscles that move the head and neck. Afferent proprioceptor fibers return from these muscles
Origin: spinal cord
Hypoglossal (XII) Motor Type: Somatic
Motor Function:
Innervate the intrinsic and extrinsic muscles of the tongue. Aid tongue movements during feeding, swallowing, and speech. Afferent proprioceptor fibers return from these muscles.
Origin: medulla
Spinal Nerves in Spinal Cord Thirty-one pairs of spinal nerves, each containing thousands of nerve fibers, attach to the spinal cord
Cervical: 8
Thoracic: 12
Lumbar: 5
Sacral: 5
Coccygeal: 1
Dorsal Root Contains sensory fibers entering the spinal cord, and it enlarges into a dorsal root ganglion, which contains the cell bodies of the sensory neurons. Each spinal nerve connects to the spinal cord by a dorsal root and a ventral root
Ventral Root Contains motor fibers exiting the spinal cord. Each spinal nerve connects to the spinal cord by a dorsal root and a ventral root. The ventral root contains the axonal processes of motor neurons whose cell bodies are located in the ventral gray column of the spinal cord.
Rootlets Segments that connect spinal nerves from spinal cord to nerve roots (both dorsal and ventral). Dorsal rootlets carry sensory impulses to spinal cord, ventral rootlets carry motor info from spinal cord to the body.
Roots they lie medial to the spinal nerves and are either strictly sensory (dorsal roots) or strictly motor (ventral root).
Rami The rami of spinal nerves are lateral branches of the spinal nerves, and each contains both sensory fibers and motor fibers.
Dorsal Ramus Innervates the muscles, joints, and skin of the back. The dorsal rami supply the dorsum of the neck and the back.
Ventral Ramus The much thicker ventral rami supply a larger area: the anterior and lateral regions of the neck and trunk, and all regions of the limbs.
Rami Communicantes Connects to the base of the ventral ramus. Small branches that connect the spinal nerve to the sympathetic trunk ganglia.
Sympathetic Trunk Ganglion Branched from rami communicantes, located along both sides of the vertebral column, and they carry sympathetic ganglia.
A cell that is releasing acetylcholine in the sympathetic trunk ganglion has its cell body in the lateral horn of the spinal cord
Intercostal Nerves The thoracic ventral rami run anteriorly, one deep to each rib. These nerves supply the intercostal muscles, the skin of the anterior and lateral thorax, and most of the abdominal wall inferior to the rib cage.
Lateral and Anterior Cutaneous Branches Along its course, each intercostal nerve gives off lateral and anterior cutaneous branches to the adjacent skin. The last (T12) lies inferior to the twelfth rib and thus is called a subcostal (“below the ribs”) nerve rather than an intercostal nerve. The first (most superior) intercostal nerve is exceptionally small because most fibers of T1 enter the brachial plexus (discussed shortly).
Nerve Plexus A network of nerves. The ventral rami of all spinal nerves except T2-T12 branch and join one another lateral to the vertebral column, forming nerve plexuses. The fibers from different spinal nerves mix in the plexus, and the resulting nerves that emerge from the plexus contain fibers from multiple spinal nerves. These interlacing networks occur in the cervical, brachial, lumbar, and sacral regions and primarily serve the limbs. Formed by ventral rami only.
Cervical Description:
Formed by the ventral rami of the first four cervical nerves
Location:
Buried deep in the neck, under the sternocleidomastoid muscle, and extends into the posterior triangle of the neck
Function:
Carry sensory impulses from the skin of the neck, the back of the head, and the most superior part of the shoulder. Other branches carry motor innervation to muscles in the anterior neck region
Spinal Nerves: C1-C5
Key Nerves:
Superficial
– Lesser occipital
– Greater auricular
– Transverse cervical
– Supraclavicular (anterior, middle, + posterior)
Deep
– Ansa Cervicalis (superior + inferior roots)
– Segmental + other muscular branches
– Phrenic
Brachial Description:
has 4 components (from medial to lateral):
ventral rami → trunks → divisions → cords
Location:
Partially in neck & partially in axilla (armpit)
Deep to SCM muscle
Function:
Give rises to almost all nerves that supply the upper limb
Spinal Nerves: C5 – C8, T1
Key Nerves:
Anterior compartment:
– musculocutaneous
– median
– ulnar
Posterior compartment:
– radial
– axillary
Lumbar Description:
anterior rami of spinal nerves L1-L4 and contains several major nerves including the femoral nerve, obturator nerve, and the lumbosacral trunk.
Location:
First four lumbar spinal nerves (L1-L4) and lies within the psoas major muscle in the posterior abdominal wall.
Function:
Its smaller branches innervate parts of the abdominal wall and the psoas muscle itself, but the main branches descend to innervate the anterior thigh.
Spinal Nerves: L1-L4
Key Nerves:
Iliohypogastric
Ilioinguinal
Genitofemoral
Lateral femoral cutaneous
Sacral Description:
Formed by the anterior rami of spinal nerves L4-S4
Location:
Lies immediately caudal to the lumbar plexus
Function:
Supplies all of the lower limb except the anterior and medial regions of the thigh. The tibial nerve and its branches supply almost all muscles in the posterior region of the lower limb and supply cutaneous innervation to (1) the skin of the sole of the foot, through the plantar nerves, and (2) a vertical strip of skin along the posterior leg through a sural nerve, to which the common fibular nerve also contributes
Spinal Nerves: L4-S4
Key Nerves:
Superior gluteal
Inferior gluteal
Posterior femoral cutaneous
Pudendal
Phrenic Part of What Plexus: cervical
Spinal Nerves: C3-C5
Main Structures:
Motor: innervates diaphragm (critical for breathing)
Sensory: diaphragm – hiccups
Musculocutaneous Part of What Plexus: brachial
Spinal Nerves: C5-C7
Main Structures:
Motor: coracobrachialis, biceps brachii, and brachialis
Median Part of What Plexus: brachial
Spinal Nerves: C5-T1
Main Structures:
most muscles of the anterior forearm and the lateral palm, most muscles in the flexor compartment of the forearm, five intrinsic muscles in the lateral part of the palm
Ulnar Part of What Plexus: brachial
Spinal Nerves: C8, T1
Main Structures:
Branches off the medial cord of the brachial plexus, descends along the medial side of the arm; supplies the flexor carpi ulnaris and the medial (ulnar) part of the flexor digitorum profundus; innervates most of the intrinsic hand muscles and the skin on the medial side of the hand
Radial Part of What Plexus: brachial
Spinal Nerves: C5-T1
Main Structures:
Motor: Innervates extensor muscles of arm and forearm
Sensory: skin of dorsolateral surface of hand
Axillary Part of What Plexus: brachial
Spinal Nerves: C5-C6
Main Structures:
Runs posterior to the surgical neck of the humerus and innervates the deltoid and teres minor muscles; supply the capsule of the shoulder joint and the skin covering the inferior half of the deltoid muscle
Femoral Part of What Plexus: lumbar
Spinal Nerves: L2,L3,L4
Main Structures:
Runs deep to the inguinal ligament to enter the thigh, descends vertically through the center of the femoral triangle; innervates the muscles of the anterior compartment of the thigh, including the quadriceps femoris; cutaneous branches serve the skin of the anterior thigh and the medial surface of the leg form the knee to the foot
Obturator Part of What Plexus: lumbar
Spinal Nerves: L2,L3,L4
Main Structures:
Motor: Innervates adductor muscle group of medial thigh
Sensory: skin of superomedial thigh
Sciatic Part of What Plexus: sacral
Spinal Nerves: L4, L5, S1, S2 and S3
Main Structures:
Supplies all of the lower limb except the anterior and medial region of the thigh; has the tibial and common fibular nerves that descends through the posterior thigh deep to the hamstrings, which it innervates
Tibial Part of What Plexus: sacral
Spinal Nerves: L4, L5, S1, S2 and S3
Main Structures:
Courses through the popliteal fossa, descends through the calf deep to the soleus muscle; supply almost all muscles in the posterior region of the lower limb and supply cutaneous innervation to the skin of the sole of the foot and a vertical strip of skin along the posterior leg through a surval nerves
Common Fibular Part of What Plexus: sacral
Spinal Nerves: L4, L5, S1, S2
Main Structures:
Supplies most structures on the anterolateral aspect of the leg; descends laterally from its point of origin in the popliteal fossa and enters the superior part of the leg
Cutaneous Nerves Most branches of the cervical plexus that carry sensory impulses from the skin of the neck, the back of the head, and the most superior part of the shoulder.
Ventral Rami Divide into anterior and posterior divisions, and these divisions combine to form the lateral, medial, and posterior cords of the brachial plexus. The ventral rami from spinal segments C59T1 form the roots* of the brachial plexus.
Trunks (Upper, Middle, Lower) Branches from the ventral rami, continues into divisions
Upper trunk: a combination of C5 and C6 roots.
Middle trunk: continuation of C7 root.
Lower trunk: continuation of C8 & T1 roots.
Divisions (Anterior, Posterior) Each trunk splits into two divisions, anterior and posterior
Anterior: give rise to nerves that innervate the anterior compartment muscles in the upper limb (flexor muscles) and skin on the anterior surface.
Posterior: serve the limb’s posterior compartment muscles (extensor muscles) and skin on the posterior surface.
Cords (Lateral, Medial, Posterior) These six divisions then converge to from three cords
Lateral: The anterior divisions from the upper and middle trunks give rise to the lateral cord
Medial: the anterior division from the lower trunk forms the medial cord
Posterior: composed of the posterior divisions of all three trunks
Hilton’s Law Any nerve that innervates a muscle producing movement at a joint also innervates the joint itself (and the skin over it). This means that the nerves that innervate a joint also innervate the muscles that move that joint, as well as the skin that covers those muscles.
This law allows us to determine which nerves innervate a joint by examining which muscles move the joint and which skin covers those muscles.
Dermatones The area of skin innervated by the cutaneous branches from a single spinal nerve, literally a “skin segment.” All spinal nerves except C1 participate in dermatomes.
Autonomic Nervous System (ANS) The ANS is the system of motor neurons that innervate the smooth muscle, cardiac muscle, and glands of the body. By controlling these effectors, the ANS regulates such visceral functions as heart rate, blood pressure, digestion, and urination, which are essential for maintaining the stability of the body’s internal environment. The ANS is the general visceral motor division of the peripheral nervous system and is distinct from the general somatic motor division, which innervates the skeletal muscles
The ANS contains two types of neurons: preganglionic neurons and postganglionic neurons. Preganglionic neurons originate in the brain or spinal cord and synapse with postganglionic neurons in autonomic ganglia located outside the CNS. Postganglionic neurons extend from the ganglia to innervate the target organs.
The effectors of the ANS include smooth muscle, cardiac muscle, and glands. The SNS and PSNS have opposing effects on these effectors. For example, the SNS increases heart rate and respiratory rate, dilates pupils, and inhibits digestion, while the PSNS slows heart rate and respiratory rate, constricts pupils, and stimulates digestion.
Somatic Motor Nervous System The cell body of a somatic motor neuron is located in the ventral horn of the spinal cord.
In the somatic motor nervous system, there is a single neuron pathway from the central nervous system to the effector organ, which is the skeletal muscle
The primary neurotransmitter used in the somatic motor system is acetylcholine (ACh).
Preganglionic Neuron A type of neuron that originates in the central nervous system (CNS) and synapses with a postganglionic neuron within an autonomic ganglion. Transmit signals from the CNS to the autonomic ganglia. Responsible for releasing the neurotransmitter acetylcholine (ACh) onto the postganglionic neuron in the ganglion. Signals the postganglionic neuron.
Preganglionic Axon (Fiber) Axon of the preganglionic neuron that extends from the cell body of a preganglionic neuron to synapse with the second motor neuron, the postganglionic neuron. These axons are usually myelinated and release acetylcholine as their neurotransmitter at the synapse with the postganglionic neuron. Transmit signals from the central nervous system to the autonomic ganglion, where it synapses with the postganglionic neuron to continue the signal to the effector organ. Axons of preganglionic neurons are thin, lightly myelinated fibers
Postganglionic Neuron Stimulates muscle contraction or gland secretion in the effector organ. Transmit signals that control involuntary processes such as heart rate, digestion, and breathing. The postganglionic neurons are responsible for the second stage of the two-neuron chain of the autonomic nervous system.
Autonomic Ganglion The motor ganglion where the axon of the preganglionic neuron synapses with the postganglionic neuron. This occurs outside the CNS.
Postganglionic Axon (Fiber) Axons of postganglionic neurons are even thinner and are unmyelinated. Impulses are conducted through the autonomic nervous system more slowly than through the somatic motor system.
Sympathetic vs Parasympathetic Sympathetic:
– mobilizes the body during extreme situations (fight or flight) such as exercise, excitement, or emergencies.
– innervated mostly in thoracic and lumbar regions, has long postganglionic fibers, preganglionic fibers are highly branched which can influence many organs at once
– preganglionic neurons release acetylcholine (ACh); postganglionic neurons release norepinephrine (NE, adrenergic).
– lateral horn of grey matter from T1 to L2
– near the spinal cord and vertebral column; sympathetic ganglia, which lie in two chains along either side of the vertebral column
Parasympathetic:
– Maintenance functions; conserves and stores energy; “rest and digest.”
– fibers emerge from the brain (cranial part) and the sacral spinal cord (sacral part); short postganglionic axons; ganglia lie far from the CNS, in or near the organs innervated; fibers do not branch profusely; Effects are more localized and discrete.
– preganglionic and postganglionic neurons release acetylcholine (ACh)
– grey matter of brainstem (CN III, VII, IX, X), and sacral region of spinal cord (S2 – S4)
– ganglia near or within the target organ; lie far from the CNS, in or near the organs innervated
Oculomotor Nerve (III) Innervate smooth muscles in the eye that cause the pupil to constrict and the lens of the eye to bulge (actions that allow focusing on close objects in the field of vision). Supplies the parasympathetic innervation of the head.
Facial Nerve (VII) Stimulates secretion of glands in the head (lacrimal/tear above the eye, mucus-secreting glands in nasal cavity, and salivary glands of the mouth). Supplies the parasympathetic innervation of the head.
Glossopharyngeal Nerve (IX) Stimulate secretion of a large salivary gland, the parotid gland, which lies anterior to the ear. Supplies the parasympathetic innervation of the head.
Vagus Nerve (X) Innervates the visceral organs of the thorax and most of the abdomen (does not include the pelvic organs and the vagal innervation of the digestive tube ends halfway along the large intestine). Contains nearly 90% of the preganglionic parasympathetic fibers in the body. Stimulation of digestion, reduction in the heart rate, and constriction of the bronchi in the lungs.
Parasympathetic Nervous System Sacral Outflow Nerves involved: S2-S4
Organs: bladder, distal half of the large intestine, and reproductive organs such as the uterus, and the erectile tissues of the external genitalia
Function: stimulation of defecation, voiding of urine, and erection
Sympathetic Trunk A paired bundle of nerve fibers that runs from the base of the skull to the bottom of the vertebral column. It contains mostly preganglionic sympathetic axons.
White Rami Communicantes Join sympathetic trunk ganglia to the ventral rami of nearby spinal nerves. They lie lateral to gray rami communicantes
Gray Rami Communicantes Join sympathetic trunk ganglia to the ventral rami of nearby spinal nerves. Contain postganglionic sympathetic fibers that travel from the sympathetic trunk ganglia to the spinal nerves. Lies laterally to white rami communicantes.
Superior, Middle, and Inferior Cervical Ganglia Three ganglia located in the neck, adjacent to the cervical vertebrae. Responsible for providing sympathetic innervation to the structures in the neck region.
Stellate Ganglion Located in the neck, anterior to the transverse process of the seventh cervical vertebra. It is formed by the fusion of the inferior cervical ganglion and the first thoracic ganglion in the superior thorax. The stellate ganglion innervates the upper extremity and the head and neck region.
Sympathetic Trunk Ganglia vs Dorsal Root Ganglia Location: The sympathetic trunk ganglia are located near the spinal cord, while the dorsal root ganglia are located along the dorsal roots of spinal nerves.
Function: The sympathetic trunk ganglia contain cell bodies of postganglionic sympathetic neurons that innervate visceral organs and blood vessels, while the dorsal root ganglia contain cell bodies of sensory neurons that receive sensory information from the body and send it to the central nervous system.
Autonomic Plexuses The autonomic plexuses are the networks of nerve fibers associated with the autonomic nervous system. Several plexuses contain sympathetic postganglionic axons, parasympathetic preganglionic axons, and some visceral sensory axons. You don’t need to memorize the names or locations of the plexuses, but know the difference between collateral ganglia and autonomic plexuses, and recognize that they are involved in the innervation of organs in the thorax, abdomen, and pelvis.
Celiac Ganglia Located in the upper abdomen near the aortic plexus. They receive preganglionic fibers from the greater splanchnic nerve and lesser splanchnic nerve. They innervate the stomach, liver, spleen, pancreas, and parts of the small intestine.
Superior Mesenteric Ganglia Located in the upper abdomen near the aortic plexus. They receive preganglionic fibers from the lesser splanchnic nerve. They innervate the small intestine, ascending colon, and transverse colon.
Inferior Mesenteric Ganglia Lower middle region of the abdomen; innervates distal colon and rectum.
Inferior Hypogastric Ganglia Located in the pelvis near the internal iliac artery. They receive preganglionic fibers from the sacral splanchnic nerve. They innervate the pelvic organs, including the bladder, uterus, and prostate gland. Passes directly to these autonomic plexuses and synapses in collateral ganglia there.
Three General Pathways of Sympathetic Preganglionic Axons The preganglionic axon synapses with a postganglionic neuron in the sympathetic trunk ganglion at the same level and exits via the gray ramus communicans into the spinal nerve at that level.
The preganglionic axon ascends or descends in the sympathetic trunk to synapse in another trunk ganglion. The postganglionic fiber exits the sympathetic trunk via the gray ramus communicans at the level of the synapse.
The preganglionic axon passes through the sympathetic trunk, exits on a splanchnic nerve, and synapses in a collateral ganglion. The postsynaptic fiber extends from the collateral ganglion to the visceral organ via an autonomic nerve plexus.
Sympathetic Pathways to the Periphery of the Body innervate sweat glands, arrector pilli muscles in skin, and peripheral blood vessels
use sympathetic trunk ganglion, but some travel superiorly or inferiorly in trunk
Sympathetic Pathways to the Head innervate glands, smooth muscle, blood vessels
innervate eyelid and iris
travels from T1-T4 up sympathetic trunk, synapses in superior cervical ganglion
Sympathetic Pathways to the Thoracic Organs the preganglionic axons originate at spinal levels T1-T6
Some of these axons synapse in the nearest sympathetic trunk ganglion, and the postganglionic axons run directly to the organ being supplied
Postganglionic fibers extending from the ganglia to thoracic organs, including the heart, lungs, and bronchi
The sympathetic system increases heart rate and force of contraction, dilates bronchioles, and decreases peristalsis and secretion in the digestive system in response to stress (fight or flight response)
Inhibit the muscle and glands in the esophagus, effects that are integral to the fight-or-flight response.
Sympathetic Pathways Abdominal Organs The preganglionic axons originate in the inferior half of the thoracolumbar spinal cord, T5-L2
They synapse in the collateral ganglia, which include the celiac, superior mesenteric, and inferior mesenteric ganglia.
The celiac ganglion innervates the stomach, spleen, pancreas, and liver.
The superior mesenteric ganglion innervates the small intestine and proximal colon.
The inferior mesenteric ganglion innervates the distal colon and rectum.
The inferior mesenteric ganglion also communicates with the pelvic splanchnic nerves that innervate the descending and sigmoid colon, rectum, and bladder.
The sympathetic pathways to the abdominal organs primarily function to inhibit motility and secretion, and to constrict blood vessels, leading to decreased blood flow to these organs.
Sympathetic Pathways Pelvic Organs Preganglionic fibers from spinal cord segments T10-L2 travel through the sympathetic trunk and synapse in the inferior mesenteric ganglion.
Postganglionic fibers leave the ganglion and travel through the hypogastric plexus.
The hypogastric plexus forms the superior and inferior hypogastric plexuses.
The superior hypogastric plexus gives rise to the hypogastric nerves, which provide sympathetic innervation to the pelvic viscera.
The inferior hypogastric plexus gives rise to the pelvic splanchnic nerves, which provide parasympathetic innervation to the pelvic viscera.
The sympathetic pathways to the pelvic organs are involved in regulating blood flow, glandular secretion, and smooth muscle contraction in the reproductive and urinary systems. They also play a role in the fight-or-flight response.
Adrenal Medulla On the superior aspect of each kidney lies an adrenal (suprarenal) gland. The internal portion of this gland — the adrenal medulla — is a major organ of the sympathetic nervous system. The adrenal medulla is a specialized sympathetic ganglion containing a collection of modified postganglionic neurons that completely lack nerve processes. These neuron-derived cells secrete great quantities of two excitatory hormones into the blood of nearby capillaries during the fight-or-flight response.
The hormones secreted are norepinephrine (the chemical secreted by other postganglionic sympathetic neurons as a neurotransmitter) and greater amounts of a related excitatory molecule called epinephrine (adrenaline). Once released, these hormones travel throughout the body in the bloodstream, producing the widespread excitatory effects that we have all experienced as a “surge of adrenaline.”
The cells of the adrenal medulla are stimulated to secrete by preganglionic sympathetic fibers that arise from cell bodies in the T8-L1 region of the spinal cord. From there, they run in the thoracic splanchnic nerves and pass through the celiac plexus before reaching the adrenal medulla. The adrenal medulla has a more concentrated sympathetic innervation than any other organ in the body.
Visceral Sensory Neurons Relative to Autonomic Neurons The visceral division of the PNS contains sensory and motor neurons. General visceral sensory neurons monitor stretch, temperature, chemical changes, and irritation within the visceral organs. The brain interprets this visceral information as feelings of hunger, fullness, pain, or nausea. Almost all of the receptors for these visceral senses are free (nonencapsulated) nerve endings widely scattered throughout the visceral organs.
Visceral sensations tend to be difficult to localize with precision. These sensory neurons are located in the sensory ganglia adjacent to the autonomic ganglia, often called visceral sensory ganglia.
Visceral pain fibers enter the spinal cord through dorsal roots and synapse on interneurons in the dorsal horn. From there, the signals may be transmitted to higher brain centers, including the thalamus and cortex, or to reflex centers in the brainstem and spinal cord.
Most visceral inputs travel along the spinothalamic (and spinoreticular) pathways to the thalamus. Neurons in the thalamus relay visceral sensory information to the visceral sensory cortex in the insula lobe for conscious perception. Visceral sensory information also reaches and influences the visceral control centers in the hypothalamus and medulla oblongata
Referred Pain People suffering from visceral pain often perceive this pain to be somatic in origin—that is, as if it originated from the skin or outer body.
It is known that both the affected organ and the region of the body wall to which the pain is referred are innervated by the same spinal segments.
Visceral Reflex Arc A visceral reflex arc is made up of visceral sensory and autonomic neurons that lead to the regulation of some functions of visceral organs.
Examples: The gastrocolic reflex, which occurs when the presence of food in the stomach stimulates contractions in the colon to facilitate the movement of feces through the digestive tract.
Exteroceptors Sensitive to stimuli arising outside the body. Located at or near the body surface and include receptors for touch, pressure, pain, and temperature in the skin and most receptors of the special sense organs.
Interoceptors Also called visceroceptors, receive stimuli from the internal viscera, such as the digestive tube, bladder, and lungs. Different interoceptors monitor a variety of stimuli, including changes in chemical concentration, taste stimuli, the stretching of tissues, and temperature. Their activation causes us to feel visceral pain, nausea, hunger, or fullness.
Proprioceptors Located in the musculoskeletal organs, such as skeletal muscles, tendons, joints, and ligaments. Monitors the degree of stretch of these locomotory organs and send input on body movements to the CNS.
Mechanoreceptors respond to mechanical forces e.g touch, pressure, stretch, vibrations.
Thermoreceptors respond to temperature change
Chemoreceptors respond to chemicals in solution (such as molecules tasted or smelled) and to changes in blood chemistry.
Receptors for taste (gustation) and smell (olfaction) are chemoreceptors
Taste belongs to the somatic sensory division of the nervous system, while smell belongs to the visceral sensory division.
Photoreceptors responds to light in the eye
Nociceptors respond to harmful stimuli that result in pain
Free Nerve Ending of Sensory Neuron Structural Class: Free nerve ending
Functional Class (Location): Exteroceptors, interoceptors, and proprioceptors
Functional Class (Stimulus Type):
Nociceptors (pain), thermoreceptors (heat and cold), mechanoreceptors (pressure), chemoreceptors
Description / Special Features:
they monitor the affective senses, those to which people have an emotional response
Examples / Body Location:
Most body tissues; most dense in connective tissues (ligaments, tendons, dermis, joint capsules, periosteum) and epithelia (epidermis, cornea, mucosae, and glands)
Epithelial Tactile Complexes Structural Class: Free nerve ending
Functional Class (Location): Exteroceptors
Functional Class (Stimulus Type):
Mechanoreceptors (slow adapting, light pressure, continue to fire)
Description / Special Features:
Associated with tactile cell / Merkel discs
Examples / Body Location:
Basal layer of epidermis
Hair Follicle Receptors Structural Class: Free nerve ending
Functional Class (Location): Exteroceptors
Functional Class (Stimulus Type):
Mechanoreceptors (hair deflection), rapidly adapting
Description / Special Features:
free nerve endings that wrap around hair follicles
Examples / Body Location:
In and surrounding hair follicles
Tactile Corpuscles Structural Class: Encapsulated nerve ending
Functional Class (Location): Exteroceptors
Functional Class (Stimulus Type):
Mechanoreceptors (light pressure, discriminative touch, vibration of low frequency), rapidly adapting
Description / Special Features:
a few spiraling nerve endings are surrounded by Schwann cells, which in turn are surrounded by an eggshaped capsule of connective tissue
Examples / Body Location:
Dermal papillae of hairless skin, particularly nipples, external genitalia, fingertips, eyelids
Lamellar Corpuscles Structural Class: Encapsulated nerve ending
Functional Class (Location): Exteroceptors and interoceptors
Functional Class (Stimulus Type):
Mechanoreceptors (deep pressure and stretch)
Description / Special Features:
Rapidly adapting
Examples / Body Location:
Dermis, hypodermis, joints, fingers, soles of feet, external genitalia, nipples
Bulbous Corpuscles Structural Class: Encapsulated nerve ending
Functional Class (Location): Exteroceptors and proprioceptors
Functional Class (Stimulus Type):
Mechanoreceptors (deep pressure and stretch); slowly adapting or nonadapting
Description / Special Features:
contain an array of nerve endings enclosed in a thin, flattened capsule. They adapt slowly and thus can monitor continuous pressure placed on the skin.
Examples / Body Location:
Deep in dermis, hypodermis, and joint capsules
Muscle Spindles Structural Class: Encapsulated nerve ending
Functional Class (Location): Proprioceptors
Functional Class (Stimulus Type):
Mechanoreceptors (stretch)
Description / Special Features:
Made of modified muscle cells
Examples / Body Location:
Skeletal muscle
Tendon Organs Structural Class: Encapsulated nerve ending
Functional Class (Location): Proprioceptors
Functional Class (Stimulus Type):
Mechanoreceptors (tendon stretch)
Description / Special Features:
they monitor tension within tendons
Examples / Body Location:
Tendons
Joint Kinesthetic Receptors Structural Class: Encapsulated nerve ending
Functional Class (Location): Proprioceptors
Functional Class (Stimulus Type):
Mechanoreceptors and nociceptors
Description / Special Features:
monitor stretch in the synovial joints
Examples / Body Location:
Joint capsules of synovial joints
Papillae peglike projections on tongue mucosa
Fungiform Papillae scattered over entire tongue, taste buds found on apical surface
Vallate Papillae Large, circular papillae located on the posterior part of the tongue. They contain up to 100 taste buds each
Foliate Papillae On the posterolateral surface of the tongue, the taste buds are in the side walls
Gustatory Epithelial Cells contains gustatory hairs that project through the taste pore to stimulate taste (can be scraped or burned off via eating)
Basal Epithelial Cells divides and replenishes the gustatory epithelial cells. When an entire taste bud is destroyed it grows back after the nerve ending grows.
Gustatory Hair Long microvilli project from the gustatory epithelial cells and extend through a taste pore to the surface of the epithelium. There, these microvilli are bathed in saliva containing the dissolved molecules that stimulate taste. Such molecules bind to the plasma membrane of the microvilli, inducing the gustatory epithelial cells to generate impulses in the sensory nerve fibers that innervate them.
Taste Pore Allows gustatory hair to come into contact with dissolved chemicals in the mouth
Olfactory Epithelium pseudostratified columnar epithelium
Olfactory Sensory Neurons millions of bipolar neurons; contain receptors for odor molecules; one of the few neurons in the body that undergo replacement throughout adult life.
Olfactory Stem Cells At the base of the epithelium lie short olfactory stem cells, undifferentiated neuroepithelial cells that continually form new olfactory sensory neurons.
Olfactory Cilia (Hairs) Long hairs radiating from the knobs of the apical dendrite projecting to the epithelial surface; Act as the receptive structures for smell by binding odor molecules to receptor proteins located in the plasma membrane of the cilia; Largely immotile; Mucus that captures and dissolves odor molecules from the air is renewed continuously, flushing away old odor molecules so that new odors always have access to the olfactory cilia.
Olfactory Bulb Overlying part of the forebrain; the olfactory nerve axons branch profusely and synapse with neurons called mitral cells in complex synaptic clusters called glomeruli. The mitral cells then relay the olfactory information to other parts of the brain.
Olfactory Tract This is a bundle of axons that extends from the olfactory bulb to the primary olfactory cortex in the temporal lobe of the brain. The olfactory tract carries information about odor perception from the olfactory bulb to higher brain regions. Mitral cells transmit the impulses along the olfactory tract to
Filaments of the Olfactory Nerve The axons of the olfactory sensory neurons that converge to form the olfactory nerve. They pass through the cribriform plate of the ethmoid bone and enter the olfactory bulb, where they synapse with mitral cells and tufted cells. Axons gather into nerve bundles which penetrate the cribriform plate of the ethmoid bone.
Eyebrow Location:
superciliary arches
Structural Features:
coarse hair
Function:
blocks sunlight and sweat
Eyelids (Palpebrae) Location:
Anteriorly; Meets at medial and lateral angles, or eye corners
Structural Features:
Thin, skin-covered folds supported internally by connective tissue structures called tarsal plates
Function:
protect and cover the eyes
Lacrimal Caruncle Location:
The medial angle
Structural Features:
reddish elevation at the inner corner of each eye
Function:
It contains glands that produce a whitish, oily secretion to help lubricate the eyes.
Eyelashes Location:
Projecting from the free margin of each eyelid
Structural Features:
The follicles of these hairs are richly innervated by nerve endings
Function:
Helps protects the eyes from dust
Tarsal Glands Location:
embedded in the tarsal plate, gland of eyelid
Structural Features:
modified sebaceous glands
Function:
helps lubricate eye and add lipids to tears
Conjunctiva Location:
covers the inner surfaces of the eyelids
Structural Features:
transparent mucous membrane
Function:
helps to keep the eye moist and lubricated.
Lacrimal Apparatus Location:
keeps eye surface moist
Structural Features:
consists of glands + ducts
Function:
drain lacrimal fluid into the nasal cavity
Lacrimal Gland Location:
in the orbit superolateral to the eye
Structural Features:
gland
Function:
produces lacrimal fluid (tears) that help to keep the eye moist and clean
Lacrimal Punctum Location:
At the medial angle / inner corner of each eyelid
Structural Features:
tiny opening
Function:
Tears drain into these openings and flow into the lacrimal canals
Lateral Rectus Muscle Location:
outer side of each eye
Function:
moves eye laterally (outward)
Medial Rectus Muscle Location:
inner side of each eye
Function:
moves eye medially (inward)
Superior Rectus Muscle Location:
above each eye
Function:
elevates and turns eye medially + superiorly
Inferior Rectus Muscle Location:
below each eye
Function:
depresses and turns eye medially + inferiorly
Superior Oblique Muscle Location:
originates posteriorly near the common tendinous ring, runs anteriorly along the medial orbit wall; above each eye, near the nose
Function:
depresses the eye and turns it laterally (down and out)
Inferior Oblique Muscle Location:
originates on the anteromedial part of the orbit floor and angles back to insert on the posterolateral part of the eye; below each eye, near the nose.
Function:
elevates the eye and turns it somewhat laterally (up and out)
Poles, Segments, Layers, and Humors in the Eyes Two poles: anterior and posterior
Two segments:
Anterior segment: contains liquid aqueous humor
Posterior segment: contains jellylike vitreous humor
Three external layers of the eye:
fibrous layer (most external)
vascular layer
inner layer (most internal)
Sclera Opaque white, tough; Forms the posterior five-sixths of the fibrous layer; “white of the eye”; Protects the eyeball and provides shape and a sturdy anchoring site for the extrinsic eye muscles; Corresponds to the dura mater that covers the brain
Cornea The anterior sixth of the fibrous layer; Transparent; Light enters the eye and forms part of the light-bending apparatus of the eye; This round window bulges anteriorly from its junction with the sclera; Consists of a thick layer of dense connective tissue (hundred of sheets of collagen fibers) sandwiched between a superficial corneal epithelium and a deep corneal endothelium; Avascular; Richly supplied with nerve endings, most are pain receptors
Choroid Highly vascular, darkly pigmented membrane that forms the posterior five-sixths of the vascular layer. Its many blood vessels nourish the other layers of the eye. The brown color of the choroid is produced by melanocytes, whose pigment, melanin, helps absorb light, thereby preventing light from scattering within the eye and creating visual confusion. The choroid layer of the eye corresponds to the arachnoid and pia mater around the brain.
Ciliary Body Anteriorly, the choroid is continuous with a thickened ring of tissue that encircles the lens. Consists chiefly of ciliary muscle
Ciliary Muscle Smooth muscle which acts to focus the lens
Ciliary Zonule The halo of fine fibrils that extends from around the entire circumference of the lens and attaches to the ciliary processes
Iris The visible, colored part of the eye. Contains only brown pigment. It lies between the cornea and lens, and its base attaches to the ciliary body. Contains both circularly arranged and radiating smooth muscle fibers, the sphincter and dilator pupillae muscles, that act to vary the size of the pupil. In bright light and for close vision, the sphincter pupillae contracts to constrict the pupil. In dim light and for distant vision, the dilator pupillae contracts to widen the pupil, allowing more light to enter the eye. Constriction of the pupil is controlled by parasympathetic fibers; dilation of the pupil is controlled by sympathetic fibers.
Pupil Round central opening of the Iris allows light to enter the eye
Autonomic Nervous System – Pupil In bright light and for close vision, the sphincter pupillae contracts to constrict the pupil. In dim light and for distant vision, the dilator pupillae contracts to widen the pupil, allowing more light to enter the eye. Constriction of the pupil is controlled by parasympathetic fibers; dilation of the pupil is controlled by sympathetic fibers.
Retina Two layers: a thin pigmented layer and a far thicker neural layer. The neural and pigmented layers of the retina are held together by a thin film of extracellular matrix, but they are not tightly fused. Contains interneurons—including amacrine cells and horizontal cells—that process and modify visual information before it is sent to higher brain centers for further processing.
Optic Nerve Runs from the eye to the brain
Axons of the ganglion cells exit the eye here. Cranial nerve II
Pigmented Layer Outer layer of the retina that lies against the choroid. A single layer of flatto-columnar melanocytes. Functions to absorb light and prevent it from scattering within the eye. Does not play a direct role in vision; Supports the photoreceptive cells
Neural Layer The much thicker inner layer of the retina. A sheet of nervous tissue that contains the light-sensitive photoreceptor cells. Plays a direct role in vision.
Neurons in the Inner Layer From external to internal, these are the photoreceptor cells, bipolar cells, and ganglion cells. When stimulated by light, the photoreceptor cells signal the bipolar cells, which then signal the ganglion cells to generate nerve impulses potentials. Axons from the ganglion cells run along the internal surface of the retina and converge posteriorly to form the optic nerve
Photoreceptor Cells Rods: many rods, sensitive to light, allows us to see in dim light. *does not provide image sharpness and color
Cones: operate in bright light to see color (blue, red, green)
Specialized Regions on the Posterior Part of the Retina Lying precisely at the eye’s posterior pole is the macula lutea. At the center of the macula lutea is a tiny pit called the fovea centralis. The fovea contains only cones and provides maximal visual acuity. Because the fovea lies directly in the anterior-posterior axis of the eye, we see things most clearly when we look straight at them. The macula contains mostly cones and the density of cones declines with increasing distance from the macula. For this reason, peripheral vision is not as sharp as central vision.
A few millimeters medial to the fovea is the optic disc, a circular elevation where the axons of ganglion cells converge to exit the eye as the optic nerve. The optic disc is called the blind spot because it lacks photoreceptors, and light focused on it cannot be seen.
Posterior Segment Filled with the clear vitreous humor
Vitreous Humor A jellylike substance that contains fine fibrils of collagen and a ground substance that binds tremendous amounts of water (98%). Transmits light, support the posterior surface of the lens and hold the neural retina firmly against the pigmented layer, and helps maintain intraocular pressure (the normal pressure within the eye), thereby counteracting the pulling forces of the extrinsic eye muscles.
Anterior Segment Divided into an anterior chamber between the cornea and iris, and a posterior chamber between the iris and lens. Renewed continuously and is in constant motion and is filled with a watery fluid called the aqueous humor.
Anterior Chamber The space between the cornea and the iris.
Posterior Chamber The space between the iris and the lens
Aqueous Humor A clear fluid similar to blood plasma. Enters the posterior chamber, flows through the pupil into the anterior chamber, and drains into a large vessel at the corneoscleral junction, the scleral venous sinus, which returns it to the blood. An equilibrium in the rates at which the aqueous humor forms and drains results in a constant intraocular pressure, which supports the eyeball internally. Furthermore, the aqueous humor supplies nutrients and oxygen to the avascular lens and cornea.
Lens The lens is a thick biconvex disc that changes shape to direct and precisely focus light. It is encapsulated by an elastic capsule and if held in place by the ciliary zonule.
Eye Accommodation (Autonomic Nervous System) Although the lens is not as powerful as the cornea in bending light, its curvature is adjustable. This adjustability allows the eye to focus on nearby objects—a process called accommodation. A resting eye, with its lens stretched along its long axis by tension in the ciliary zonule, is “set” to focus the almost-parallel rays from distant points. Therefore, distance vision is the natural state. The diverging rays from nearby points must be bent more sharply if they are to focus on the retina. To accomplish this, the lens is made rounder: The ciliary muscle contracts in a complex way that releases most of the tension on the ciliary zonule. No longer stretched, the lens becomes rounder as a result of its own elastic recoil.
Accommodation is controlled by the parasympathetic fibers that signal the ciliary muscle to contract.
Optic Chiasma Lies anterior to the hypothalamus. X-Shaped. The axons from the medial half of each eye decussate and then continue in an optic tract.
Optic Tract Axons from the area of the retina lateral to the fovea continue to the ipsilateral optic tract. The paired optic tracts sweep posteriorly around the hypothalamus and send most of their axons to the lateral geniculate nucleus of the thalamus, where they synapse with thalamic neurons
Damage to the Optic Nerve, Optic Chiasma, or Optic Tract Destruction of one eye or one optic nerve eliminates true depth perception and causes a loss of peripheral vision on the side of the damaged eye.
However, if damage occurs beyond the optic chiasma—in an optic tract, the thalamus, or the visual cortex—then the entire opposite half of the visual field is lost. For example, a stroke affecting the left visual cortex leads to blindness (blackness) throughout the right half of the visual field