Human Muscular and Nervous Systems: Anatomy and Function
The Muscular System
Muscles make up approximately 40% of our body weight. We have over 630 muscles that facilitate movement, including over 30 facial muscles. Eye muscles are the busiest in the body, potentially moving more than 100,000 times a day. The largest muscle is the gluteus maximus in the buttocks.
The muscular system constitutes nearly half the weight of the human body. Muscles provide the forces that enable movement. They stretch across joints, linking bones and working in groups to respond to nerve impulses. Muscles provide strength, balance, posture, movement, and heat to keep the body warm. Muscles can be classified by function, constitution, and position.
By Function
Voluntary Muscles: These muscles, such as the twins in the calf, cover the bones and are under our control.
Involuntary Muscles: These muscles, such as those in the stomach and intestines, are not under our control and generally found in organs.
By Constitution
Skeletal Striated Muscles: Attached to the skeleton, they work in pairs. One muscle moves the bone in one direction, and the other moves it back. These muscles are formed by fibers with stretch marks and are voluntary. A thought initiates a movement, which is then sent to the nervous system to contract the appropriate muscle.
Smooth Muscles: Found in internal organs like the digestive system, blood vessels, bladder, respiratory organs, and the uterus in females. Smooth muscles can stretch and maintain tension over extended periods. They are involuntary, controlled automatically by the nervous system.
Cardiac Muscles: Found only in the heart, it can stretch like smooth muscle and contract like skeletal muscle. It is a twitch muscle, performing short, single contractions. Cardiac muscles have stretch marks and are involuntary.
By Position
Superficial or Cutaneous: Found under the skin.
Deep: Found in the bones and other organs.
Facial Muscles
- Orbicularis oculi: Muscles that move the eyes.
- Frontalis Temporalis: Muscles that move the forehead and sides of the head.
- Zygomaticus and Masseter: Muscles that move the jaw and upper lip area.
- Orbicularis oris: Muscles that move the lips.
Neck Muscles
The neck is primarily moved by the Sternohyoid and Sternocleidomastoid, which allow the head to move left and right. They work with the platysma muscle to control the range of motion. The trapezius allows the head to move up and down and extends down to the shoulder and thorax area. The trapezius is named for its trapezoid shape.
Shoulder Muscles
A group of muscles, including the trapezius, deltoid, infraspinatus, teres major, and rhomboid major, work together to move the shoulder area.
Arm Muscles
The bicep brachii allows you to bring your forearm close to your body. The tricep brachii and brachialis are also located in the arm region and enable push-ups.
Forearm Muscles
Muscles in the forearm, such as the brachioradialis major, palmaris longus, and flexor carpi radialis, help control the arm. The flexor carpi radialis is named for the carpals it helps move and the radius bone it is attached to.
Thorax Muscles
The trapezius and latissimus dorsi support the head, arms, stomach, and upper body. These muscles can be easily damaged without proper stretching or heavy lifting.
Abdomen Muscles
The abdominal area consists of muscles that allow you to bend down and move your waist. The internal oblique and external oblique move the body from left to right. The transversus abdominus and rectus abdominus, along with the trapezius and latissimus dorsi, allow you to bend down and grab objects.
Hip Muscles
The gluteus medius and gluteus maximus make up the hip area and are primarily used for sitting.
Pelvis and Thigh Muscles
The pelvis area, or upper part of the leg, includes muscles like the pectineus and illiopsoas. Thigh muscles, rich in capillaries, support the body. The upper thigh muscles are the abductor longus, gracilis, sartorius, and tensor fasciae latae. The lower thigh muscles are the rectus femoris, vastus lateralis, and medialis. The hamstrings, located in the back of the leg, help with running, jumping, and walking.
Leg Muscles
The leg region, including the gastrocnemius, soleus, peroneus longus, and tibialis anterior, helps support the body and absorb impact during walking and running.
The Nervous System
The nervous system is a complex network of nerves and cells that carry messages to and from the brain and spinal cord to various parts of the body. It includes the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).
The Central Nervous System
The CNS is divided into the brain and spinal cord.
Brain
The brain, located within the skull, consists of four principal parts: the brain stem, cerebrum, cerebellum, and diencephalon. It weighs approximately 1.3 to 1.4 kg and contains neurons and glia. The brain has grey matter, which receives and stores impulses, and white matter, which carries impulses to and from grey matter.
The brain stem, also known as the medulla oblongata, is located between the pons and the spinal cord.
The cerebrum forms the bulk of the brain and is divided into two hemispheres, each controlling the opposite side of the body.
The cerebellum is located behind and below the cerebrum.
The diencephalon, also known as the forebrain stem, includes the thalamus and hypothalamus. The thalamus is where sensory impulses coalesce, and the hypothalamus is a smaller part of the diencephalon.
Spinal Cord
The spinal cord is a tube-like structure extending from the brain. It is composed of 31 segments, each with a pair of spinal nerves. The region from which a pair of spinal nerves originates is called the spinal segment. Both motor and sensory nerves are located in the spinal cord. The spinal cord is about 43 cm long in adult women and 45 cm long in adult men.