Human Body Systems: Nervous, Endocrine, Sensory & Musculoskeletal

Posted on Nov 13, 2024 in Biology

Relationship: The Nervous System

Maintaining Internal Balance

The nervous system plays a crucial role in maintaining internal balance, also known as homeostasis. It coordinates the activities of organs, apparatuses, and systems throughout the body. This complex network consists of specialized cells, structures, and chemicals that work together to feel sensations, perform simple operations, and respond to internal and external stimuli.

Nerve Cells: Neurons

Neurons are highly specialized cells responsible for transmitting nerve impulses. Each neuron has a membrane, cytoplasm, and a nucleus. Its unique shape allows for rapid message transmission across long distances. A neuron consists of a cell body, which houses the nucleus, and extensions called dendrites and axons.

• Dendrites: Small and numerous, dendrites capture nerve impulses from receptors or other neurons.
• Axon: Long and singular, the axon conducts nerve impulses to distant areas of the body. A set of axons forms a nerve.

Propagation Speed of Nerve Impulses

Nerve impulses enter through the dendrites, pass through the cell body, and exit through the axon. The speed of transmission varies depending on whether the axon is surrounded by Schwann cells.

Schwann cells have the following characteristics:

• Elongated cytoplasm
• Wrapped around the axon membrane
• Contain myelin, a white substance that:
  • Prevents the passage of nerve impulses except in areas between Schwann cells (saltatory conduction)

Myelinated neurons have thicker axons and faster impulse transmission. Unmyelinated neurons lack myelin and transmit impulses more slowly (e.g., pain receptors).

The Central Nervous System

The central nervous system (CNS) is the control center of the body, responsible for perceiving stimuli and developing responses. It consists of the brain and spinal cord.

• Brain: Controls voluntary and conscious responses. It is protected by the skull and three membranes, with cerebrospinal fluid (CSF) circulating between the inner membranes. The brain has three main regions:
  • Cerebrum: The largest part, divided into two connected hemispheres.
  • Cerebellum: Located beneath the cerebrum, also divided into two connected hemispheres.
  • Medulla Oblongata: The brain stem, composed of nuclei and neuronal axons.
• Spinal Cord: A cord of nerve tissue located dorsally, protected by membranes and the spine. Motor and sensory nerves originate here.

The Peripheral Nervous System

The peripheral nervous system (PNS) consists of sensory and motor nerves. Cranial nerves originate from the brain (e.g., optic, acoustic, facial), while spinal nerves originate from the spinal cord (e.g., sciatic).

• Somatic Nervous System: Innervates skeletal muscles and controls voluntary movements like walking.
• Autonomic Nervous System: Controls involuntary actions like breathing and functions of internal organs.

Gray and White Matter

White matter contains myelinated axons and is located inside the brain and spinal cord. Gray matter contains cell bodies lacking myelin and is located in the cortex of the brain and within the spinal cord.

The Spinal Cord and Involuntary Actions

Reflexes

A reflex is the quickest response to a stimulus. It is an involuntary action that doesn’t require conscious awareness. Reflexes originate in the spinal cord and follow the shortest path from receptor to effector.

A reflex arc is the chain of neurons involved in a reflex, consisting of a sensory neuron, interneuron, and motor neuron.

Levels of Nervous Integration

The nervous system is a hierarchical structure where higher levels control lower levels. This organization allows for autonomous operation and adaptation to changing conditions.

The Endocrine System

The endocrine system controls slower changes like growth and can also be involved in rapid responses (e.g., adrenaline release). It consists of cells that secrete hormones directly into the bloodstream. These cells are grouped into endocrine glands, which are controlled by the nervous system.

Hormones circulate throughout the body but only act on specific target cells. Therefore, the same hormone can have different effects depending on the target cell.

Key Endocrine Glands

• Pituitary Gland: Secretes hormones that control the activity of other glands.
• Hypothalamus: Controls hormone release and acts as a liaison between the nervous and endocrine systems. It receives information from the brain, detects hormone levels in the blood, and secretes hormones that act on the pituitary gland through negative feedback loops.

Taste and Smell

Taste

Taste receptors are located in taste buds on the tongue, palate, and pharynx. Each taste bud recognizes a basic taste: bitter, salty, sweet, sour, and umami. Taste and smell work together to create flavor perception.

Smell

Olfactory receptors are located in the olfactory epithelium on the roof of the nasal cavity. These receptors detect chemical stimuli dissolved in the air. Like taste, smell contributes to flavor perception.

Skin and Hair

Skin

The skin responds to various stimuli, including touch, pressure, temperature, and pain.

• Touch: Modified dendrites surrounded by connective tissue.
• Deep Touch/Pressure: Specialized receptors detect deeper pressure.
• Thermal: Thermoreceptors detect changes in temperature.
• Pain: Nociceptors detect painful stimuli.

Each receptor type is distributed unevenly across the skin.

Hair

Hair follicles also contribute to the sense of touch, particularly light touch and deep pressure.

The Eye

The eye is a spherical organ located in the orbit.

• Cornea: Transparent outer layer at the front of the eye.
• Aqueous Humor: Clear fluid between the cornea and lens.
• Pupil: The opening in the center of the iris, its diameter controlled by light.
• Iris: Regulates pupil size and gives the eye its color.
• Vitreous Humor: Gelatinous substance filling most of the eyeball.
• Sclera: Tough, white outer layer of the eye.
• Blind Spot: Where the optic nerve exits the eye, lacking photoreceptors.
• Optic Nerve: Transmits visual information to the brain.
• Fovea: Area of the retina with high concentration of photoreceptors.
• Retina: Inner layer containing photoreceptor cells (rods and cones).
• Choroid: Thin membrane with blood vessels.
• Lens: Transparent, elastic structure that focuses light onto the retina.

Hearing

The ear is responsible for hearing and balance.

• Outer Ear (OE):
  • Pinna: Collects sound waves.
  • Ear Canal: Channels sound waves to the eardrum.
• Middle Ear (OM):
  • Eardrum: Vibrates in response to sound waves.
  • Ossicles: Transmit vibrations to the inner ear.
  • Eustachian Tube: Connects the middle ear to the pharynx.
• Inner Ear (OI):
  • Cochlea: The organ of hearing, containing auditory receptors.
  • Auditory Nerve: Transmits auditory signals to the brain.
  • Semicircular Canals, Utricle, Saccule: Responsible for balance.

The Musculoskeletal System

The musculoskeletal system consists of bones and muscles. The nervous system controls movement by sending impulses to muscles, causing them to contract.

Skeletal System

Functions of the skeletal system:

• Supports the body
• Protects organs and soft tissues
• Produces blood cells
• Stores calcium
• Facilitates movement

The skeletal system is divided into two parts:

• Axial Skeleton: Skull, vertebral column, sternum, and ribs.
• Appendicular Skeleton: Upper and lower limbs, shoulder girdle, and pelvic girdle.

Muscular System

Muscles work in conjunction with bones to produce movement.

Joints

Joints connect bones and are classified into three types:

• Immobile: Fibrous joints that prevent movement (e.g., skull bones).
• Semi-Mobile: Cartilaginous joints that allow limited movement (e.g., vertebrae).
• Mobile: Synovial joints that allow a wide range of motion (e.g., wrist).

Tendons

Tendons attach muscles to bones, enabling movement.

Muscle Contraction

• Biceps Contraction: Bends the elbow.
• Triceps Contraction: Extends the arm.