The Nervous System: Structure, Function, and Components

Nervous System Overview

Definition

The nervous system is a complex network of specialized cells, tissues, and organs that coordinates and controls the activities of the body. It is responsible for transmitting signals between different parts of the body and integrating information from both the external and internal environments to ensure appropriate responses.

Importance of the Nervous System

• Regulation of Actions: Controls all voluntary and involuntary actions, ensuring proper coordination of bodily functions.

• Sensory Processing: Processes sensory input from the environment to enable appropriate reactions.

• Homeostasis: Maintains internal stability by regulating physiological functions such as temperature and blood pressure.

• Cognition: Supports cognitive functions including learning, memory, and decision-making.

• Reflexes: Enables quick responses to stimuli, including protective reflex actions.

• Emotional Regulation: Processes emotions and contributes to mood regulation.

• Adaptation: Ensures survival through adaptive responses to environmental changes.

• Endocrine Interaction: Works with the endocrine system to regulate growth, metabolism, and reproduction.

Structure and Components

Major Divisions

• Central Nervous System (CNS): Consists of the brain and spinal cord. Responsible for processing and integrating information.

• Peripheral Nervous System (PNS): Composed of peripheral nerves that connect the CNS to limbs and organs, facilitating communication between the body and the CNS.

Components of the CNS

• Brain: Constitutes about one-fifth of body weight and lies within the cranial cavity. Main parts include:

  • Cerebrum

  • Brain Stem (midbrain, pons, medulla oblongata)

  • Cerebellum

• Spinal Cord: Extends from the brain stem and transmits signals throughout the body.

Cerebrum

Structure

• Largest part of the brain, occupying the anterior and middle cranial fossae.

• Comprises two cerebral hemispheres and their cortices (outer layer of grey matter) and underlying regions of white matter.

• The cerebral cortex is the outer covering of grey matter over the hemispheres.

Lobes of the Cerebrum

• Frontal Lobe: Personality, decision-making, voluntary movement.

• Parietal Lobe: Sensory perception, spatial relationships, touch, and sensation.

• Temporal Lobe: Hearing, memory, language.

• Occipital Lobe: Visual processing.

Brain Stem

Structure and Functions

The brain stem is the stem-like part at the base of the brain, connecting to the spinal cord. It is essential for basic life functions.

Region	Main Function
Midbrain	Visual reflexes
Pons	Involved in respiratory functions
Medulla oblongata	Regulates heart and lung function, swallowing, coughing, vomiting, sneezing

Cerebellum

Structure

• Located behind the pons and below the posterior portion of the cerebrum.

• Ovoid in shape, with two hemispheres separated by the vermis.

Functions

• Coordination of voluntary muscular movement, posture, and balance.

• Involved in cognitive functions such as language.

• Adapts and fine-tunes motor programs for accurate movements through trial-and-error processes.

Meninges

Protective Membranes

• Dura Mater: Outer tough fibrous membrane attached to the skull; contains dural venous sinuses.

• Arachnoid Mater: Middle web-like membrane; CSF is contained in the subarachnoid space.

• Pia Mater: Innermost layer wrapped around brain tissue; blood vessels pass through, supplying the brain.

Limbic System

Functions and Components

The limbic system integrates sensory and cognitive inputs to influence emotional responses and decision-making, playing a vital role in survival instincts and social interactions.

• Hippocampus: Learning and memory

• Amygdala: Emotion

• Mammillary bodies: Memory processing

• Fornix: Fiber bundle interconnecting hippocampus and mammillary bodies

Ventricular System

Structure

• Four irregular-shaped communicating cavities (ventricles) containing cerebrospinal fluid (CSF):

  • Lateral ventricles (right and left)

  • Third ventricle

  • Fourth ventricle

Cerebrospinal Fluid (CSF)

Functions and Circulation

• CSF is a clear, colorless fluid that surrounds the brain and spinal cord, providing mechanical and chemical protection.

• Produced primarily by the choroid plexuses in the brain's ventricles.

• CSF circulates through the ventricles, spinal cord, and subarachnoid space, cushioning the brain and spinal cord, regulating intracranial pressure, delivering nutrients, and removing waste.

• Reabsorbed into the bloodstream via arachnoid granulations to maintain balance.

Spinal Cord

Structure and Function

• Continuous above with the medulla oblongata.

• Long, thin bundle of nervous tissue (~45 cm in adult males).

• Receives and transmits electric signals throughout the body.

White and Gray Matter

• White matter tracts: Highways for nerve impulse propagation.

• Sensory input: Travels toward the brain via afferent neurons.

• Motor output: Travels from the brain to muscles and effector tissues via efferent neurons.

• Gray matter: Receives and integrates incoming and outgoing information.

Cells of the CNS

Types of Cells

• Neurons: Signaling units responsible for transmitting nerve impulses.

• Glial cells: Supporting units providing metabolic support and immune protection.

Neuron Structure

• Cell body (soma): Synthesizes proteins (neurotransmitters); groups called nuclei in CNS and ganglia in PNS.

• Dendrites: Extensions that bring information to the cell body.

• Axon: Takes information away from the cell body.

• Axon terminals: Make synaptic connections with other neurons or effector cells.

Neuron Classification

Type	Structure
Multipolar	3 or more processes (1 axon, multiple dendrites)
Bipolar	1 axon, 1 dendrite
Unipolar	1 process splits (double-ended axon)

Functional Classification

Type	Main Function
Sensory Neurons	Generally unipolar, some bipolar; carry messages from sensory receptors to CNS
Motor Neurons	Multipolar; deliver impulses to effector organs (glands/muscles)
Associative Neurons	Most are multipolar; integrate sensory and motor in CNS

Glial Cells

Functions

• Provide metabolic support and immune protection for neurons.

• Outnumber neurons by about 10:1 in the CNS.

• Do not generate or conduct nerve impulses.

Types of Glial Cells in the CNS

• Astrocyte: Main supporting tissue; found adjacent to blood vessels and neurons; essential for blood-brain barrier (BBB) formation and maintenance.

• Oligodendrocyte: Myelinating cells; provide and maintain myelin sheaths around axons.

• Ependymal cell: Line cavities of CNS; produce CSF; form barriers between CSF and brain extracellular space.

• Microglial cell: Derived from monocytes; activated in response to injury; present antigens to lymphocytes in response to infection.

Types of Neuroglia in the PNS

• Schwann cell: Provide myelination to axons in the PNS; have phagocytic activity and clear debris for regrowth.

• Satellite cell: Surround neurons in ganglia; regulate external chemical environment; sensitive to injury and inflammation.

Peripheral Nervous System (PNS)

Divisions

• Somatic Division: Controls muscle action and carries sensory information to the CNS.

  • Cranial nerves: 12 pairs

  • Spinal nerves: 31 pairs

• Autonomic Division: Governs smooth muscle and gland secretion.

  • Parasympathetic: Supports activities that increase energy

  • Sympathetic: Arousal and expenditure of energy

Cranial Nerves

Functions

• Olfactory nerve: Smell

• Optic nerve: Vision

• Oculomotor nerve: Eye muscle movement, pupil response

• Trochlear nerve: Controls superior oblique muscle (eye movement)

• Trigeminal nerve: Sensation to face, chewing

• Abducens nerve: Controls lateral rectus muscle (eye movement)

• Facial nerve: Facial expression, taste, gland secretion

• Vestibulocochlear nerve: Hearing, balance

• Glossopharyngeal nerve: Swallowing, taste, saliva production

• Vagus nerve: Heart rate, digestion, speech, swallowing

• Accessory nerve: Muscles and movements of head, neck, shoulders

• Hypoglossal nerve: Tongue movement

Spinal Nerves

Structure and Function

• Mixed nerves connecting the spinal cord to the rest of the body, facilitating sensory and motor communication.

• 31 pairs categorized into:

  • 8 Cervical

  • 12 Thoracic

  • 5 Lumbar

  • 5 Sacral

  • 1 Coccygeal

• Each spinal nerve emerges from the spinal cord via dorsal and ventral roots, serving specific body regions for sensation and movement.

Region	Number of Pairs
Cervical	8
Thoracic	12
Lumbar	5
Sacral	5
Coccygeal	1

Example

The phrenic nerve (C3-C5) is a cervical spinal nerve that innervates the diaphragm, essential for breathing.

Additional info: The notes have been expanded with definitions, examples, and tables for clarity and completeness.