Fundamentals of the Nervous System: Structure and Function

The Nervous System

Overview and Organization

The nervous system is the master control and communication system of the body, responsible for monitoring stimuli, processing information, and dictating responses. It is composed of nervous tissue and is divided into two main anatomical divisions:

• Central Nervous System (CNS): Consists of the brain and spinal cord. It serves as the integration and command center.

• Peripheral Nervous System (PNS): Includes all neural structures outside the CNS, such as cranial nerves, spinal nerves, and ganglia (clusters of neuronal cell bodies). Peripheral nerves link all regions of the body to the CNS.

Functional Organization

The nervous system functions through three main processes:

• Sensory Input: Information gathered by sensory receptors in response to a stimulus (a change detected inside or outside the body).

• Integration: The CNS processes and interprets sensory input, determining the appropriate response.

• Motor Output: The response dictated by the CNS, activating effector organs (muscles or glands).

Classification of Nerve Fibers and Subdivisions

Somatic and Visceral Regions

Nerve fibers are classified according to the body region they serve:

• Somatic: Relates to the body wall and limbs.

• Visceral: Relates to internal organs.

This results in four main subdivisions:

• Somatic Sensory

• Visceral Sensory

• Somatic Motor

• Visceral Motor (autonomic nervous system)

Somatic & Visceral Sensory

General Somatic Senses

Receptors are distributed throughout the outer tube of the body and detect:

• Touch

• Pain

• Vibration

• Pressure

• Temperature

Proprioceptive senses provide information about body position and movement in space.

General Visceral Senses

• Detect stretching, pain, temperature, nausea, and hunger.

• Widely felt in digestive and urinary tracts, and reproductive organs.

Somatic & Visceral Motor

General Somatic Motor

• Signals contraction of skeletal muscles.

• Under voluntary control ("voluntary nervous system").

Visceral Motor

• Regulates contraction of smooth and cardiac muscle.

• Makes up the autonomic nervous system ("involuntary nervous system").

• Controls function of visceral organs.

Neurons: Structure and Function

Neuron Anatomy

• Cell Body (Soma): Size varies from 5 to 140 μm; contains organelles and other structures. Most are located in the CNS (nuclei), with some in the PNS (ganglia).

• Dendrites: Extensively branched processes that transmit electrical signals toward the cell body.

• Axons: Each neuron has one axon, which acts as an impulse generator and conductor, transmitting impulses away from the cell body.

Synapses

Neuronal Communication

• Synapse: Site at which neurons communicate.

• Signals pass across most synapses in one direction only.

• Presynaptic neuron: Conducts signal toward a synapse.

• Postsynaptic neuron: Transmits electrical activity away from a synapse.

Classification of Neurons

Structural Classification

Type	Processes	Location
Multipolar	More than two (numerous dendrites, one axon)	Most abundant, CNS
Bipolar	Two (one dendrite, one axon)	Rare, special sensory organs
Unipolar (pseudounipolar)	One short, single process	PNS; start as bipolar during development

Functional Classification

• Sensory (Afferent) Neurons: Transmit impulses toward the CNS; virtually all are unipolar; cell bodies in ganglia outside CNS. The single process divides into:

  • Central process: Terminates in the CNS.

  • Peripheral process: Extends to sensory receptors.

• Motor (Efferent) Neurons: Carry impulses away from the CNS to effector organs; most are multipolar; cell bodies within the CNS; form junctions with effector cells.

• Interneurons (Association Neurons): Most are multipolar; lie between motor and sensory neurons; confined to the CNS; link into chains to form complex neuronal pathways.

Neuroglia (Glial Cells)

Supportive Functions

• Provide supportive functions for neurons.

• Cover non-synaptic regions of neurons.

• Four types in the CNS (make up half the mass of the brain, can divide throughout life): Oligodendrocytes (form myelin sheaths), others not listed here.

• Two types in the PNS: Schwann cells (form myelin sheaths), Satellite cells.

Myelin Sheaths

Structure and Function

• Segmented structures composed of the lipoprotein myelin.

• Surround thicker axons, forming an insulating layer that prevents leakage of electrical current.

• Increase the speed of impulse conduction along the axon.

• CNS: Myelin sheaths formed by oligodendrocytes.

• PNS: Myelin sheaths formed by Schwann cells.

Formation and Properties

• Oligodendrocytes (CNS): Multiple processes coil around several different axons.

• Schwann cells (PNS): Wrap in concentric layers around the axon, covering it in tightly packed coils of membranes.

Myelin Sheath Gaps (Nodes of Ranvier)

• Gaps along the axon, more widely spaced in the PNS.

• Speed up nerve transmission by allowing saltatory conduction.

Myelinated vs. Nonmyelinated Axons

• Thick axons: Myelinated, conduct impulses rapidly.

• Thin axons: Nonmyelinated, conduct impulses more slowly. In the PNS, Schwann cells surround but do not wrap; in the CNS, covered by long processes of glial cells.

CNS: Gray and White Matter

Gray Matter

• "Gray"-colored tissue that surrounds hollow central cavities of the CNS.

• Mixture of neuron cell bodies, dendrites, short nonmyelinated axons, and neuroglia.

• Site where synapses occur.

White Matter

• White color due to high myelin content.

• External to the gray matter of the CNS.

• Contains myelinated axons and tracts (bundles of axons traveling to similar destinations).

Example: Saltatory Conduction

In myelinated axons, action potentials "jump" from node to node, greatly increasing conduction velocity. The equation for conduction velocity ($v$) in myelinated axons is approximately:

$v \propto \sqrt{d}$

where $d$ is the diameter of the axon.

Additional info: The notes above provide a foundational overview of nervous system structure and function, suitable for introductory college-level Anatomy & Physiology. For more advanced study, students should explore synaptic transmission mechanisms, neurotransmitter types, and detailed neuroglial cell functions.