The Nervous System: General Overview

Introduction

The nervous system is a complex network responsible for rapid and specific cellular communication throughout the body. It enables immediate responses to internal and external stimuli, maintaining homeostasis and coordinating bodily functions.

• Cellular communication: The nervous system transmits signals quickly and precisely.

• Immediate responses: Allows the body to react to changes in the environment.

Functions of the Nervous System

Three Overlapping Functions

The nervous system performs three main functions that often overlap:

1. Sensory input: Gathering information from sensory receptors about internal and external changes.

2. Integration: Processing and interpreting sensory input to determine an appropriate response.

3. Motor output: Activating effector organs (muscles and glands) to produce a response.

Example: Touching a hot object triggers sensory input (pain receptors), integration (processing in the brain), and motor output (muscle contraction to withdraw the hand).

Organization of the Nervous System

Principal Divisions

The nervous system is divided into two main parts:

• Central Nervous System (CNS): Consists of the brain and spinal cord; responsible for integration and command.

• Peripheral Nervous System (PNS): Includes all neural structures outside the CNS; connects the CNS to the rest of the body.

Peripheral Nervous System (PNS) Subdivisions

• Sensory (afferent) division: Transmits sensory information to the CNS.

• Motor (efferent) division: Carries motor commands from the CNS to effector organs.

The motor division is further subdivided into:

• Somatic nervous system: Controls voluntary movements via somatic motor nerve fibers.

• Autonomic nervous system (ANS): Regulates involuntary functions (e.g., heart rate, digestion) and consists of two functional subdivisions: sympathetic and parasympathetic.

Additional info: The ANS also includes the enteric nervous system, which governs gastrointestinal function.

Neuroglia (Glial Cells)

Overview

Neuroglia are non-neuronal cells that provide support, protection, and nutrition to neurons. They are essential for maintaining the environment required for neuronal function.

• Neuroglia (glial cells): Support and protect neurons.

• Neurons (nerve cells): Specialized for signal transmission.

Neuroglia of the CNS

• Astrocytes: Most abundant; maintain the chemical environment, support neurons, and participate in information processing.

• Microglial cells: Small, defensive cells; monitor neuron health and can transform into phagocytes.

• Ependymal cells: Line central cavities; produce and circulate cerebrospinal fluid (CSF).

• Oligodendrocytes: Form myelin sheaths around CNS nerve fibers.

Neuroglia of the PNS

• Satellite cells: Surround neuron cell bodies in ganglia; regulate environment.

• Schwann cells: Form myelin sheaths around PNS nerve fibers; vital for repair and maintenance.

Neurons (Nerve Cells)

Characteristics

Neurons are highly specialized cells responsible for transmitting electrical signals throughout the body.

• Large, specialized cells with high metabolic rates.

• Amitotic: Most neurons do not divide after maturation.

• High metabolic rate: Require continuous supply of oxygen and glucose.

Neuron Structure

• Cell body (soma): Contains nucleus and organelles; site of synthesis and metabolic activity.

• Dendrites: Numerous, branched processes; receive signals and conduct graded potentials toward the cell body.

• Axon: Single, long process; conducts impulses away from the cell body.

Tracts refer to bundles of axons in the CNS, while nerves are bundles in the PNS.

Axon Structure and Function

• Axon hillock: Cone-shaped region where the axon originates.

• Axon collaterals: Branches of the axon.

• Terminal branches: Endings that form synapses with other cells.

• Conducting region: Generates and transmits nerve impulses.

• Neurotransmitters: Chemicals released at axon terminals to communicate with other cells.

• Axonal transport: Movement of materials along the axon; occurs in both directions (anterograde and retrograde).

Clinical Note: Homeostatic Imbalance

Certain viruses and toxins can damage neural tissue by using retrograde axonal transport. Examples include polio, rabies, and herpes simplex viruses.

• Retrograde transport: Movement from axon terminals toward the cell body.

• Research: Investigating gene therapy using retrograde transport for genetic diseases.

Myelin Sheath

Structure and Function

The myelin sheath is a fatty layer that insulates axons, increasing the speed of nerve impulse transmission.

• Composed of myelin: Produced by oligodendrocytes in the CNS and Schwann cells in the PNS.

• Function: Insulation and rapid signal conduction.

• Myelinated fibers: Axons covered with myelin; conduct impulses quickly.

• Nonmyelinated fibers: Axons without myelin; conduct impulses more slowly.

Myelination in the PNS

• Schwann cells: Wrap around axons to form myelin sheaths.

• Myelin sheath gaps (Nodes of Ranvier): Gaps between Schwann cells; facilitate saltatory conduction.

• Nonmyelinated fibers: Surrounded by Schwann cells but not wrapped in myelin.

Myelination in the CNS

• Oligodendrocytes: Form myelin sheaths around multiple axons.

• White matter: Regions of the CNS with dense myelinated fibers.

• Gray matter: Contains neuron cell bodies and nonmyelinated fibers.

Classification of Neurons

Structural Classification

Neurons are classified based on the number of processes extending from the cell body:

• Multipolar: Many processes; most common type in CNS.

• Bipolar: Two processes; found in sensory organs (e.g., retina).

• Unipolar (pseudounipolar): Single process that splits; common in PNS sensory neurons.

Functional Classification

Neurons are also classified by function:

• Sensory (afferent) neurons: Transmit impulses toward the CNS.

• Motor (efferent) neurons: Carry impulses away from the CNS to effectors.

• Interneurons (association neurons): Connect sensory and motor neurons; most numerous and found within the CNS.

Summary Table: Neuroglia Types and Functions

Key Equations and Terms

• Axonal transport rate: (where k is a transport constant; additional info: actual rate depends on molecular motors and energy availability)

• Saltatory conduction: (where v is conduction velocity, d is axon diameter, is membrane resistance, is membrane capacitance)

These notes provide a comprehensive overview of the nervous system, its organization, cellular components, and functional classifications, suitable for college-level Anatomy & Physiology students.