Study Notes: Structure and Function of the Nervous System

Nervous System Overview

Learning Outcomes

The nervous system is essential for controlling and communicating information throughout the body. These study notes cover its structure, function, and key cellular components.

• List the basic functions of the nervous system: Sensory input, integration, and motor output.

• Explain the structural and functional divisions: Central and peripheral nervous systems, somatic and autonomic divisions.

• Identify types of neuroglia and their locations: Astrocytes, microglia, ependymal cells, oligodendrocytes (CNS); satellite cells, Schwann cells (PNS).

• Define neurons and their components: Soma, dendrites, axon, axon terminals.

• Differentiate nuclei vs. ganglia, nerve vs. tract: Nuclei and tracts are CNS; ganglia and nerves are PNS.

• Explain myelin sheath importance: Insulates axons, increases speed of impulse transmission.

• Describe membrane potential and its basis: Electrical charge differences across neuron membranes.

Functions of the Nervous System

Main Functions

The nervous system is the master control and communication system of the body, using electrical and chemical signals for rapid, specific responses.

• Sensory Input: Information gathered by sensory receptors about internal and external changes.

• Integration: Processing and interpretation of sensory input.

• Motor Output: Activation of effector organs (muscles and glands) to produce a response.

Structural Divisions of the Nervous System

Central Nervous System (CNS)

The CNS consists of the brain and spinal cord, serving as the integration and control center.

• Brain: Located in the cranial cavity.

• Spinal Cord: Located in the vertebral cavity.

Peripheral Nervous System (PNS)

The PNS includes all neural structures outside the CNS, such as nerves and ganglia.

• Spinal nerves: Extend from the spinal cord.

• Cranial nerves: Extend from the brain.

Functional Divisions of the PNS

• Sensory (Afferent) Division: Conveys impulses from sensory receptors to the CNS.

• Motor (Efferent) Division: Transmits impulses from the CNS to effector organs (muscles and glands).

Motor Division Subdivisions

• Somatic Nervous System: Voluntary control of skeletal muscles.

• Autonomic Nervous System: Involuntary control of smooth muscle, cardiac muscle, and glands. Subdivided into sympathetic and parasympathetic divisions.

Neuroglia (Glial Cells)

Definition and Types

Neuroglia are supporting cells in nervous tissue, providing structural and functional support to neurons.

• Astrocytes: Most abundant, versatile, and highly branched. Support neurons, regulate chemical environment, and form blood-brain barrier.

• Microglial Cells: Small, phagocytic cells that monitor neuron health and remove debris.

• Ependymal Cells: Line brain ventricles and spinal cord central canal; produce and circulate cerebrospinal fluid (CSF).

• Oligodendrocytes: Form myelin sheaths in CNS.

Neuroglia of the PNS

• Satellite Cells: Surround neuron cell bodies in PNS; similar function to astrocytes.

• Schwann Cells: Form myelin sheaths in PNS; aid in regeneration of damaged peripheral nerve fibers.

Neurons

Structure and Characteristics

Neurons are the structural units of the nervous system, specialized for conducting impulses.

• Longevity: Can last a person's lifetime.

• Amitotic: Most do not divide after development.

• High Metabolic Rate: Require continuous supply of oxygen and glucose.

Neuron Cell Body (Soma)

• Contains nucleus and most organelles.

• Most located in CNS (nuclei); in PNS, grouped in ganglia.

Neuron Processes

• Dendrites: Receptive regions; convey incoming messages toward cell body as graded potentials.

• Axon: Conducting region; transmits impulses away from cell body.

• Axon Collaterals: Occasional branches off the main axon.

• Axon Terminals: Distal endings; release neurotransmitters.

Axon Functional Characteristics

Impulse Transmission

Axons transmit electrical impulses along their length to axon terminals, where neurotransmitters are released.

• Excite or inhibit: Neurotransmitters can excite or inhibit target cells.

• Axonal transport: Movement of molecules and organelles along axon via cytoskeletal elements.

Types of Axonal Transport

Type	Direction	Examples
Anterograde	Away from cell body	Mitochondria, cytoskeletal elements, membrane components, enzymes
Retrograde	Toward cell body	Organelles to be degraded, signal molecules, viruses, bacterial toxins

Myelin Sheath

Structure and Function

The myelin sheath is a whitish, protein-lipid substance that electrically insulates axons and increases the speed of nerve impulse transmission.

• Myelinated fibers: Segmented sheath surrounds most long or large-diameter axons.

• Unmyelinated fibers: Conduct impulses more slowly.

Membrane Potential

Electrical Basis

Neurons communicate via changes in membrane potential, which is the difference in electrical charge across the cell membrane.

• Resting membrane potential: Typically -70 mV in neurons.

• Action potential: Rapid change in membrane potential that propagates along the axon.

Formula:

$V_m = V_{inside} - V_{outside}$

Where $V_m$ is the membrane potential, $V_{inside}$ is the potential inside the cell, and $V_{outside}$ is the potential outside the cell.

Summary Table: Neuroglia Types and Functions

Cell Type	Location	Main Function
Astrocytes	CNS	Support neurons, regulate chemical environment, form blood-brain barrier
Microglia	CNS	Phagocytosis, monitor neuron health
Ependymal Cells	CNS	Produce and circulate CSF
Oligodendrocytes	CNS	Form myelin sheaths
Satellite Cells	PNS	Support neuron cell bodies
Schwann Cells	PNS	Form myelin sheaths, aid regeneration

Additional info:

• Neurons are classified functionally as sensory (afferent), motor (efferent), or interneurons (association neurons).

• Action potentials are all-or-none electrical events that travel along axons to communicate with other cells.

• Diseases such as multiple sclerosis involve damage to myelin sheaths, impairing nerve transmission.