Central Nervous System: Structure, Function, and Integration

Central Nervous System Overview

Introduction

The central nervous system (CNS) consists of the brain and spinal cord. It is responsible for processing sensory information, coordinating movement, and integrating higher cognitive functions such as learning, memory, and emotion. The CNS is protected by specialized cells, connective tissue, and bone, ensuring its resilience against physical trauma and disease.

General Anatomy of the Central Nervous System

Glial Cells and Neurons

While neurons are the primary signaling cells of the CNS, glial cells (or neuroglia) make up 75–90% of CNS cells and provide essential support functions. There are four major types of glial cells:

• Schwann cells

• Oligodendrocytes

• Microglia

• Astrocytes

Astrocytes are the most abundant and play a key role in maintaining the extracellular environment, supporting neurons, and forming the blood-brain barrier. They also store molecules for neurons and synthesize neurotransmitters such as glutamate.

Blood-Brain Barrier

The blood-brain barrier is a selective barrier formed by astrocytes and tight junctions between capillary endothelial cells. It restricts the movement of substances from the blood into the CNS, protecting neural tissue from toxins and pathogens.

Glial Cells in Neurodegenerative Diseases

Glial cells are implicated in several neurodegenerative diseases:

• Multiple sclerosis: Loss of myelin in the CNS due to autoimmune attack on oligodendrocytes.

• Alzheimer's disease: Loss of cholinergic neurons and abnormal protein accumulation; astrocytes and microglia play roles in disease progression.

• Parkinson's disease: Degeneration of dopaminergic neurons; glial cells contribute to neuroinflammation.

Physical Support of the CNS

The CNS is protected by the skull and vertebral column, as well as three connective tissue membranes called meninges:

• Dura mater: Outermost, tough layer

• Arachnoid mater: Middle, web-like layer

• Pia mater: Innermost, delicate layer

Between the arachnoid and pia mater is the subarachnoid space, which contains cerebrospinal fluid (CSF).

Cerebrospinal Fluid (CSF)

CSF is a clear, watery fluid that cushions the CNS and fills cavities called ventricles. It is produced by the choroid plexus and circulates through the ventricles and subarachnoid space, providing nutrients and removing waste.

• Functions of CSF:

  • Acts as a shock absorber

  • Provides nutrients

  • Removes waste products

  • Maintains chemical environment for neurons and glia

CSF volume in adults is about 125–150 mL, and it is replenished several times per day.

Key Terms and Concepts

• Tight junctions: Specialized connections between endothelial cells that restrict passage of substances.

• Neurotransmitters: Chemical messengers released by neurons to communicate across synapses.

• Neural structure: Organization of neurons and glial cells within the CNS.

• Communication at synapses: Transmission of signals between neurons via neurotransmitters.

• Neural integration: The process by which the CNS combines sensory input, motor output, and higher cognitive functions.

Examples and Applications

• Example: Astrocytes store glycogen, which can be broken down to lactate and used as an energy source by neurons during periods of high activity.

• Application: The blood-brain barrier prevents many drugs and toxins from entering the CNS, which is important for pharmacology and treatment of neurological diseases.

Table: Major Glial Cell Types and Functions

Glial Cell Type	Main Function	Location
Astrocytes	Support neurons, maintain environment, form blood-brain barrier	CNS
Oligodendrocytes	Form myelin sheaths around axons	CNS
Schwann cells	Form myelin sheaths around axons	PNS
Microglia	Immune defense, remove debris	CNS

Formulas and Equations

• CSF Production Rate:

$\text{CSF Production Rate} = \frac{\text{Total CSF Volume}}{\text{Number of Replenishments per Day}}$

Summary

• The CNS is composed of the brain and spinal cord, protected by bone, meninges, and CSF.

• Glial cells play essential roles in supporting neurons, maintaining the environment, and protecting against disease.

• The blood-brain barrier and CSF are critical for CNS function and protection.

• Neurodegenerative diseases often involve dysfunction of glial cells.