Nervous System: Structure, Function, and Cellular Organization

Nervous System Overview

Introduction

The nervous system is a complex network responsible for coordinating the body's activities by transmitting signals to and from different parts of the body. It is divided into central and peripheral components, each with specialized functions and cellular structures.

Nervous System Subdivisions

Main Divisions

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

• Peripheral Nervous System (PNS): Includes all neural tissue outside the CNS. Connects the CNS to limbs and organs.

Functional Subdivisions

• Afferent (Sensory) Division: Brings sensory information to the CNS from receptors.

• Efferent (Motor) Division: Carries motor commands from the CNS to effectors (muscles and glands).

• Somatic Nervous System (SNS): Controls voluntary movements by innervating skeletal muscles.

• Autonomic Nervous System (ANS): Regulates involuntary functions (e.g., cardiac muscle, smooth muscle, glands). Subdivided into:

  • Sympathetic Division: Prepares the body for 'fight or flight' responses.

  • Parasympathetic Division: Promotes 'rest and digest' activities.

Cells of Nervous Tissue

Types of Cells

• Neurons: Excitable cells that transmit electrical signals (nerve impulses).

• Neuroglia (Glial Cells): Supporting cells that protect, nourish, and insulate neurons.

Neuroglia in the CNS

• Astrocytes: Anchor neurons to blood capillaries, maintain the blood-brain barrier, and regulate the chemical environment.

• Microglia: Specialized macrophages that protect the CNS by engulfing microorganisms and debris.

• Ependymal Cells: Line the ventricles of the brain and central canal of the spinal cord; involved in cerebrospinal fluid (CSF) production and circulation.

• Oligodendrocytes: Form myelin sheaths around CNS axons, increasing the speed of impulse conduction.

Neuroglia in the PNS

• Schwann Cells: Form myelin sheaths around PNS axons and assist in axonal regeneration; also act as phagocytes.

• Satellite Cells: Surround neuron cell bodies in ganglia; regulate the chemical environment for neuron survival.

Neuroglia Table

Cell Type	Location	Main Function
Astrocytes	CNS	Support neurons, anchor to capillaries, maintain environment
Microglia	CNS	Phagocytosis of debris and pathogens
Ependymal Cells	CNS	Line ventricles, produce and circulate CSF
Oligodendrocytes	CNS	Form myelin sheaths
Schwann Cells	PNS	Form myelin sheaths, aid regeneration
Satellite Cells	PNS	Regulate environment around neuron cell bodies

Neurons: Structure and Function

Characteristics of Neurons

• Extreme Longevity: Can function for a lifetime with proper nutrition.

• Amitotic: Lose the ability to divide (no mitosis) after maturity.

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

Parts of a Neuron

• Cell Body (Soma): Contains the nucleus and nucleolus; site of most metabolic activity.

• Dendrites: Short, branched processes that receive signals and conduct them toward the cell body.

• Axon: Long process that conducts electrical impulses away from the cell body to other neurons or effectors. Each neuron has only one axon, which may branch into axon collaterals.

Functional Zones of a Neuron

• Input Zone: Dendrites and cell body (receive signals).

• Trigger Zone: Axon hillock (initiates action potential).

• Conducting Zone: Axon (transmits action potential).

• Output Zone: Axon terminals (release neurotransmitters to communicate with other cells).

Myelin Sheath

• Definition: A whitish, fatty, segmented covering around many nerve fibers.

• Function: Increases the speed of nerve impulse conduction.

• Formation in PNS: Formed by Schwann cells; gaps between cells are called Nodes of Ranvier (only found in PNS).

• Formation in CNS: Formed by oligodendrocytes; CNS myelin lacks a neurilemma.

• Myelinated fibers: Conduct impulses rapidly (white matter).

• Unmyelinated fibers: Conduct impulses slowly (gray matter).

Bundles of Neuron Processes

• Tracts: Bundles of neuron processes in the CNS.

• Nerves: Bundles of neuron processes in the PNS.

Classification of Neurons

Structural Classification

• Multipolar: Many dendrites, one axon; most common in CNS.

• Bipolar: One dendrite, one axon; found in special sense organs (e.g., retina, olfactory epithelium).

• Unipolar (Pseudounipolar): Single process that splits into two branches; mainly sensory neurons in the PNS.

Functional Classification

• Sensory (Afferent) Neurons: Transmit impulses from sensory receptors toward the CNS.

• Motor (Efferent) Neurons: Carry impulses away from the CNS to effectors (muscles or glands).

• Interneurons (Association Neurons): Connect sensory and motor neurons; found only in the CNS and involved in integration.

Neuron Classification Table

Type	Structure	Location	Function
Multipolar	Many dendrites, one axon	CNS	Majority of neurons; integration and motor output
Bipolar	One dendrite, one axon	Special sense organs	Sensory (e.g., vision, smell)
Unipolar	Single process splits into two	PNS	Sensory input

Nerves and Connective Tissue Organization

Structure of a Nerve

• Endoneurium: Surrounds individual axons.

• Perineurium: Surrounds bundles of axons (fascicles).

• Epineurium: Surrounds the entire nerve.

Regeneration of Nervous Tissue

Peripheral Nervous System (PNS)

• Damaged axons in the PNS can regenerate if the cell body is intact and the damage is not severe.

• Regeneration steps:

  1. Separated ends of the axon seal and swell.

  2. Myelin sheath distal to the injury degenerates (Wallerian degeneration).

  3. Schwann cells and macrophages remove debris.

  4. Schwann cells align to form regeneration tubes, guiding new axonal sprouts.

  5. Axonal regeneration occurs at a rate of 1–5 mm/day.

• The greater the distance between severed ends, the less likely full recovery will occur.

Central Nervous System (CNS)

• Axonal regeneration does not occur in the CNS.

• Oligodendrocytes do not form regeneration tubes; astrocytes form glial scars that block regrowth.

• Microglia remove debris, but no guiding channels are formed.

• Recent research suggests CNS axons may regrow through segments of peripheral nerve containing Schwann cells, but this is not typical in vivo.

Summary Table: Regeneration Capacity

System	Regeneration Ability	Supporting Cells Involved
PNS	Possible (if cell body intact)	Schwann cells
CNS	Not possible (in vivo)	Oligodendrocytes, astrocytes (form glial scar)

Example: Peripheral Nerve Injury

If a peripheral nerve is cut, the axon distal to the injury degenerates, but Schwann cells can guide regrowth of the axon if the cell body is intact. In contrast, CNS injuries do not regenerate due to the formation of glial scars and lack of supportive regeneration tubes.

Additional info: The speed of nerve impulse conduction is influenced by axon diameter and myelination. Larger, myelinated axons conduct impulses faster due to saltatory conduction at the nodes of Ranvier.