Nervous Tissue: Structure and Function of Neurons and Neuroglia

Posted on May 2, 2024 in Biology

Nervous Tissue: General Organization of the Nervous System

The nervous system is divided into two main parts:

Central Nervous System (CNS)

The CNS is responsible for integrating information and includes the brain and spinal cord.

Peripheral Nervous System (PNS)

The PNS transmits information to and from the CNS and consists of nerves, ganglia, nerve endings, and sensory receptors.

Histologically, the nervous system is composed of nervous tissue, which is derived from the ectoderm and consists of neurons and glial cells specialized in communication.

Neurons

Neurons are the fundamental units of the nervous system, responsible for transmitting information. They consist of:

Cell Body (Soma)

  • Typically polyhedral in shape with extensions.
  • Contains a large, rounded nucleus with dispersed chromatin and a nucleolus.
  • Rich in rough endoplasmic reticulum (RER) and mitochondria.
  • Cytoskeleton composed of neurotubules and neurofilaments.

Dendrites

  • Numerous, short, and branched extensions that receive signals from other neurons.
  • Contain RER, mitochondria, neurotubules, and neurofilaments.

Axon

  • A single, long extension that transmits signals to other neurons or effector cells.
  • May have collaterals and ends in terminal boutons.
  • Contains neurotubules, neurofilaments, and mitochondria.

Transportation of Chemicals and Structures

The neuronal cytoskeleton facilitates the transport of chemicals and structures along the axon, both anterograde (away from the soma) and retrograde (towards the soma).

Types of Neurons

Neurons can be classified based on:

  1. Number of extensions:
    • Unipolar: 1 extension
    • Pseudounipolar: 1 extension divided into a “T” shape
    • Bipolar: 2 extensions
    • Multipolar: Several dendrites and one axon (Golgi type I and II)
  2. Shape of the soma: Stellate, pyramidal, fusiform
  3. Dendritic arborization pattern

Nerve Fibers

A nerve fiber consists of an axon surrounded by supporting cells. Groups of nerve fibers form nerves in the PNS and tracts in the CNS.

Myelin Sheath

Many axons are surrounded by a myelin sheath, which is formed by Schwann cells in the PNS and oligodendrocytes in the CNS. The myelin sheath is interrupted at regular intervals by nodes of Ranvier.

Functions of the Myelin Sheath

  • Insulates the axon
  • Increases the speed of nerve impulse conduction (saltatory conduction)

Neuroglia

Glial cells are supporting cells in the nervous system. They are derived from the ectoderm and can be classified into:

Glia of the CNS

  • Astrocytes: Stellate cells with numerous extensions. They provide structural support, contribute to the blood-brain barrier, and modulate neuronal activity.
  • Oligodendrocytes: Cells with few extensions responsible for myelin formation in the CNS.
  • Microglia: Small cells with phagocytic activity, involved in immune defense.
  • Ependymal cells: Line the cerebral ventricles and form a barrier between the cerebrospinal fluid and the brain tissue.

Glia of the PNS

  • Schwann cells: Form the myelin sheath around axons in the PNS.
  • Satellite cells: Surround neuron cell bodies in ganglia.