Nervous Tissue

Nervous tissue is the fourth and last of the four fundamental animal tissue types, and it is specialised not for covering, packing or contracting, but for rapidly receiving a stimulus and conducting an electrical impulse in response, allowing the different parts of the body to communicate with one another almost instantaneously. Nervous tissue is composed of two broad cell categories: neurons (or nerve cells), which are the actual conducting units, and neuroglia, a variety of smaller supporting cells that nourish, insulate and protect the neurons but do not themselves conduct impulses.

A typical neuron, of the multipolar type most common in the vertebrate body, can be described in terms of three main parts. The cell body, also called the cyton or soma, contains the nucleus together with the usual cell organelles and characteristic clumps of rough endoplasmic reticulum called Nissl granules; it is the metabolic and synthetic centre of the neuron, producing the proteins and other materials the rest of the cell needs. Projecting from the cell body are a number of short, branching processes called dendrites, which are specialised to receive incoming signals — either from a sensory receptor or from the axon terminal of another neuron — and to conduct the resulting impulse inward, towards the cell body. In contrast, a neuron typically has only one long process extending from the cell body, called the axon; the axon conducts the nerve impulse away from the cell body, often over a considerable distance, towards its target, which may be another neuron, a muscle fibre, or a gland. Many axons are wrapped in a fatty, insulating myelin sheath, produced by Schwann cells, with periodic gaps in this sheath called nodes of Ranvier; myelination greatly speeds up impulse conduction. At its far end, the axon divides into fine terminal branches ending in synaptic knobs. …