Nerve

Nerve is a specialized tissue through which impulses pass back and forth between the brain and the spinal cord. And the other parts of the body. Nerve tissue consists mainly of cells called neurons. A typical neuron comprises a central body, a long threadlike extension called an axon, and a number of shorter processes called dendrites.

Nerves are made up of long, rope-like bundles of axons whose central bodies are located in the brain or spinal cord or in clusters elsewhere in the body. The gray matter in brain consists mostly of central bodies and dendrites. The axons are covered with a layer of white, fatty material called myelin. Impulses are received in its central body through the dendrites and leave by way of the axon. The free end of the axon is somewhat spread out, like a frayed end of string. This end comes close to the dendrites of other neurons but does not quite touch them. A nerve impulse must jump across this tiny gap, which is called synapse. 

Neurons have two kinds, sensory and motor. Sensory or afferent, neurons carry impulses inward to the spinal column and brain, where they are interpreted as information about the outer world. Motor, or afferent, neurons carry impulses from the central nervous system to the various parts of the body, where they are translated into action. At the outer end of the motor neuron, the frayed end of an axon spreads out to form a so-called end plate, which connects with muscle fibers. Similar structures at the end of sensory nerves are concentrated in the sense organs and irregularly dispersed in the skin. 

It is the impulses that travels along the nerve by a series of reactions that are partly chemical, partly electrical. The impulses travel at varying speeds - up to about 200 miles an hour.