Nerve Cells and Nerve Fibres

The structural unit of the nervous system is a highly specialised cell consisting of a cell body (termed the nerve cell) and a series of processes; the complete unit is called a neuron (Fig. 5).

The functional unit consists of a pair of such neurons (Fig. 6). One is destined for the reception of impulses from a sensory surface; it is generally described as the sensory neuron, since it is frequently associated with a sensation. The other discharges an impulse - as a rule to some motor mechanism, and is hence commonly known as the motor neuron.

The processes of every neuron, although similar in structure, are functionally of two kinds, and this is often emphasised by their grouping at two roles of the cell. One set of processes receives the stimulus from without and transmits the impulse to the cell; they are known as the dendrites or afferent (centripetal) processes. As a rule they emerge from the cell body as short thick stems which rapidly break up into series of fine branched filaments. The other group is usually represented by one long process which gives off side branches (collaterals) at intervals, and finally terminates in an arborisation like that of the dendrites. This process is known as the neurite, axon, or efferent (centrifugal) process.

It has already been indicated that each neuron has one long process directed towards the periphery of the body; it will now be evident that in the case of the sensory neuron this process is a dendrite, while in the case of the motor neuron it is an axon.