Nature of the Nerve Impulse

The nerve impulse, which travels through the nerve cell and its processes, results from the action of some stimulus which has the power of liberating energy stored up within the protoplasm.

Practically the only evidence of an actual liberation of energy from the material of the nerve fibre is the electrical change - the wave of negativity that passes along the nerve. The particular form which this energy takes is quite unknown. For a long time it was believed to be electrical but the measurement of its velocity easily disproves this. A nerve impulse travels at the rate of about 120 metres per second in man; this is exceedingly slow compared with the velocity of electricity or of light, but comparable with that of sound which is due to movements of a ponderable medium.

The passage of the nerve impulse has been likened to that of a spark along a train of gunpowder leading to a magazine. The essential difference is that the powder is completely destroyed in the process, while there is no apparent change in the nerve. If a nerve cell, however, be repeatedly stimulated diffusive changes become evident in the tigroid bodies (chromatolysis). There is no perceptible alteration in the axis-cylinder process, but this is supposed to be due to the fact that the axis-cylinder sheath, i.e. the white substance of Schwann and neurilemma, serves as a nutritive reservoir which rapidly replaces any material destroyed. The medullary substance has another function: it acts in a manner similar to that of the insulating material round an electric wire, and prevents the diffusion of the nerve impulse.

Nerve cells possess considerable powers in the direction of modifying the impulses which reach them. They may either reinforce an impulse or they may partially or completely block it; this latter effect is known as inhibition. They may collect weak impulses and combine them (summation) into one effective impulse; or in place of sending out one impulse as the result of a single stimulus they may send out several at more or less regular intervals (rythmical impulses). All recent experiments tend to show that this last form is that generally accomplished by the nerve cells, and further that each nerve cell or group of nervecells, has its own particular rhythm which remains unaltered whatever be the form of stimulus employed.