Intensity Factors in Reflex Control

However, a study of even the simplest of reflexes shows that the existence of an appropriate nerve-net is not the only thing which underlies the process. In the first place, the intensity of the stimulus plays an important part. The reflex is not aroused by all intensities of the stimulus whatsoever, but requires a certain minimal intensity, which is known as the threshold. If the intensity is increased beyond this value, the vigor of the reaction will also be augmented until a limiting condition is reached. This quantitative variation in the reflex may not be considered to be of very great interest when only a single reflex tendency is in operation, but when two or more tendencies are aroused simultaneously and come into conflict, the ratio of their intensities may determine the outcome. In this case, we have to regard the intensities of the stimuli, or of the reflex tendencies themselves, as the determining factors for the resultant response.

In this connection we may note another complicating feature, namely, that the intensity of the reaction is not deducible from that of the stimulus by any simple law such as that of the conservation of energy. The nerve current does not consist merely in the propagation, along the network, of energy or tension which is applied to it by the stimulus. Frequently the energy of the latter is excessively minute, and is only sufficient to pull a trigger which liberates the inherent energies of the nervous apparatus, and these continue the process. Furthermore, the forcefulness of the response activity at any point along the conduction is to a very large extent a function of the latent energies of the nerve mechanism at this particular point. That this is true will be clear from a consideration of the simple fact that muscles exert forces of many pounds, whereas the forces of the nerve current can only be measured by the use of delicate electrical apparatus. Nevertheless, these considerations do not disturb our conception of the response process as a propagation of a disturbance from one point to another along a predetermined path. They mean only that we may be forced to compare the response chain to a telephone system involving a cascade of amplifiers, rather than a simple line of wire.