Inhibition

The synapses, or adjustors, are ordinarily mechanisms by which a nerve current or excitation is passed positively from an afferent to an efferent channel, although usually with radical modification in the nature of the process. However, it appears that in certain cases the action of the synapse, or adjustor is algebraically opposite to excitation, producing an effect known as inhibition. Inhibition is not merely a failure to excite, but involves a repression or blocking of excitations which would otherwise have occurred. The process of inhibition is of particular interest in connection with the interaction of two or more reflexes, since it is a prerequisite to the creation of conflict between them. However, a central inhibitory effect is found even in the case of single reflexes. For example, such a movement of the hind limb of the dog as is involved in the scratch reflex, requires not only an excitation of the nerve fibres which energize the flexor muscles, but a simultaneous inhibition of the fibres controlling the extensor muscles, which are mechanically opposed to the flexors. Accordingly, the adjustor which is in control of this reflex must inhibit the extensors while it is exciting the flexors, and there is conclusive evidence that the inhibitory effect occurs within the spinal cord, rather than in the muscles.