This section is from the book "A Text-Book Of Pharmacology, Therapeutics And Materia Medica", by T. Lauder Brunton. Also available from Amazon: A text-book of pharmacology, therapeutics and materia medica.
Supposing nervous stimuli to consist of vibrations like those of light or sound, the action which any nerve-cell would have upon the others connected with it would be stimulant or inhibitory according to its position in relation to them. If its relation be such that a stimulus passing from it to another cell will there meet with a stimulus from another quarter in such a way that the waves of which they consist coincide, the nervous action will be doubled; but if they interfere the nervous action will be abolished. If they meet so as neither completely to coincide nor to interfere, the nervous action will be somewhat increased, or somewhat diminished, according to the degree of coincidence or interference between the crests of the wave.
Thus if the relations of the nerve-cells s, s' and m, m' in the diagram (Fig. 65) are such that when a stimulus passes from a sensory nerve s to a motor nerve m, one part of it travels along the path s, s, m, m, and another along s, s, s', m, m, or s, s, s', m', m, m, at such a rate that the crests of the waves coincide at the motor cell m, they will increase each other's effect. If they interfere, the effect of both will be diminished or destroyed, i.e. inhibition will occur.
Fig. 65. - Diagram to illustrate inhibition in the spinal cord. s, s', and s" are sensory nerves, m, m', and m" are motor nerves, s, s', and s" are sensory cells, m, m', and m" are motor cells in the spinal cord, sb is a sensory, and mb a motor cell in the brain.