Fig. 6.

Concepts of inhibition as depicted by Lorente de Nó, Morgan and Milner. (a) A diagram explaining the production of reflex reversal by concurrent stimulation of two fibers (f₁ and f₂ or f₂ and f₃) from different peripheral sense organs and its maintenance by the impulses conducted by the closed chain C, after fiber f₃, which initiated the response of cell 3, ceases conducting. Each one of the links in the closed chain represents a multiple chain of (see M in Fig. 4A of this paper). Collateral d by lowering the threshold of cell a and thus causing two impulses to cross through cell 3 in quick succession may produce inhibition, for cell 3 will acquire a high subnormal threshold. (From [124], page 231). (b) A diagram of neurons and synaptic connections involved in reciprocal inhibition (innervation). Excitation at the synapses is widely held to be proportional to the number of active endings. For the sake of argument, the minimal number of endings which must be active for excitation to occur is arbitrarily taken to be two. If a neuron, b, common to two pathways is switched out of one pathway when it is taken up by another, the necessary condition for reciprocal innervation would be fulfilled. Let us suppose that rhythmic stimulation of fiber I is maintaining a flexor reflex. Neurons b and c are excited, and their discharges arriving synchronously at F cause it to respond. Then let us suppose that in the course of this response an extensor reflex is set up through stimulation of fiber II. The latter can excite b in the intervals between the responses to I, because of the stronger excitation which it is able to deliver through its three endings. No discharges can result therefrom in F, as the impulses in b are out of time with those in c; and I is no longer able to excite b and c in unison, because of the raised threshold of the former. Neuron b is dominated by fiber II. Its discharges are caused to be synchronous with those in a, instead of those in c, and activity begins in E. Thus, when innervation of the extensor muscles starts it must be withdrawn from the flexor muscles. (From Gasser [46] and reprinted in Lorente de Nó [124], page 231, and Morgan [139], page 67). (c) Types of recurrent inhibitory connections. It is proposed that the short-axon inhibitory cells which receive recurrent collaterals from a long-axon cell have fewer inhibitory connections to that particular long-axon cell than they do to other long-axon cells in the region. When Cell A is firing it causes the inhibition of its neighbors, B, C, and D, but is not itself inhibited. In fact, because the surrounding long-axon cells cannot now be fired, there is no way in which the short-axon cells discharging onto A can be fired. Therefore, as long as A continues to fire, it protects itself from being inhibited. (From [133], page 246). a and b Reprinted from Lorente de No R. Analysis of the activity of the chains of internuncial neurons. J Neurophysiol. 1938;1:207–44 [124]. Copyright (1938), with permission from the American Physiological Society. c Reprinted from Milner PM. The cell assembly: mark II. Psychol Rev. 1957;64:242–52 [133]. Copyright (1957), with permission from the American Psychological Association