Studies on the flexor reflex. VI.—Inhibition

Abstract

The inhibition of flexor motoneurones by a single contralateral volley has been investigated. A reduction in the size of the reflex response, i.e., inhibition, is not usually observed until the excitatory volley follows the inhibitory by 8[sigma] or more. After its first appearance the amount of the inhibition increases for 25[sigma] to 70[sigma] and then progressively decreases, but is usually still observable at 200[sigma]. In some exps. a given inhibitory volley has inhibited a weaker reflex more than a stronger. Apparently with motoneurones the prevention of reflex discharge by inhibition depends on the intensity of the excitation. Two similar inhibitory volleys in the same afferent nerve produce a greater inhibition than either volley alone, even when the interval between them is as long as 60[sigma], and a repetitive series of inhibitory volleys produces a gradually increasing inhibition, showing summation of the inhibitory effects of successive afferent volleys. A volley passing antidromically up motor nerve fibres to inhibited motoneurones has no effect on that inhibition. Excitatory impulses incident on inhibited motoneurones diminish the inhibition of a reflex evoked by a subsequent excitatory volley. Inhibition is evidently a long-lasting stage and it is inactivated by excitatory impulses. It must further be assumed that the inhibitory impulses from a single inhibitory volley are incident on motoneurones during a considerable period. This temporal dispersion, often at least 100[sigma] in duration, is attributed to delay paths. It is suggested that the central inhibitory state has no direct effect on the motoneurone, its effect being entirely due to the mutual inactiyation resulting when it and the central excitatory state interact.