EFFECTS INDUCED IN A MONOSYNAPTIC REFLEX PATH BY ITS ACTIVATION

Abstract

A monosynaptic reflex path (afferents in group I fibers) has been conditioned by setting up in it a number (widely ranging from 1 to 4500) of volleys at high frequency (300-500 per sec.), and the test response has been elicited by a single volley in the same reflex path and at a wide range of intervals after the conditioning[long dash]from a few msec. to several minutes. The test response has been recorded either focally by a microelectrode adjacent to the motor nucleus or from the emerging ventral root. Thus the conditioning was sampled by four sets of cor-relatable exptl. data; the presynaptic volley focally recorded from the motor nucleus, the focal synaptic potential, the ventral root synaptic potential, and the monosynaptic reflex spike in the ventral root. Conditioning by fewer than ten volleys gave a reflex depression which passed off in several seconds. After a longer conditioning tetanus (30-300 volleys) the initial post-tetanic depression quickly reversed at 30-50 msec. to a post-tetanic potentiation which showed a brief initial phase (peak at 100-200 msec), and, after a phase of relative depression, a much longer potentiation phase reaching a maximum in several seconds and finally disappearing in several minutes. After more severe conditioning tetani of over 500 volleys, all trace of the initial potentiation disappeared. The more pro-longed post-tetanic depression reversed to potentiation after several hundred msec. The synaptic potentials recorded either focally or from the ventral root conformed closely with the observations on reflex spike sizes, both being increased in approx. proportion to the reflex potentiation. It is concluded that the close parallelism in behavior after diverse conditioning indicates that the ""synaptic potentials"" recorded focally and from the ventral root are generated by the same synaptic excitatory process. After severe conditioning tetani, potentiation of the presynaptic spike was observed to run much the same time course as potentiation of the synaptic potential or reflex spike (confirming Lloyd). After less severe tetani this correlation broke down, and with still briefer conditioning tetani no correlation whatever was possible, there being, if anything, an inverse relationship. Thus after tetani of 30-300 volleys the presynaptic spike was usually diminished during the large initial phase of potentiation and not significantly modified during the later phase. It is argued that the focally recorded presynaptic spike gives a valid measure of the size of the presynaptic volleys, because, after conditioning by a single ""mixed"" afferent volley, there was a parallelism in the depressions of presynaptic spike and synaptic potential and in recovery therefrom. Potentiation of the presynaptic volley thus fails to explain post-tetanic potentiation after all degrees of conditioning tetani. Several alternative explanations are discussed and it is concluded that probably the presynaptic impulse becomes a more effective synaptic excitor because repetitive stimulation temporarily alters the spatial relation-ship of the synaptic knobs to the postsynaptic membrane, the knobs becoming larger and/or in closer opposition thereto. The relationship of such a change to more enduring plastic states of the nervous system is discussed.