Central inhibitory action attributable to presynaptic depolarization produced by muscle afferent volleys

Frank & Fuortes (1957) and Frank (1959 and personal communication) have shown that muscle afferent volleys produce inhibition by diminishing the size of the monosynaptic excitatory post-synaptic potential (EPSP) of motoneurones without having any other demonstrable action on those motoneurones. Thus there was no change in the ionic permeability of the post-synaptic membrane, such as occurs with other types of inhibition (Coombs, Eccles & Fatt, 1955a), for there was no associated membrane potential change either at the normal resting potential, or when the membrane potential was altered by a background depolarizing or hyperpolarizing current. Furthermore, there was no associated change in motoneuronal excitability as tested either by the intracellular application of current pulses or by the response to invasion by an antidromic impulse in the motor axon. Evidently it should be concluded that the diminution of the EPSPs is due to a diminished excitatory action of the Group Ia presynaptic impulses. However, Frank (1959) in addition proposed an alternative explanation, which attributed the EPSP depression to an action exerted so far out on the dendrites of the motoneurone that no trace of the inhibitory influence itself could be detected by a micro-electrode in the motoneuronal soma. Since both these alternative explanations would locate inhibitory action at a site remote from the motoneurone soma, Frank (1959) employed the term 'remote inhibition' for the phenomenon. It is proposed here to use the non-committal term 'EPSP depression' instead, until a more appropriate term can be developed later in this paper. There are many suggestions in the literature that central inhibitory action occurs in the presynaptic pathway, there being block or depression of presynaptic excitatory impulses (Barron & Matthews, 1935, 1938; Renshaw, 1946; Brooks, Eccles & Malcolm, 1948; Howland, Lettvin, McCulioch, Pitts & Wall, 1955). These various suggestions will be examined