The inhibitory suppression of reflex discharges from motoneurones

In a preceding paper (Coombs, Eccles & Fatt, 1955b) an account was given of the ionic events (increases in membrane permeability and the consequent ionic fluxes) that account for the potential changes which are produced in motoneurones by inhibitory impulses, i.e. the inhibitory post-synaptic potential (i.p.s.p.). The present paper provides evidence on two further questions that arise in an inquiry into central inhibitory action: What is the time course of the postulated change in membrane permeability? How does this membrane change together with the consequent ionic fluxes cause the inhibitory suppression of reflex discharges from motoneurones? The intensity-time course of this inhibitory suppression has been thoroughly studied with direct inhibitory action on monosynaptic reflex discharges (Lloyd, 1946; Laporte & Lloyd, 1952; Bradley, Easton & Eccles, 1953). Actually the intensity of inhibition is indirectly measured by the depression of the size of a testing monosynaptic reflex discharge which is applied at various times during the direct inhibitory action of a single afferent volley. The time course of the intensity so measured may be called an inhibitory curve. It will be shown below that it is possible to explain in detail the inhibitory curve that is produced with direct inhibitory action on a population of motoneurones.

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