Post‐synaptic potentials in a population of motoneurones following activity of single interneurones in the cat.

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Abstract
The technique of recording post-synaptic potentials from a population of motoneurons, by recording from ventral roots perfused with isotonic sucrose, was applied to investigate the action of single last-order interneurons; the target motoneurons were in either caudal [lumbar] L7 or [sacral] S1 segments. Using spike-triggered averaging, the inhibitory action of 70% of previously identified last-order interneurons (Renshaw cells and lamina VII Ia inhibitory interneurons) was detected. Previous observations had suggested that interneurons mediating dysynaptic non-reciprocal inhibition from group I muscle afferents should be characterized by location in laminae V-VI, monosynaptic group I input and ascending collateral axonial projection to upper lumbar segments. Of the interneurons with these characteristics 65% were found to inhibit motoneurons. Spike-triggered averaging from this group of laminae V-VI interneurons sometimes revealed a depolarizing potential which preceded the inhibitor potential evoked by the interneuron. The depolarizing potential is interpreted as being due to the action of some presynaptic fibers which branch to innervate both the investigated interneurons and motoneurons.