Decrement of ventral root electrotonus and intracellularly recorded PSPs produced by iterated cutaneous afferent volleys.

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Abstract
In an extension of previous studies on the phenomena of short term "habituation" and "dishabituation" of the flexor twitch in the acute spinal preparation, motor neuron post synaptic potential [PSPs] (recorded both intracellularly and electrotonically from ventral roots) were analyzed during repetitive activation of cutaneous afferents. Using both methods it was found that the depolarizating components showed decrement during "habituation" and an increase during "dishabituation." There was no shift in intracellularly recorded membrane potential during repetitive activation of cutaneous afferents but there were sometimes waves of background depolarization following a strong extrastimulus. The effects of I.V. administration of strychnine and picrotoxin, which are known to affect post-and pre-synaptic inhibition respectively, were analyzed with respect to decrement and restoration of the flexor twitch. It was shown that, provided stimulus intensity is reduced to give responses corresponding to those before injection of these drugs, comparable decrements may still be obtained. The data presented are discussed with regard to possible neuronal mechanisms which might be responsible for "habituation" of the flexor twitch.

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