PROGRESSIVE CHANGES IN EFFERENT UNIT RESPONSES TO REPEATED CUTANEOUS STIMULATION IN SPINAL CATS

Abstract

Responses of motor units isolated from the L6 ventral roots of acutely prepared spinal cats have been studied during repetitions of a brief tetanic stimulus applied to one hind paw. Stimulation was generally similar in intensity, duration and intertrial intervals to the US utilized to establish overt conditioned flexion responses in behavioral conditioning. Phasic efferent units showed, predominantly, a progressive decrease in reflex responses to repeated stimulus while tonic units showed, in some cases, a progressive decrease, but in others, a progressive increase in response. The altered motor unit responses were shown to be independent of progressive changes in the cutaneous and subcutaneous tissues underlying the stimulating electrodes and were, likewise, shown to be independent of blood pressure changes. Thus, the altered reflex responses were most probably due to changes within the spinal reflex pathway. The development of these progressive changes in reflex responses to a repeated stimulus suggests that during behavioral training the US may similarly produce changes within the spinal reflex pathway which would serve to facilitate conditioned motoneuron discharge.