Alterations in responsiveness of ascending and reflex pathways activated by iterated cutaneous afferent volleys.

Abstract

With repetitive cutaneous stimulation at 1 sec intervals there was usually a slight but statistically significant reduction of N1 and P wave components of the cord dorsum potential. Following a strong extrastimulus, however, only the P wave showed a return toward the control level. Using the technique of dorsolateral and ventral column dissection developed by La Porte, Lundberg and Oscarsson, it has been shown that activity ascending within these systems exhibits only a small decrement to cutaneous afferent volleys repeated at 1 sec intervals and shows no restoration when a strong extrastimulus is presented. Under these same conditions, however, reflex responses to the same stimuli showed much greater decrement and clearcut restoration following a strong extrastimulus. There were never any significant changes in monosynaptic response amplitude during response decrement produced by repeated cutaneous volleys, but occasionally there were increases following strong extrastimuli. It was concluded that depressed motor neuron responsiveness probably plays no role in "habituation" of the flexion reflex, but that increased responsiveness may contribute to the phenomenon of "dishabituation.".