Inhibitory interaction between motoneurons of adjacent segments in the frog spinal cord

Abstract

Inhibition of motoneurons through impulses in recurrent axon collaterals of neighboring motoneurons in the spinal cord of Xenopus laevis was investigated. The method of monosynaptic testing was employed. A slow dorsal root potential (A-DRP) was elicited by the same antidromic volley of impulses that caused recurrent inhibition of monosynaptic reflex responses. The presence of depolarization of dorsal root fibers indicates the operation of a presynaptic inhibitory mechanism. The time course of the inhibitory effect produced by a recurrent volley on the excitability of neighboring motoneurons closely resembled the time course of the A-DRP elicited by the same recurrent volley. The inhibitory effect of an antidromic stimulus depended on the pathway through which the motoneurons under investigation were excited. If recurrent inhibition was postsynaptic, the inhibitory effect would have been independent of the testing pathway. The results presented indicate that recurrent inhibition in the frog is a presynaptic process. The mechanism by means of which recurrent inhibition is effected in the frog, therefore differs from the postsynaptic Renshaw mechanism involved in recurrent inhibition of spinal motoneurons in mammals.