Spontaneous release of transmitter from ‘repressed’ nerve terminals in axolotl muscle

Abstract

DENERVATION of the limb of a lower vertebrate is followed by an ordered reinnervation, so that the normal pattern of muscle use is restored¹. Higher vertebrates, however, seem to be much less selective in their reinnervation². There is no absolute restriction on inappropriate reinnervation in lower vertebrates³; the return of normal function is due, at least in part, to multiple reinnervation of muscle fibres by different nerves, followed by a repression of function of less appropriate contacts⁴. Yip and Dennis⁵ have shown that the repression of function in inappropriate nerve-muscle junctions in the newt, Notopthalmus viridescens, involves a reduction in the number of quanta of synaptic transmitter released by each nerve impulse. If the initiating event in this process is reduction or loss of the ability of inappropriate nerve terminals to release transmitter^6,7, there should be evidence of the continued presence of these terminals even after transmission from the inappropriate nerve has been repressed. In this paper we present such evidence as a result of our studies in the axolotl.