Patterns of motor innervation in the pectoral muscle of adult Xenopus laevis: evidence for possible synaptic remodelling.

Abstract

An anatomical and electrophysiological study was performed on the pectoral muscle of X. laevis. Ag-impregnated preparations revealed immature endings, collateral and terminal sprouting and signs of synaptic regression. Of the fibers 20% received a dual innervation from 2 different nerves. The synapses of 25% of these fibers are formed in close vicinity. Some of the singly innervated and most of the dually innervated end-plates generated only subthreshold electrical activity. Synaptic efficacy in dually innervated muscles fibers with closely spaced or distant endings was, on the average, 1/3 and 2/3, respectively, of that obtained in singly innervated fibers. Fibers with subthreshold electrical activity displayed normal ACh [acetylcholine] sensitivity. The existence of non-transmitting synapses, of dually innervated end-plate sites and of morphological signs of the sprouting of new endings and the degeneration of old ones suggests that synaptic remodelling may occur in normal adult muscles.