DUAL INNERVATION OF ENDPLATE SITES AND ITS CONSEQUENCES FOR NEUROMUSCULAR-TRANSMISSION IN MUSCLES OF ADULT XENOPUS-LAEVIS

Electrophysiological study of dually innervated end-plate sites was carried out in Xenopus laevis pectoral muscle fibers. End-plate potentials (epp) and miniature end-plate potentials (mepp) were recorded in Mg-blocked preparations. The mean quantal content (m) of each epp at dually innervated end-plates was significantly smaller than the corresponding values obtained at singly innervated ones. At a given doubly innervated end-plate site the values of m tended to be inversely related, so that the compound value of m (obtained by adding them) was in the same range as that found in singly innervated junctions. The existence of an upper limit in the average amount of transmitter released at a synaptic site is suggested. It was found that neither intermittent presynaptic conduction block, nor particular muscle fiber properties could account for the low values of m in dual end-plates. The small size of the nerve terminals appears to be the most likely explanation. The sensitivity to acetylcholine and muscle fiber electrical properties were investigated; no differences were found between fibers with sub- or suprathreshold epp. The nature of the factors responsible for this presumed small size of the nerve endings (competition between nerve endings for a limited synaptic space on the muscle membrane or reciprocal interaction between closely located terminals) and the possible origins of polyinnervation are discussed.