The relationship between end‐plate size and transmitter release in normal and dystrophic muscles of the mouse.

Abstract

The morphology of nerve terminals and sub-neural apparatuses was examined in the muscles of normal and dystrophic adult mice of the Bar Harbor 129 ReJ strain. Nerve terminals were larger in dystrophic muscles than in normal muscles and nerve terminal sprouting was evident in about 50% of the dystrophic muscle fibers. End-plate area was positively correlated with muscle fiber diameter in both normal and dystrophic muscles. Polyneuronal innervation was found in only 1% of dystrophic muscle fibers impaled with micro-electrodes. Miniature end-plate potential [m.e.p.p.] amplitude was positively correlated with muscle fiber input resistance in both normal and dystrophic muscles. There was a greater than normal variation in the amplitudes of m.e.p.p. recorded from individual dystrophic muscle fibers. Quantum contents of e.p.p. were estimated in normal and dystrophic mouse nerve-muscle preparations partially blocked with d-tubocurarine. The quantum content of e.p.p. seemed to be related to muscle fiber diameter and in dystrophic muscles the characteristics of evoked release were indistinguishable from normal. The nerve terminal sprouting and the expansion of end-plate area which were observed in dystrophic muscles are not a consequence of any form denervation, but represent an attempt by the axon to expand the area of synaptic contact in hypertrophied muscle fibers.