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

1. 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 muscles fibres. End-plate area was positively correlated with muscle fibre diameter in both normal and dystrophic muscles. 2. Polynueronal innervation was found in only 1% of dystrophic muscle fibres impaled with micro-electrodes. 3. Miniature end-plate potential amplitude was positively correlated with muscle fibre input resistance in both normal and dystrophic muscles. There was however, a greater than normal variation in the amplitudes of m.e.p.p.s recorded from individual dystrophic muscle fibres. 4. Quantum contents of end-plate potentials were estimated in normal and dystrophic mouse nerve-muscle preparations partially blocked with D-tubocurarine. The quantum content of e.p.p.s seemed to be related to muscle fibre diameter, and in dystrophic muscles the characteristics of evoked release were indistinguishable from normal. 5. It was concluded that the nerve terminal sprouting and the expansion of end-plate area which were observed in dystrophic muscles are not a consequence of any form of denervation, but represent an attempt by the axon to expand the area of synaptic contact in hypertrophied muscle fibres.