The regulation of synaptic strength within motor units of the frog cutaneous pectoris muscle

The physiological properties of frog neuromuscular junctions may vary widely in a single muscle. In order to understand the factors that contribute to this variation, we have studied populations of synapses belonging to individual motor units of the frog cutaneous pectoris muscle. Motor units in this muscle differ widely in twitch strength. A motor axon's synaptic contacts could be found throughout the muscle, at both singly and polyneuronally innervated endplates. Indeed, over 36% of the endplates contacted by each isolated motor axon were polyneuronally innervated. Comparisons of synapses on muscle fibers in large twitch motor units with those in small twitch motor units reveal that endplate potential amplitude, transmitter release, and muscle fiber diameter are positively correlated with the strength of the motor unit contraction. Large and small twitch motor units differ more in their transmitter release than in their nerve terminal length, indicating that larger twitch motor units have a higher release per unit length of terminal. Among motor units of roughly similar twitch tension, transmitter release at endplates receiving only one axonal input is remarkably constant, independent of postsynaptic muscle fiber input resistance, or, presumably, nerve terminal size. In cases where two different motor axons contribute to a single endplate, the synaptic strength of each input is again related to properties of the contributing motoneuron, although the individual synaptic inputs are markedly reduced in strength and size relative to singly innervated endplates. Additionally, the diameter of polyneuronally innervated muscle fibers appears related to properties of both innervating motoneurons. Thus, the pre- and postsynaptic characteristics of neuromuscular junctions may be determined both by the motoneuron and by peripheral interactions between motoneurons.