Correlated electrophysiological and ultrastructural studies of a crustacean motor unit.

Structural and functional interrelationships between the pre- and postsynaptic elements of a singly motor innervated crab muscle (stretcher of Hyas araneus L.) were examined using electrophysiological and electron microscopic techniques. Excitatory postsynaptic potential (EPSP) amplitude at 1 Hz was found to be inversely related to the extent of facilitation, and directly related both to the amount of transmitter released at 1 Hz and the muscle fiber input resistance (R(in)). The extent of facilitation (F(e)), taken as the ratio of the EPSP amplitude at 10 Hz to that 1 Hz, was inversely related to muscle fiber R(in), tau(m), and sarcomere length. Sarcomere length was directly related to R(in) and tau(m). The excitatory nerve terminals of low F(e) muscle fibers had larger neuromuscular synapses than did those of high F(e) fibers. Inhibitory axo-axonal synapses were more often found in low F(e) muscle fibers. These structural features may account for the greater release of transmitter at low frequencies from the low F(e) nerve terminals as well as provide for a greater amount of presynaptic inhibition of low F(e) muscle fibers. The implications of these findings for the development and physiological performance of the crustacean motor unit are discussed. It is proposed that both nerve and muscle fiber properties may be determined by the developmental pattern of nerve growth.