Excitatory synapses of blue crab gastric mill muscles

Summary Physiological and ultrastructural studies were made of neuromuscular synapses in stomach muscles, especially two gastric mill muscles of the blue crab innervated by neurons of the stomatogastric ganglion. These muscles depolarized and contracted with application of glutamate, but not acetylcholine, whereas the dorsal dilator muscles of the pyloric region depolarized and contracted in acetylcholine, but not in glutamate. Large excitatory postsynaptic potentials (EPSP's) of 5–20 mV were recorded in the gastric mill muscles. At low frequencies of activation, individual synapses released on average about 2 quanta of transmitter for each nerve impulse. Facilitation of EPSP's after a single nerve impulse could be detected for at least 10 s. Synapses were found on enlarged terminals of the motor axon; their contact areas ranged from 0.2 m² up to 3 m². Both electron-lucent, round synaptic vesicles and dense-cored vesicles occurred near these synapses. A possible correlation between contact area of a synapse and output of transmitter, is discussed.Supported by grants from the National Research Council of Canada and the Muscular Dystrophy Association of Canada to H.L. Atwood and C.K. Govind. We thank Kazuko Hay, Eva Yap-Chung and Irene Kwan for technical assistance with electron microscopy and reconstruction of nerve terminals from micrographs