Neuromuscular synapses of a crab motor axon

Neuromuscular transmission of the “fast” motor axon of a crab, Pachygrapsus crassipes , was investigated, using extracellular microeletrodes to record electrical events at individual synaptic regions, and intracellular microelectrodes to record simultaneously the postsynaptic potentials of the muscle fiber. At 1/sec stimulation of the motor axon, the transmitter quantum content per stimulus was estimated at about 2 for many synapses, with relatively slight facilitation at higher frequencies. At some synapses a lower quantum content was measured at 1/sec, and more facilitation was apparent at higher frequencies. Fatigue of transmission with maintained stimulation was rapid. Observations of time course and amplitude of extracellularly recorded potentials suggested heterogeneity of these events. Electron micrographs of synapse‐bearing nerve endings showed them to be in clefts or embedded beneath the surface of the muscle fiber. The presynaptic vesicles were usually present at relatively low density, which could account for the rapid fatigue of transmission. Calculated volume distributions of the vesicles did not correspond to amplitude distributions of extracellularly recorded spontaneous potentials; possible reasons for the lack of agreement are discussed.