Electrophysiological and electron microscopic studies on the neuromuscular junction of the crayfish stretch receptors.

Abstract

1. Investigations on the inhibitory neuromuscular junction of crayfish stretch receptor muscles were performed electrophysiologically and electron microscopically.2. Fast receptor muscles responded to direct stimulation with a transient depolarization which exhibited the property of a propagated action potential, but did not show any overshoot. On the other hand, slow muscles did not generate an action potential but showed excitatory junction potentials of 10-30mV in amplitude. EJPs of fast receptor muscles were summated by repetitive stimulation until all-or-nothing spikes were generated.3. When a nerve bundle was carefully isolated and one of the isolated filaments were stimulated, inhibitory junction potentials could be obtained from fast stretch receptor muscles by the microelectrode inserted near the receptor neuron. They had a reversal potential approximately at-60mV. Inhibitory junction potentials completely inhibited excitatory junction potentials and spikes when inhibitory nerves were stimulated at high frequency of 100c/s for several seconds.4. Application of GABA of 10-4-10-5M showed similar effects to stimulation of inhibitory nerves. It's effect was due to the increase of postsynaptic membrane conductance.5. Two different types of synapses were identified electron microscopically at fast stretch receptor muscles. The one contained spherical synaptic vesicles, the other ellipsoidal ones. The latter was observed within the limit of the vicinity of the neuron soma. From UCHIZONO'S (1965, 1966) previous studies and the present electrophysiological experiments, it was concluded that the latter was the terminal of inhibitory neuromuscular junctions.