Variation in Physiological Properties of Crustacean Motor Synapses

Abstract

THE properties of individual synapses of the motor axon which innervates the opener (dactylopodite abductor) muscle in the crayfish walking leg have been investigated by Dudel and Kuffler¹, who placed an external microelectrode close to the synaptic region to record the flow of current through the post-synaptic muscle fibre membrane during the action of the neuromuscular transmitter². With low frequencies of stimulation of the motor axon individual synapses showed frequent failures of transmission, and the synaptic currents generated by successful transmission were small, often equal in size to a single externally recorded spontaneous miniature potential. The latter event was found to equal one quantal unit of transmitter action. At higher frequencies of stimulation, the probability of failure of transmission decreased, and the synaptic currents attained a larger average size, indicating that the output of transmitter substance from the presynaptic terminals had increased. The postsynaptic potentials recorded internally from the muscle fibres were correspondingly larger at the higher frequencies of stimulation. These observations indicate that facilitation at the crayfish neuromuscular junction is the result of some process which gives rise to a higher probability of release of transmitter substance at the higher frequencies of stimulation or during the course of a train of closely spaced stimuli.