Input–Output Relation of a Single Synapse

Abstract

THE rate of secretion of a transmitter substance from a nerve terminal is known to be controlled by the membrane potential of the terminal. This has been shown, for example, at the neuromuscular junction by registering the frequency of miniature end-plate potentials when electric currents are sent through the motor nerve endings¹. More recently, the use of tetrodotoxin² has made it possible to eliminate initiation of impulses on either side of the junction, and so to study the graded relation between electrical “input” and “output” of a synapse, undisturbed by any regenerative potential change. With this method it has been found that brief pulses of depolarization, locally imposed on a motor nerve ending, elicit graded end-plate potentials the size of which can reach or even surpass that normally produced by a nerve impulse³. To obtain additional information, it was necessary to use a preparation in which direct measurements of the membrane potential can be made on both sides of the junction. The so-called giant synapse of the squid⁴ is suitable for this purpose: it is possible to insert microelectrodes in the pre- as well as post-synaptic nerve fibres of this preparation, close to their region of synaptic contact, and there is now good evidence that chemical transmission operates at this synapse⁵. The experiments reported here were made on the stellate ganglion of the squid Loligo vulgaris at the Naples Zoological Station. Similar work carried out at Woods Hole has been reported by Bloedel, Gage, Llinás and Quastel⁶.