PROPERTIES OF A SINGLE SYNAPSE IN THE STELLATE GANGLION OF SQUID

Abstract

In a prepn. of the isolated stellate ganglion with pre- and postganglionic nerves it is possible to record impulses from the giant pre- and post-fibers and from the synapse between them in such a way that a single junction is active. Transmission is polarized; delay may be less than 0.5 msec. (24 C). Transmission is always 1:1; after-discharge has not been observed; temporal summation occurs during fatigue; the junction can follow at least as high as 475 presynaptic impulses per sec. being limited by the refractory period of the prefiber. Fatigue does not change the action current of the prefiber as far into the ganglion as it can be traced. The postspike is exclusively affected, apparently by a depression of the synaptic potential. This is recognized, before the spike drops out, as a notch in the rising phase and is left by itself after the spike fails, when its characteristics are readily studied. It is a\ graded, local negativity rising to a crest in about 0.5 msec., falling to 37% in about the same time, reaching 20% of the height of a spike without initiating a spike and decrementing to half amplitude in about 3 mm. from the ganglion. The synaptic potential becomes smaller with fatigue but, unlike the spike, does not arise later. Nor does its time course or rate of decrement change. Significant variations in the area occupied by the synaptic potential from moment to moment in the same prepn. were observed. It is tentatively concluded that the potential is propagated for a short distance, decrementally, rather than being conducted entirely electro-tonically. Abortive and growing spikes are described. The local potential can be eilcited by direct simulation of the ganglion when its amplitude shows the expected nonlinear increase with increasing subthreshold cathodal shocks. An absolute refractory period may be altogether absent in fresh prepns. or it may be present and of various durations depending on the state of fatigue. The time course of excitability after a local response passes through a supernormal, a subnormal and then another supernormal phase in suitable prepns. The consequences of antidromic interaction with normally transmitted impulses are described and resemble those in simple nerve fiber. Cold can greatly prolong the synaptic delay and separately depress the local potential. The latter results in an even greater delay in the spike which may not be initiated for 6 msec. Anoxia has a similar action. DFP blocks, reversibly, in a manner similar to its action on the peripheral giant fiber and in about the same concn. No hyperexcitable phase occurs. The great similarity between the behavior of the prepn. and the ephapse of Arvanitaki and the isolated vertebrate nerve-muscle junction of Kuffler is considered significant. The implications of a simple 1:1 relay-type synapse for normal function in the animal are pointed out.