Further Studies on Synaptic Transmission in Insects

Abstract

1. Some integrative properties of the chemical synapses between the sensory cereal afferent fibres and a giant axon are studied at the unitary level by external recording. 2. Unitary monosynaptic EPSPs are related to cereal receptor activity, one presynaptic spike inducing one EPSP. Unitary IPSPs are not directly related to cereal activity. They seem to originate within a ganglion. 3. Many receptors make synaptic contacts with a given giant axon. When a single receptor is strongly stimulated, the summation of elementary induced EPSPs gives a postsynaptic depolarization which is not able to reach the threshold. The simultaneous stimulation of several receptors provides a considerable amount of postsynaptic depolarization, thus triggering one or two spikes, rarely more. 4. The postsynaptic responses appear to be more or less different according to the afferent firing pattern: phasic, phasic-tonic and tonic. Some factors modulate these schemes: (a) Potentiation, which occurs only with phasic patterns, thus increasing their postsynaptic effect, is followed by a longer component of depression. (b) Limiting factors, such as fatigue phenomenon, are thought to be essentially related to depletion of an available transmitter. 5. Synchronous electrical stimulation induces a monosynaptic response, later followed by a complex polysynaptic phenomenon. These responses decrease with repetitive stimulation, but the former is far less labile than the latter. 6. These studies seem to indicate that a giant interneurone in the cockroach fires only in special conditions, namely when a strong mechanical stimulation occurs after a noticeable period of rest. This spike is then rapidly conducted to higher centres without relay.