Presynaptic Gain Control in a Locust Proprioceptor

Summary

A common finding in both vertebrates and invertebrates is that the central terminals of their mechanosensory neurones receive synaptic inputs. These inputs are often caused by neurones that release GABA, and change the chloride conductance which, at the normal resting potential, causes a depolarisation. These inputs in turn reduce the efficacy with which the sensory spikes release transmitter onto postsynaptic neurones. The consequence of this presynaptic inhibition is that the information coded in the spikes of the sensory neurones may not be reliably transmitted to postsynaptic neurones. This paper suggests a new function for some of these presynaptic inputs, from an analysis of proprioceptive afferents in a locust. It proposes that they can form part of an automatic gain control mechanism that limits the efficacy of the proprioceptive signals, when activated by the spikes in other afferents from the same sense organ responding to the same movement. The result is that the actions of one sensory neurone are interpreted by the central nervous system only in the context of the network actions of all the other sensory neurones responding to the same stimulus.