Input and output connections of an anteromedial group of spiking local interneurons in the metathoracic ganglion of the locust

Abstract

Intracellular recordings were made from an anteromedial group of spiking local interneurons in the metathoracic ganglion of the locust to determine the input connections that shape their mechanoreceptive fields on a hindleg and the output connections that define their effects on hindleg motor neurons. The receptive fields of these interneurons may contain excitatory and inhibitory regions. An excitatory region on the ipsilateral hind leg is formed by direct excitatory connections of the afferents from exteroceptors. Afferent spikes consistently evoke EPSPs in interneurons with a central synaptic latency of 1.2-1.5 msec. The connections appear to be direct and chemically mediated. An inhibitory region of a receptive field is not formed by direct afferent connections. Instead, spiking local interneurons of a midline group, which also receive direct excitatory inputs from the afferents, make direct inhibitory connections with anteromedial interneurons. Spikes in particular midline interneurons consistently evoke IPSPs in anteromedial interneurons with a central synaptic latency of about 1.0 msec. The connections appear to be direct and chemically mediated. By contrast, spikes in anteromedial interneurons could not be linked to any synaptic potentials in midline interneurons, so that direct connections are unidirectional and inhibitory. The large inhibitory regions to the receptive fields of anteromedial interneurons result from convergent inhibitory inputs of several midline interneurons. Each midline interneuron may make inhibitory output connections with several anteromedial interneurons, but there is a high degree of specificity in these divergent connections. Anteromedial interneurons make excitatory output connections with specific hind leg motor neurons. EPSPs in motor neurons follow spikes in the interneurons with short and consistent synaptic latencies of about 1.0 msec. The connections appear to be direct and chemically mediated. Two or more anteromedial interneurons with overlapping receptive fields may converge onto one motor neuron, and, in turn, one interneuron may make divergent connections with several motor neurons of the same pool. This pattern of connections suggests that these interneurons are essential elements in the local circuitry responsible for the expression of local reflexes of a leg. They provide an excitatory drive to the motor neurons in parallel to that from nonspiking local interneurons when particular arrays of exteroceptors are stimulated.