Organization of receptive fields of spiking local interneurons in the locust with inputs from hair afferents

Abstract

The receptive fields of spiking local interneurons in the locust were defined by making intracellular recordings from them while stimulating mechanoreceptors on the surface of a hindleg. All the interneurons tested have their cell bodies near the ventral midline, in the so-called "midline" group. Those described here receive inputs only from external mechanoreceptors; others receive inputs from internal proprioceptors alone or from receptors of both kinds. The receptors on the surface of a hindleg that contribute to the receptive field of an interneuron may be clustered together in a discrete area or be distributed in separate regions that provide either excitation or inhibition. An interneuron may have a receptive field that is wholly excitatory or one with both excitatory and inhibitory regions. Excitatory but not inhibitory effects are mediated by direct connections between afferents and interneurons. The longitudinal boundaries of most receptive fields occur along one of the major axes of the leg. For example, hairs on the anterior half of a leg can be excitatory, while hairs on the posterior half are inhibitory; or those on the dorsal half are excitatory, and those on the ventral half are inhibitory. The proximal-distal boundaries of a receptive field often correspond to the articulations between the segments of a leg, although they may also occur within a segment of a leg where there are no other obvious anatomical discontinuities. The receptive fields of these interneurons often overlap, and an individual afferent from a hair excites more than one interneuron. In this way a particular region on the surface of a hindleg may be mapped onto as many as 12 interneurons. The size of a receptive field is not correlated with its position along the proximal-distal axis of the leg, but the smallest fields occur on the distal tibia or span the femorotibial joint.