Proprioceptive inputs to nonspiking local interneurons contribute to local reflexes of a locust hindleg

Local reflexes of a leg of the locust Schistocerca gregaria (Forskal) can be elicited by selective stimulation of a proprioceptor (the femoral chordotonal organ) at the femorotibial joint. Motor neurons are either excited or inhibited, so that a coordinated reflex response of a leg results. At the same time, some nonspiking local interneurons are either excited or inhibited by the inputs from these proprioceptive afferents. Altering the membrane potential of an individual, nonspiking interneuron can either increase or decrease the response of the participating motor neurons to the proprioceptive stimulus and thereby alter the gain of the reflex. To determine the pathways, and to understand the role of the nonspiking interneurons in mediating these reflex effects, recordings were made simultaneously from these interneurons and afferent neurons. The excitation of a particular nonspiking local interneuron is produced monosynaptically by the afferent neurons. Chemically mediated EPSPs consistently follow sensory spikes with a latency that is the same as that for the known parallel, direct connections made by these sensory neurons with motor neurons (Burrows, 1987a). The chordotonal afferents and the branches of the local interneurons project to the same regions of neuropil. In contrast, the simplest inhibitory pathway is disynaptic, involving spiking local interneurons. The afferents make direct excitatory connections with some of these spiking interneurons, which then make direct inhibitory connections with a nonspiking interneuron. Interactions between the local interneurons add to the complexity of the pathways.