Connections between descending visual interneurons and metathoracic motoneurons in the locust

Summary

Connections between the four DMD neurons and metathoracic motoneurons in the locustSchistocerca were examined by recording extracellularly from the interneurons in the pro-mesothoracic connectives and intracellularly from seventeen motoneurons. A DIMD or DCMD spike causes an EPSP in the fast extensor tibiae motoneuron, which can be modified by changing the membrane potential. The EPSP always follows spikes at frequencies up to 200 Hz and with a latency of 0.9 ms, suggesting that the connections are monosynaptic and chemically mediated. EPSPs from the DIMD or DCMD arrive at the same time, their axons having the same conduction velocity, and appear simultaneously in the fast extensor tibiae motoneurons on both sides of the ganglion. There is spatial and temporal summation between the inputs but on no occasion did the motoneurons spike. Three inhibitory neurons are depolarized by DMD inputs and may on occasion spike, but it is not known whether these connections are direct. Similarly the slow excitatory motoneuron to the anterior coxal adductor muscle is hyperpolarized by DMD input. Other leg, flight or ventilatory motoneurons examined received no inputs from the DMD neurons. The connections shown are consistent with the hypothesis that the DMD neurons are in some way involved with initiation of a jump, but to achieve this must act synergistically with other inputs.