Reciprocal excitation between identified flight motor neurons inDrosophila and its effect on pattern generation

Summary

Intracellular recordings from the motor neurons innervating the dorsal longitudinal flight muscle (DLM) show that when one motor neuron fires, a response is transmitted to the other motor neurons (especially between the pairs innervating muscle fibers 1–2 and 3–4). The transmitted response often had a very similar waveform to the neuron's own response, consisting of a prepotential, spike-like component, and hyperpolarized phase. It is suggested that this response is transmitted electrically rather than chemically.

A spike in one motor neuron had an observable effect on the firing time of another motor neuron only when that other neuron was very close to threshold (i.e., when the other neuron was about to fire soon after the first neuron). Two possible effects were observed at this time: (1) Most frequently, it would cause the other motor neuron to fire earlier and synchronously with it; (2) occasionally, it would cause a delay in the other motor neuron's firing time. It is suggested that this delay might be due to a partial resetting of the other neuron's timing, triggered by a ‘local’ excitatory response caused by firing in the first neuron. Thus, both effects could be due to an excitatory (depolarizing) input from the earlier-firing cell.

These interactions between the DLM motor neurons are apparently responsible for the observation that no DLM motor unit fires immediately after any other. The observations were incompatible with a system of mutual inhibition between the motor neurons.