Phase-specific premotor inhibition modulates leech rhythmic motor output

Understanding how motoneuron activity is finely tuned remains an open
question. Leeches are a highly suitable organism for studying motor
control due to their well-characterized behaviors and relatively simple
nervous system. On solid surfaces, leeches display crawling, a rhythmic
motor pattern that can be elicited in the isolated nerve cord or even in
single ganglia isolated from it. This study aimed to learn how this motor
output is shaped by concurrent premotor signals. Specifically, we analyzed
how electrophysiological manipulation of a premotor nonspiking (NS)
neuron, which forms a recurrent inhibitory circuit (analogous to that
formed by vertebrate Renshaw cells), shapes the leech crawling motor
pattern. The study included a quantitative analysis of putative motor
units active throughout the fictive crawling cycle, which shows that the
rhythmic motor output in isolated ganglia mirrors the phase relationships
observed in vivo. Taken together, the study reveals that the premotor NS
neurons, under the control of the segmental pattern generator, modulated
the degree of excitation of motoneurons during crawling in a
phase-specific manner.