Figure 1.

An integrative model for backward locomotion: local reversal oscillators are phase-coupled via proprioception, and dually regulated by descending inputs. (a) The A-class motor neurons exhibit intrinsic, oscillatory activities that are sufficient to drive backward movement. (Top panel) Calcium oscillation in the posterior A motor neuron was observed in animals where chemical synaptic transmission and all premotor interneurons were removed from the nervous system. Left, sample traces; right, raster plot of recording from multiple animals. (Bottom panel) A dissected ventral cord muscle preparation from an animal where all premotor interneurons were removed exhibited anterior A-class motor-dependent rhythmic postsynaptic currents (rPSCs) and action potential (AP) bursts, both denoted by red arrowheads. (b) The A-class motor neurons may use intrinsic proprioceptive properties to self-organize phase-coupling during backward movement. (Top panels) A comparison of calcium activities exhibited by posterior A-class motor neurons in an immobilized (left) and a freely moving (reversal) animal (right), where all premotor interneurons were removed from the nervous system. Movements strengthened both calcium oscillation of and phase-coupling among A-class motor neurons. (c) The AVA premotor interneurons provide descending inputs that dually regulate the A-class motor neuron's oscillation through a mixed gap junction and chemical synapse configuration. Gap-junction-mediated coupling between AVA and A-class motor neurons shunts their intrinsic oscillation, whereas chemical synapses allow optogenetically activated AVA to potentiate their oscillation. (d) A model: backward movement is driven by oscillation from a chain of distributed CPGs (the A-class motor neurons), phase-coupled by proprioceptive feedback and regulated by descending inputs. Figure panels adapted from [39,53].