Abstract
In all vertebrates, excitatory spinal interneurons execute dynamic adjustments in the timing and amplitude of locomotor movements. Currently, it is unclear whether interneurons responsible for timing control are distinct from those involved in amplitude control. Here, we show that in larval zebrafish, molecularly, morphologically and electrophysiologically distinct types of V2a neurons exhibit complementary patterns of connectivity. Stronger higher-order connections from type I neurons to other excitatory V2a and inhibitory V0d interneurons provide timing control, while stronger last-order connections from type II neurons to motor neurons provide amplitude control. Thus, timing and amplitude are coordinated by distinct interneurons distinguished not by their occupation of hierarchically-arranged anatomical layers, but rather by differences in the reliability and probability of higher-order and last-order connections that ultimately form a single anatomical layer. These findings contribute to our understanding of the origins of timing and amplitude control in the spinal cord.
Highlights
In all vertebrates, excitatory spinal interneurons execute dynamic adjustments in the timing and amplitude of locomotor movements
Source data are provided in a Source Data file. h, As in panel g, but for a type II V2a neuron allow for rapid adjustments in posture (Fig. 1b)[22]
We find that distinct types of V2a neurons are not organized as anatomically distinct higher-order and last-order multi-layer arrays that stratify timing and amplitude control
Summary
Excitatory spinal interneurons execute dynamic adjustments in the timing and amplitude of locomotor movements. Timing and amplitude are coordinated by distinct interneurons distinguished not by their occupation of hierarchically-arranged anatomical layers, but rather by differences in the reliability and probability of higher-order and last-order connections that form a single anatomical layer These findings contribute to our understanding of the origins of timing and amplitude control in the spinal cord. Since navigation during swimming requires differential control of the timing and amplitude of rhythmic activity in axial motor pools[15,16,17,18], a means to stratify locomotor control using a single layer architecture presumably exists, but has yet to be revealed If so, this would provide better insight into the evolutionary origins of timing and amplitude control in the spinal cord. This drive could originate from two sources, V2a neurons with descending projections within spinal cord and V2a neurons with descending and supraspinal projections, both of which appear to make perisomatic last-order connections[23]
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