Continuous shifts in the active set of spinal interneurons during changes in locomotor speed

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Abstract
The authors here show that two completely different classes of spinal premotor interneurons drive motoneurons during slow and fast swimming of zebrafish larvae. As the fish accelerate, the 'slow' interneurons are progressively silenced, while the 'fast' interneurons take over, and vice versa. The classic 'size principle' of motor control describes how increasingly forceful movements arise by the recruitment of motoneurons of progressively larger size and force output into the active pool. We explored the activity of pools of spinal interneurons in larval zebrafish and found that increases in swimming speed were not associated with the simple addition of cells to the active pool. Instead, the recruitment of interneurons at faster speeds was accompanied by the silencing of those driving movements at slower speeds. This silencing occurred both between and within classes of rhythmically active premotor excitatory interneurons. Thus, unlike motoneurons, there is a continuous shift in the set of cells driving the behavior, even though changes in the speed of the movements and the frequency of the motor pattern appear to be smoothly graded. We conclude that fundamentally different principles may underlie the recruitment of motoneuron and interneuron pools.