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.
Bibliographical noteFunding Information:
We are extremely grateful to S. Higashijima for providing us with the alx-gfp transgenic fish. We also thank L. Heller for fish care and M. Koyama for comments on the manuscript. This work was supported by fellowships from the US National Institutes of Health (NS44728 to D.L.M. and NS44758 to M.A.M.) and grants from the National Science Foundation (DMS-0107893 to W.B.L.) and US National Institutes of Health (NS26539 to J.R.F.).