David L. McLean
Development and Plasticity of Motor Networks
Walking or running can at times seem automatic. This ability relies on networks of rhythmically active neurons in our spinal cord. We are interested in how rhythmic movements, like locomotion, develop and we study spinal networks in zebrafish to answer this question. In hatchling zebrafish, interneurons in the spinal cord responsible for activating motoneurons during swimming are topographically organized according to the speed/force of movement to which they contribute. Interneurons nearest the ventral edge of the spinal cord generate the slowest speeds of swimming, with progressively more dorsal interneurons activated as the animal speeds up. Critically, as dorsal neurons are engaged, ventral interneurons are inhibited. So in contrast to motoneurons where the active pool simply increases in size with increasing speeds, there are continuous shifts in the active pools of interneurons during smooth variations in movement intensity.
One important feature of this functional pattern is related to the birth order of neurons. Spinal neurons driving fast/coarse movements in larval fish emerge first during development, with cells driving progressively slower/refined movements emerging later. Because the position of the neurons in spinal cord reflects their time of differentiation, the result is an orderly map of movement speed that also represents the temporal emergence of behavior. This observation raises fundamental questions about the neural control of behavior. Why do cells emerge in this order? Is there something about the functional properties of cells and circuits that necessitates a wiring strategy from fast to slow or strong to weak? Do the same rules apply for excitatory and inhibitory neurons? How is this pattern achieved and what could it tell us about the general features of circuit organization and thus behavior? My lab is currently addressing these questions using the unique technical advantages of the zebrafish model system, namely the ability to label, monitor and perturb neural circuitry in the living animal as behaviors develop.