Assembling Neural Circuits for Generating Movement

Abstract

Locomotion is essential for survival and animals need to move
early in life to protect themselves from predators and to forage. Neural circuits controlling locomotion thus need to be functional even at stages when the nervous system is immature and developing. Studies from a wide variety of experimental model systems show that motor circuits are capable of generating functional output even though the mature adult circuit may not be present. The output from this immature circuit is however qualitatively different and in many ways, infl exible as compared to the adult. Also, network activity may be driven by different sets of mechanisms in the developing animal compared to the adult. For example, at early embryonic stages, network activity can be driven by electrical coupling between neurons, which gradually decreases as the animals mature, giving place to chemical neurotransmission. The electrical activity generated by early networks acts as a regulator that provides instructional cues for developmental events such as the correct outgrowth of axons to their targets and the selection of transmitter molecules used by a neuron. Early networks have homeostatic mechanisms in place to maintain their activity levels within an operating range. Neuromodulators, present in descending and sensory projections and as circulating hormones have profound effects on neural circuit development by affecting multiple developmental processes and by fusing and uncoupling networks to produce appropriate motor patterns.