Maturation of spinal motor control through developmental changes in ion currents

The maturation of motor control by the nervous system arises from several developmental processes including neurogenesis, the establishment of synaptic connectivity, and the development of neuronal intrinsic properties. The latter, particularly via ion channels, determines the firing properties of neurons and synaptic transmission. The spinal cord plays a vital role in the control of movement. Its contributions to motor maturation have been well studied at the levels of spinal neuron development and circuit formation involving intraspinal, supraspinal and sensory inputs to form spinal circuits for movement. Less is known about how changes in ion currents can refine movements despite their importance in setting neural activity. Recent studies have advanced our understanding of which ion currents in spinal neurons undergo developmental changes. In this review, we provide an overview of the ion currents known to shape motor activity and describe how specific ion currents exhibit distinct developmental dynamics across many vertebrate species. We argue, based on recent findings, that some of these changes are important for the development of the motor repertoire by ensuring the appropriate recruitment of spinal circuits and the different muscle types they control. These findings emphasize the importance of developmental changes in intrinsic properties to the growth and refinement of the motor repertoire.