Physiology and Morphology Indicate That Individual Spinal Interneurons Contribute to Diverse Limb Movements

Abstract

Overlapping neuronal networks have been shown to generate a variety of behaviors or motor patterns in invertebrates, but the evidence for this is more circumstantial in vertebrates. The turtle spinal cord can produce multiple forms of hindlimb scratching movements as well as hindlimb withdrawal, but it is still uncertain whether individual spinal cord interneurons contribute to the motor output for more than one type of limb motor pattern. In this study, individual spinal cord interneurons were recorded intracellularly in vivo in spinal immobilized turtles, and, after characterization, were filled with Neurobiotin. Interneurons that were rhythmically activated during multiple forms of ipsilateral fictive hindlimb scratching often had axon-terminal arborizations in the ventral horn of the spinal cord hindlimb enlargement. This provides some of the strongest evidence to date that interneurons involved in multiple forms of scratching contribute directly to hindlimb motor output. Moreover, most of these interneurons were also active during contralateral fictive scratching and during ipsilateral fictive hindlimb withdrawal, suggesting that they contribute to motor output for these additional behaviors as well. Such interneurons may provide the cellular basis for the contralateral contributions to ipsilateral scratching that have been demonstrated previously. Taken together, these findings suggest that diverse vertebrate limb movements are produced by spinal cord interneuronal networks that include some shared components.