Somato-Dendritic Morphology Predicts Physiology for Neurons That Contribute to Several Kinds of Limb Movements

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Abstract
It has been difficult to predict the behavioral roles of vertebrate CNS neurons based solely on their morphologies, especially for the neurons that control limb movements in adults. We examined the morphologies of spinal interneurons involved in limb movement control, using intracellular recording followed by Neurobiotin injection in the in vivo adult turtle spinal cord preparation. We report here the first description of a class of spinal interneurons whose somato-dendritic morphologies predict their robust activity during multiple forms of ipsilateral and contralateral fictive hindlimb scratching and fictive hindlimb withdrawal. These “transverse interneurons” or T cells have a mediolaterally elongated soma and a simple dendritic tree that is extensive in the transverse plane but restricted rostrocaudally. During fictive scratching, these cells display strong rhythmic modulation with higher peak firing rates than other scratch-activated interneurons. These higher peak firing rates are at least partly caused by T cells having larger phase-locked membrane potential oscillations and narrower action potentials with briefer afterhyperpolarizations than other scratch-activated interneurons. Many T cells have axon terminal arborizations in the ventral horn of the spinal cord hindlimb enlargement. Identification of this morphological and physiological class of spinal interneurons should facilitate further exploration of the mechanisms of hindlimb motor pattern selection and generation.