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1 Neurobiology and Behavior, Cornell University, Ithaca, New York, USA
* To whom correspondence should be addressed. E-mail: rmh4{at}cornell.edu.
The interneuron populations that constitute the central pattern generator (CPG) for locomotion in the mammalian spinal cord are not well understood. We have studied the properties of a set of commissural interneurons whose axons cross and ascend in the contralateral cord (aCINs) in the neonatal mouse. During NMDA and 5-HT-induced fictive locomotion, a majority of lumbar (L2) aCINs we examined were rhythmically active; most of these fired in phase with the ipsilateral motoneuron pool, but some fired with contralateral motoneurons. Serotonin (5-HT) plays a critical role in enabling the locomotor CPG to function. We found that 5-HT increased the excitability of aCINs by depolarizing the membrane potential, reducing the post-spike afterhyperpolarization (AHP) amplitude, broadening the action potential, and decreasing the action potential threshold. Serotonin had no significant effect on the input resistance and sag amplitude of aCINs. These results support the hypothesis that aCINs play important roles in coordinating left-right movements during fictive locomotion, and thus may be component neurons in the locomotor CPG in neonatal mice.
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