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1 Center for Neurobiology and Behavior, Columbia University, New York, New York, United States
2 Department of Physiologie, Universite de Montreal, United States
* To whom correspondence should be addressed. E-mail: jm17{at}columbia.edu.
While we understand that activity- and use-dependent processes are important in determining corticospinal axon terminal development in the spinal cord, little is known about the role of these processes in development of skilled control of limb movements. In the present study we determined the effects of unilateral motor cortex activity blockade produced by muscimol infusion during the corticospinal axon terminal refinement period, between postnatal weeks 5-7, on visually guided locomotion. We examined stepping and forepaw placement on the rungs of a horizontal ladder and gait modifications as animals stepped over obstacles during treadmill walking. When cats traversed the horizontal ladder, the contralateral limb was placed significantly farther forward on the rungs than the ipsilateral limb, indicating defective endpoint control. Similarly, when animals stepped over obstacles on a treadmill, the limb contralateral to the inactivation was placed farther in front of the obstacle, but only when it was the first (i.e., leading) limb to step over the obstacle, not when it was the second (i.e., trailing) limb. This is also indicative of an endpoint control deficit. In contrast, neither during the ladder walking, nor when stepping over obstacles on the treadmill, was there any consistent evidence for any major impairment in limb trajectory. These results point to distinct, and possibility independent, corticospinal mechanisms for movement trajectory control and endpoint control. While corticospinal activity during early postnatal development is needed to refine circuits for accurate endpoint control, this activity-dependent refinement is not needed for movement trajectory control.
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