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J Neurophysiol (December 17, 2008). doi:10.1152/jn.91026.2008
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Submitted on September 13, 2008
Revised on December 5, 2008
Accepted on December 15, 2008

Activity-dependent plasticity improves M1 motor representation and corticospinal tract connectivity

Samit Chakrabarty1, Kathleen Margaret Friel1, and John H Martin1*

1 Columbia University

* To whom correspondence should be addressed. E-mail: jm17{at}columbia.edu.

Motor cortex (M1) activity between postnatal weeks 5-7 is essential for normal development of the corticospinal tract (CST) and visually-guided movements. Unilateral reversible inactivation of M1, by intracortical muscimol infusion, during this period permanently impairs development of the normal dorsoventral distribution of CST terminations and visually-guided motor skills. These impairments are abrogated if this M1 inactivation is followed by inactivation of the contralateral, initially active M1, from weeks 7-11 (termed alternate inactivation). This later period is when the M1 motor representation normally develops. The purpose of this study was to determine the effects of alternate inactivation on the M1 motor representation. We used intracortical microstimulation to map the left M1 one - two months after the end of left M1 muscimol infusion. We compared representations in the unilateral inactivation and alternate inactivation groups. Alternate inactivation converted the sparse proximal M1 motor representation produced by unilateral inactivation to a complete and high-resolution proximal-distal representation. The motor map was restored by week 11, the same age that our present and prior studies demonstrated that alternate inactivation restored CST spinal connectivity. Thus, M1 motor map developmental plasticity closely parallels plasticity of CST spinal terminations. After alternate inactivation reestablished CST connections and the motor map, an additional three weeks was required for motor skill recovery. Since motor map recovery preceded behavioral recovery, our findings suggest that the representation is necessary for recovering motor skills but additional time, or experience, is needed to learn to take advantage of the restored CST connections and motor map.




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