Activity-dependent competition between developing corticospinal terminations.

In cats, each sensorimotor cortex has bilateral corticospinal (CS) terminations during early postnatal life, with contralateral terminations denser than ipsilateral ones. Subsequent development depends on neural activity: silencing CS neurons on one side results in development of sparse terminations from that side and dense bilateral terminations from the active side. In this study we determined whether activity-dependent competition between CS terminations is important in shaping their development. We compared the effect of bilateral sensorimotor cortex activity blockade (activity deprivation alone, without competition) on CS development, with that produced by unilateral blockade (activity deprivation with competition). We found that the extent of the spinal gray matter labeled is substantially greater when the motor cortex on each side is silenced rather than when only one side is silenced. These findings support the hypothesis that competition between developing CS terminations is important in shaping the topography of CS connections.