Developing role of sensorimotor cortex and pyramidal tract neurons in contact placing in kittens.

Unilateral or bilateral removal of kitten sensorimotor (SM) cortex prior to the fourth postnatal week was followed by recovery of contact placing (CP) to stimulation of any of the four forepaw cutaneous fields. However, immediately after a removal under ethyl chloride, CP was depressed contralaterally, especially to lateral stimulation (lateral CP). When the operated kitten aged to 5--9 weeks, the likelihood of CP secondarily declined contralateral to the SM cortical extirpation, but was restored by administration of d-amphetamine. An initial extirpation after the fifth postnatal week was followed by a severe deficit in CP. 2. Cooling the SM cortex, internal capsule or bulbar pyramid during the 2nd and 3rd week was followed by reversible, contralateral loss of lateral CP, extension of the forelimb and, later, by reduced movements. Lateral CPs were often hypermetric prior to loss. By contrast, cooling SM cortex in the first week was usually ineffective. Thus, during the 2nd week, lateral CP developed a dependence on SM cortex, and in particular on PT neurons. 3. After a delay usually of 1 or more min, cooling the SM cortex reduced the resting discharge of individual neurons in buried motor cortex; the antidromic conduction time in PT neurons was significantly increased and the spike height was reduced.