The physiological identification of pyramidal tract neurons within transplants in the rostral cortex taken from the occipital cortex during development

Axons from neurons in the occipital cortex transiently extend to the pyramidal tract (PT) during the early postnatal development of rats. Normally, these axons are eliminated by the end of the third postnatal week. However, if a portion of fetal occipital cortex is transplanted to the parietofrontal region in newborn hosts then some neurons in the transplant will extend pyramidal tract axons and maintain them. Intracortical microstimulation and electrophysiological recording techniques were used to identify the physiological characteristics of the transplanted pyramidal tract cells and to determine if motor effects could be elicited from the occipital transplant. Microstimulation of the transplant did not reliably evoke movement but the low density and disarray of PT cells within the transplant might account for this. Recording from within the transplant revealed that the overall cell activity was depressed. We were able to identify neurons within the transplant which responded antidromically to stimulation of the pyramidal tract, indicating that their axons have the capacity to conduct impulses and are therefore likely to have developed some viable connections. The functional significance of such projections remains uncertain.