Functional role of regrowing pyramidal tract fibers.

When pyramidal tract axons are severed in the infant hamster, the damaged fibers regrow via a new pathway to their normal terminal sites in the medulla and spinal cord and there form synaptic connections (Kalil and Reh, '79, '82). We studied the behavior of animals with infant and adult lesions of the medullary pyramid to determine the functional significance of the new pathway in maintaining normal motor behavior. Examination of behaviors normally mediated by the pyramidal tract, particularly the manipulation of sunflower seeds during feeding, revealed a correlation between the presence of the new tract and the preservation of function. Furthermore, in the adult animal with an infant lesion, the spared behaviors were lost when the new pathway was destroyed.