An experimental study of the corticofugal system following cerebral lesions in the albino rats.

Following neonatal lesion in the sensorimotor and adjacent cortex (SMC) in the albino rats, a corresponding region in the contralateral intact cortex gave rise to a bilateral corticofugal projection to the pretectal area, superior colliculus, pons, gracile and cuneate nuclei, the nucleus of Bischoff, caudal medullary reticular formation and the spinal cord. The anomalous growth response was greatest when the initial lesion was made within the first three days after birth, but became much reduced as the initial lesion was made at a progressively older age. A significant amount of anomalous projection could, however, still be demonstrated in the pons even in animals first lesioned as late as 20 days postnatal. The sprouting phenomenon was precise and ordered and in the various regions studied, the pattern of anomalous projection mirrored that of the normal projection. The anomalous fibres reached their specific target cells either by deflecting ipsilaterally at the pyramidal decussation to reach the spinal cord and the caudal medulla or by running across the mid-line to the contralateral pons, superior colliculus, pretectal area and the thalamic ventrobasal complex. In contradistinction to the findings of Hicks and D'Amato (1975) no anomalous fibres could be traced to the trigeminal complex and the rostral reticular formation in the medulla. A mechanism of competitive occupation of vacant synaptic sites was proposed to explain the sprouting phenomenon.