PERIPHERAL INFLUENCES ON THE SIZE AND ORGANIZATION OF SOMATOTOPIC REPRESENTATIONS IN THE FETAL-RAT CORTEX

Nerve lesions at different fetal ages and on the day of birth were use d to determine the role of the periphery in establishing territories d evoted to representations of different portions of the body surface in rat somatosensory cortex. Transection of the infraorbital nerve (ION) , the trigeminal branch that supplies the whisker pad, resulted in a s ignificant reduction in the area within the primary somatosensory cort ex devoted to the representation of the mystacial vibrissae in fetal, but not newborn, rats. Such lesions in fetal, but not neonatal, rats a lso resulted in significant increases in the cortical area devoted to the representation of the lower lip and jaw. There was a significant p ositive correlation between the reduction in the vibrissae representat ion and the expansion of that of the lower lip and jaw. Damage to the ION in either neonatal or fetal rats failed to increase significantly the amount of cortex devoted to the representation of the forepaw. The se results indicate that the primary afferent innervation of the perip hery does influence the amounts of cortex devoted to representations o f different parts of the body surface and that the representation of o ne region can expand significantly when that of another body part is r educed.