LIMITS ON PLASTICITY IN SOMATOSENSORY CORTEX OF ADULT-RATS - HINDLIMBCORTEX IS NOT REACTIVATED AFTER DORSAL COLUMN SECTION

1. To better understand the limits and extents of plasticity in sensor y systems of adult mammals, we unilaterally sectioned the dorsal funic ulus at thoracic levels in nine adult rats to deactivate ascending aff erents from the hindpaw and lower body. After postsurgical recovery pe riods of 3 h to 3 mo, the region of primary somatosensory cortex (S1) representing the limbs and trunk was extensively mapped with microelec trodes. 2. Recording sites were later identified as being within the h indlimb representation and other parts of S1 by relating locations of microlesions to the cytochrome oxidase pattern in sections of cortex c ut tangential to the pial surface. The extent and effectiveness of spi nal, cord lesions were evaluated by injecting cholera toxin B subunit conjugated with horseradish peroxidase (B-HRP) at various sites in the deafferented hindpaw. 3. In five animals with complete section of the dorsal funiculus, we failed to detect any response to cutaneous stimu lation of any part of the body in the deafferented hindlimb cortex. In four other animals with incomplete lesions, neurons in some penetrati ons could be activated by hindlimb stimulation, but not by stimulating other body parts. In those cases without activation of hindlimb corte x, B-HRP was detected in the spinal cord only caudal to the lesion, an d it was not transported to the nucleus gracilis. Limited transport pa st the lesion to nucleus gracilis was detected in cases with incomplet e lesions. 4. The results indicate that forelimb inputs do not substit ute for missing hindlimb inputs in primary somatosensory cortex in rat s and that the potential for somatotopic reorganization is more limite d than previously thought.