BIPHASIC REORGANIZATION OF SOMATOTOPY IN THE PRIMARY MOTOR CORTEX FOLLOWS FACIAL-NERVE LESIONS IN ADULT-RATS

Effects of facial nerve transection were studied on muscle responses e voked by electrical stimulation in the primary motor cortex (MI) of ad ult rats. In intact animals, activated muscles varied according to the somatotopic representation map, and responses were restricted to the contralateral side. Unilateral transection of the facial nerve extingu ished contralateral vibrissal responses, while ipsilateral vibrissae b egan to respond within 4 min. This abnormal response (primary change) was transient and gradually disappeared within hours to days, Instead, contralateral movements of forepaw and eye/eyelid muscles could be ev oked from increasing portions of the former vibrissal field (secondary change), in which many points became unresponsive. After 4 days, the former vibrissal field had shrunk to a small central part, where ipsil ateral vibrissae responsiveness remained. The secondary modification w as stable for at least 2 weeks. Since the primary change is rapid, tra nsient and may be mimicked by picrotoxin, it may be based on disinhibi tion of commissural connections, while the secondary change is longlas ting and therefore may include some form of reorganization of associat ional synapses.