Receptive fields (RFs) of neurons in the primary somatosensory (SI) cortex were defined before, during, and after electrical stimulation of myelinated fibers in the dorsal cutaneous branch of the ulnar nerve in adult pentobarbital sodium-anesthetized cats. 2. This stimulation resulted in an approximately threefold increase of SI multiunit RF sizes. Substantial changes were first recorded within 1-2 h of stimulation. RFs typically enlarged continuously over a several-hour stimulation period, then stabilized. 3. RF-area increases were observed within both the forepaw and hindpaw representational zones in the SI cortex contralateral to the stimulated forepaw nerve. RF sizes did not increase in the ipsilateral SI body surface representation or in sham-stimulation control animals. 4. Preliminary studies indicate that stimulation-induced changes can be halted and often reversed by the intravenous administration of the opiate antagonist naloxone. 5. These observations suggest a global naloxone-sensitive modulatory system that operates on large-diameter afferent inputs in the cat somatosensory system. The increases in RF size occur under nerve-stimulation conditions similar to those that result in the generation of widespread analgesia (Chung et al. 1984a,b; Gamble and Milne 1986; Toda and Ichioka 1978).
