Visceral region in the rat primary somatosensory cortex identified by vagal evoked potential

Recent noninvasive human studies have reconfirmed the presence of a viscerally responsive region in the most lateral part of the primary somatosensory cortex (S1). The present electrophysiological study identified the corresponding area in rats as a vagal afferent projection region and examined the cytoarchitecture. Electrical stimulation of the cervical vagus nerve elicited a field potential comparable in waveform, latency, and amplitude to the simultaneously evoked potential in the insular visceral sensory cortex. The potential field adjoined the S1 trigeminal region without overlap, and was rostroventral to the lip representation barrel field, which was identified histochemically, and rostrodorsal to the tongue representation region, which was identified electrophysiologically. The vagal potential underwent a phase reversal in the middle layers; thus, the current sink site was cytoarchitectonically identified as the most rostral part of the parietal granular cortex or the S1, where no somatosensory input has previously been demonstrated. The rat S1 contains a region representing general visceral information, topographically located as if the visceral organs protruded from the mouth.