Somatotopic distribution of trigeminal nociceptive specific neurons within the caudal somatosensory thalamus of cat.

Trigeminal nociceptive specific (NS) neurons were found within the shell region of the caudal part of nucleus ventralis posterior medialis (VPM) of cat. They showed a somatotopic organization. Namely, the mandibular division was represented along the border between the VPM proper and nucleus ventralis posterior medialis parvocellularis (VPMpc). The ophthalmic division was represented dorsolaterally. The maxillary division fell in between. No other types of trigeminal nociceptive neurons were found in the caudal VPM where trigeminal NS neurons were encountered. After cooling the dorsolateral surface of the medulla oblongata caudal to the obex, responses of NS neurons within the shell region of the caudal VPM to noxious stimulation of the peripheral receptive field reversibly disappeared. The responses were also eliminated by the contralateral trigeminal tractotomy at the level of the obex. It was inferred that trigeminal NS neurons within the shell region of the caudal VPM receive nociceptive input relayed via NS neurons located within the marginal layer of the trigeminal subnucleus caudalis.