Single units responsive to noxious mechanical stimulation of orofacial receptive fields were recorded within the ventrobasal complex of the rat thalamus. The induced activities were compared before and after deafferentation of the subnucleus caudalis by a trigeminal tractotomy performed at the obex level. The receptive fields activated by noxious stimulation were classified as 'oral' when included in the oral, perioral or paranasal areas, and as 'facial' when included in facial regions distant from the oral cavity. After tractotomy, the unit responses to noxious stimulation of an oral field remained unchanged in 8 cases, decreased in 3 cases, and were suppressed in 4 cases. For units responding to noxious stimulation of a facial field, the responses were suppressed in 8 cases, decreased in two cases and remained unchanged in two other cases. So it appears that the rostral part of the trigeminal sensory complex (1) receives nociceptive afferents mainly from the oral and perioral areas and (2) is a relay in ascending pathways which convey painful sensations. © 1988.
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