Inputs to trigeminal brain stem neurones from facial, oral, tooth pulp and pharyngolaryngeal tissues: I. Responses to innocuous and noxious stimuli.

Responses evoked in anaesthetized or decerebrate cats by stimulation of afferents supplying the face, mouth, pharynx, larynx, tooth pulp and jaw muscles were recorded from single neurones located in the trigeminal (V) main sensory nucleus, V nucleus oralis, and adjacent regions. Many cells (both V-thalamic relay and non-relay with localized V mechanoreceptive cutaneous fields could be activated by stimulation of a number of these afferents. A particularly prominent short-latency (often monosynaptic) input was noted from the canine tooth pulp, stimulation of which is generally considered to elicit only responses of pain in man. Control experiments showed that pulp-evoked responses were not the result of stimulus spread to tissues outside the pulp. The interaction of these various inputs to neurones at this level of the V brain stem complex typically resulted in a prolonged period of inhibition that was sometimes preceded by a short-lasting facilitatory phase. This inhibitory effect was also apparent in neurones located outside the complex, although a late facilitatory phase was frequently also noted. Our findings indicate a significant nociceptive input to V main sensory-oralis neurones, a proportion of which relay directly to the ventrobasal thalamus. The interactions described may be involved in perceptual and reflex aspects of responses to noxious and innocuous V stimuli.