PHYSIOLOGICAL IDENTIFICATION OF JAW-MOVEMENT-RELATED NEURONS IN THE TRIGEMINAL NUCLEUS OF CATS

Although neurons responsive to jaw movements have been identified in most parts of the trigeminal brainstem nuclei, little is known about how this information is relayed to the thalamus and ultimately to the cortex for kinesthetic functions and sensorimotor integration. The present extracellular recording experiments showed that a substantial amount of movement-related information is relayed to the thalamus through the caudal part of subnucleus interpolaris (V(i)) in adult cats. Vertical jaw displacements, natural mechanical stimuli, and electrical stimulation of the masseter nerve were used to determine the receptive fields and response properties of movement-related neurons. Movement-related responses were observed in 161 units. The receptive fields of these units were located in the masseter muscle, other deep structures, hairy skin, oral mucosa, or some combination of these structures (i.e., convergent). The latency of units responding to masseter nerve stimulation ranged from 1.0 msec to 20 msec, which suggested that some movement-related information was provided by smaller-diameter muscle afferents. Movement responses were either tonic or phasic. Tonic units showed continuous firing at some jaw position; some of these showed a ''dynamic'' response to jaw displacement. Phasic units were only active, or showed increased activity, when the jaw moved through a specific position. Seventy-one movement-related units were activated by stimulation from the contralateral ventroposteromedial nucleus (VPM) of the thalamus. Most of the brainstem recording sites were located in the dorsal part of V(i) between the caudal pole of the facial motor nucleus and the obex. Neurons in caudal V(i) may be important for facial kinesthesia. This supports current ideas that this composite sensory experience involves integration of information from many types of peripheral inputs.