Effects of lateral reticular nucleus stimulation on trigeminal sensory and motor neuron activity related to the jaw opening reflex.

The effects of lateral reticular nucleus (LRN) stimulation on the responses to tooth pulp (TP) stimulation of neurons located in the trigeminal (V) sensory (39 units) and motor (33 units) nuclei were assessed in anesthetized rats. Only neurons which responded to TP stimulation with bursts of spikes that were in a constant temporal relationship with the digastric EMG signal were studied. The LRN-stimulating electrodes were positioned at optimal sites to suppress the TP-evoked jaw-opening reflex (JOR) recorded simultaneously with the neuronal activity related to it. It was found that: (1) the neurons in the V nucleus oralis responded to TP stimulation with 3-8 msec latency excitatory responses that were suppressed during LRN conditioning stimulation with a time course comparable to that of the JOR suppression, and (2) the neurons in the V nucleus motor responded to TP stimulation with 5-15 msec latency excitatory responses. This activity was suppressed during LRN-conditioning stimulation with a time course that parallels the inhibition of the activity of nucleus oralis neurons and of the JOR. However, assuming that the excitatory interneurons for the V motoneurons are located in the nucleus oralis, the suppression of this input by LRN may account for the lack of response in V motor neurons. Thus, we suggest that LRN inhibits the TP-evoked JOR by acting on the sensitive afferent limb of the reflex.