Properties of rhythmically active reticular neurons around the trigeminal motor nucleus during fictive mastication in the rat.

Response properties of the neurons in the reticular formation around the trigeminal motor nucleus (MoV) were examined during cortically-induced fictive mastication (CIFM) in anesthetized and immobilized rats. Forty-three neurons were rhythmically active (RA neurons) during CIFM, most of which were located in the supratrigeminal nucleus and the reticular formation medial to the oral spinal trigeminal nucleus. The firing frequency of 36 of the RA neurons was modulated in the same rhythm as that of masseteric or digastric nerve activities during CIFM. We divided these neurons into four groups according to the phase of activation: sixteen neurons fired mainly in the phase of masseteric activity (type 1), 11 fired in the transition phase from masseteric activity to digastric activity (type 2), 5 fired in the phase of digastric activity (type 3) and 4 fired in the transition phase from digastric activity to masseteric activity (type 4). Thirty-nine (91%) of the 43 RA neurons responded to at least one of the tested peripheral stimuli. The responses were mostly excitatory but inhibitory responses were sometimes obtained, especially for types-1 and 2 neurons. RA neurons in the reticular formation medial to the oral spinal trigeminal nucleus responded to stimulation of inferior alveolar nerve at a shorter latency than RA neurons in the supratrigeminal nucleus. Fifteen (48%) of 31 RA neurons responded to triple-pulse stimulation of the contralateral cortex. In contrast, only 5(26%) of the 19 RA neurons responded to the ipsilateral cortical stimulation. Stimulation of the ipsilateral MoV was performed on 24 RA neurons, of which 9 responded antidromically (A-RA neurons) at latencies of 0.4-1.4 ms. Eight (89%) of the 9 A-RA neurons received peripheral inputs. The spike triggered averaging method was applied to 4 of the 9 A-RA neurons, ad in all cases short latency field potentials were recorded in the MoV. We conclude that trigeminal premotor neurons receive convergence from central and peripheral inputs. This integration can adjust the appropriate level of motoneuronal excitability during mastication.