Effects of whole-body rotation on masseteric motoneuron excitability

Vestibular stimulation is a popular clinical treatment for enhancing the excitability of spinal motoneurons innervating trunk and limb muscles, but whether vestibular stimulation can also influence trigeminal motoneurons is not known. We determined whether or not vestibular stimulation evoked by rotation of a seated subject would modify the excitability of masseteric motoneurons. The amplitude and frequency of occurrence of masseteric compound action potentials evoked by standard chin taps provided measures for assessing masseteric motoneuron excitability. Eleven healthy adults with no orofacial or otologic disorders served as subjects. Each sat in a motorized dental chair with his head stabilized by a halo head-piece so that chair rotation caused labyrinthine excitation. The frequency (3/s) of chin taps and their impact force were maintained constant by microcomputer control. After each tap, a 16-ms sample of EMG recorded from surface electrodes over the right masseter was digitized and stored for subsequent visual inspection. Only compound action potentials meeting rigorous criteria in terms of latency, amplitude, duration, and waveform were accepted as responses. The mean frequencies of occurrence and the mean amplitudes of the responses showed wide variability. Histogram displays of every response for each subject, however, revealed enhanced output from the masseteric motoneuron pool during the decleration and postrotation phases. In subjects not immediately retested this enhancement was persistent but decayed during the next 5 min. In five subjects the experiment was repeated after 1 min. The changes in response variables during phases 3 and 4 were significantly less than on the first trial, suggesting habituation. These results provide quantitative evidence that the dynamic input from vestibular ampullary receptors in response to rotation enhances masseteric motoneuron output.