A Sound-Evoked Vestibulomasseteric Reflex in Healthy Humans

Abstract

Averaged responses to loud clicks were recorded in the unrectified and rectified masseter electromyogram (EMG) of 18 healthy subjects. Unilateral clicks (0.1 ms, 3 Hz, 70-100 dB NHL), delivered during a steady masseter contraction, evoked bilateral responses that appeared to consist of 2 components on the basis of threshold, latency, and their appearance in rectified EMG. The lowest threshold response appeared as a p16 wave (onset 11-13 ms) in the unrectified EMG and corresponded with a 10- to 12-ms period of inhibition in the rectified EMG. Higher-intensity clicks recruited an earlier p11 response in the unrectified EMG (onset 7.0-9.2 ms) that sometimes appeared as an initial increase in the rectified EMG before suppression. The amplitude of the p11 wave scaled with background EMG level and was asymmetrically modulated by 30 degrees tilt of the whole body. The threshold of the early p11/n15 wave in masseter was the same as the threshold for click-induced vestibulocollic reflexes. Single motor unit recordings demonstrated that responses in masseters corresponded to a silent period in unit firing that began earlier and lasted longer at 100 dB than at 80 dB. We propose that loud clicks induce 2 partially overlapping short-latency reflexes in masseter muscle EMG: a p11/n15 response, which we suggest is of vestibular origin, and a p16/n21 response, which we suggest is equivalent to the previously described jaw-acoustic reflex.

Transneuronal tracing of vestibulo-trigeminal pathways innervating the masseter muscle in the rat

“…This observation provides an anatomical substrate to account for the finding that vestibular stimulation alters masseter muscle activity in animals (Tolu and Pugliatti 1993;Tolu et al 1994Tolu et al , 1996Deriu et al 1999) and humans (Hickenbottom et al 1985;Deriu et al 2000Deriu et al , 2003Deriu et al , 2005. All parts of the VN contained infected neurons following injections of PRV-BA into the masseter muscle, especially the caudal portion of the MVe and the SpVe; in addition, labeling was concentrated in the caudal PH.…”

“…Moreover, the muscle fibers associated with each masseter motor unit are limited to a relatively small region of the muscle, such that each masseter motoneuron provides innervation to a restricted territory (Weijs et al 1993). Our PRV-BA injections were made into the lower third of the superficial layer of the muscle, because this compartment receives the densest motor innervation (Godeaux and Desmedt 1975;Widmer and Lund 1989;Widmer et al 1997); in humans, the largest vestibular-induced EMG responses were detected when recording electrodes were positioned on this part of the muscle (Deriu et al 2003(Deriu et al , 2005. The compartments of the masseter muscle injected with PRV-BA in Fay and Norgren's study (Fay and Norgren 1997) were not discussed, such that the locations of their injections within the muscle could have differed from ours.…”

“…The presence of vestibular inputs to trigeminal motoneurons discovered in the animal model has been also demonstrated in humans (Hickenbottom et al 1985;Deriu et al 2000). Studies performed on healthy humans described short-duration inhibitory masseter muscle responses elicited bilaterally and symmetrically by vestibular stimulation at latencies consistent with no more than 2-3 synaptic relays (Deriu et al 2003(Deriu et al , 2005.…”

“…For example, the masseter muscle is multipennate (Eriksson 1982;Eriksson and Thornell 1983;Stålberg et al 1986), and is comprised of several functionally-distinct compartments that differ with respect to morphological and biochemical properties (Herring et al 1979;Widmer et al 2003). Vestibular stimulation differentially affected the firing of masseter muscle compartments (Deriu et al 2003(Deriu et al , 2005; vestibular-elicited responses were largest in magnitude in the compartment receiving the heaviest innervation by motoneurons, which is localized in the lower third of the superficial layer of the muscle (Godeaux and Desmedt 1975;Widmer and Lund 1989;Widmer et al 1997). Given this, it is possible that the lack of VN or PH labeling in Fay and Norgren's (1997) study was due to the fact that their pseudorabies virus injections did not include this region.…”

A monosynaptic pathway links the vestibular nuclei and masseter muscle motoneurons in rats

“…A variety of studies have shown that the activity of mandibular muscles is modulated by the vestibular system (Hickenbottom et al 1985;Tolu and Pugliatti 1993;Tolu et al 1994;Tolu et al 1996;Deriu et al 1999;Deriu et al 2003;Deriu et al 2005). In particular, experiments performed on guinea pigs demonstrated that stimulation of receptors in the semicircular canal ampullae elicits bilateral excitatory responses of masseter and digastric motoneurons and that macular inputs exert bilateral asymmetrical influences on jaw muscle activity.…”