Elsevier

Clinical Neurophysiology

Volume 121, Issue 10, October 2010, Pages 1690-1699
Clinical Neurophysiology

Invited review
Reflex responses of masseter muscles to sound

https://doi.org/10.1016/j.clinph.2009.11.093Get rights and content

Abstract

Acoustic stimuli can evoke reflex EMG responses (acoustic jaw reflex) in the masseter muscle. Although these were previously ascribed to activation of cochlear receptors, high intensity sound can also activate vestibular receptors. Since anatomical and physiological studies, both in animals and humans, have shown that masseter muscles are a target for vestibular inputs we have recently reassessed the vestibular contribution to masseter reflexes. We found that high intensity sound evokes two bilateral and symmetrical short-latency responses in active unrectified masseter EMG of healthy subjects: a high threshold, early p11/n15 wave and a lower threshold, later p16/n21 wave. Both of these reflexes are inhibitory but differ in their threshold, latency and appearance in the rectified EMG average. Experiments in healthy subjects and in patients with selective lesions showed that vestibular receptors were responsible for the p11/n15 wave (vestibulo-masseteric reflex) whereas cochlear receptors were responsible for the p16/n21 wave (acoustic masseteric reflex). The possible functional significance of the double vestibular control over masseter muscles is discussed.

Section snippets

Sound as a stimulus activating cochlear as well as vestibular receptors

Although the primary receptor specialised for the detection of sound is the cochlea, it has been known for many years that sound can also affect the vestibular system. For instance, in humans intense sound is well known to produce vestibular symptoms and illusions of movement (Parker et al., 1975). These probably arise from stimulation of otolith organs in the saccule since this is reported to be the most sensitive part of the vestibular system to sound (Young et al., 1977, Cazals et al., 1983,

Masseter responses to vestibular stimulation

There are many investigations into the vestibular interactions with muscles controlling the neck and eye. However, it seems possible that motoneurones innervating the masseter muscles might also be a possible target of vestibular inputs since these muscles, in addition to their role in mastication, phonation, respiration and swallowing, are also involved in maintaining the posture of the jaw against gravity (Lund and Olsson, 1983, Miralles et al., 1987). To date this question has only been

Masseter responses to acoustic stimulation

Responses to loud sounds were first recorded in the average surface EMG of voluntarily contracted masseter muscles using acoustic click stimuli (Deriu et al., 2005). Unilateral stimulation evokes a bilateral and symmetrical short-latency response that consists of two overlapping components distinguished by their threshold, latency and by their appearance in the rectified EMG, as summarized in Table 1. Responses to bilateral stimulation are similar, but larger.

The lower threshold response is a

Possible role for the vestibulo-masseteric reflex pathways in trigeminal motor control

In addition to mastication, the masseter muscle is involved in speech, swallowing, respiration and maintenance of the position of the mandible, by bringing it back to its physiological axis. Each of these functions require jaw muscles to perform motor tasks that differ in force produced, as well as rapidity, shape and precision of mouth movements. Furthermore, they are often required to perform these multiple and very different actions simultaneously, so that we are able, for instance to speak

Clinical prospects

The study of the vestibulo-masseteric reflex and of the acoustic masseteric reflex in people with selective lesion of vestibular or cochlear receptors definitely demonstrated that these end organs originate the p11/n15 and the p16/n21 waves, respectively (Deriu et al., 2007). This study also shows that masseter responses to sound can be used as additional tools to test saccular and cochlear function in human otological investigations. However, it seems unlikely that these responses will replace

Acknowledgements

This work was supported by grants from the Ministero dell’Istruzione, dell’Università e della Ricerca (MIUR) and from Fondazione Italiana Sclerosi Multipla (FISM 2008/R/9). Dr. Elena Giaconi was supported by a grant from MIUR (PRIN).

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