Exteroceptive reflexes in jaw-closing muscle EMG during rhythmic jaw closing and clenching in man

Exteroceptive jaw reflexes might play a role in normal functions of the mouth such as mastication. Until now these reflexes have only been studied under isometric conditions. The aim of this study was to compare exteroceptive reflexes in jaw muscle EMG during the closing phase of rhythmic open-close movements and clenching, at the same jaw gape and with similar muscle EMG. Reflexes consisting of successive waves of decreased and increased muscle activity (the Q, R, S and T waves of the post-stimulus electromyographic complex (PSEC)), evoked by light noxious electrical stimulation of the vermillion border of the lower lip, were recorded from the jaw closing muscles of 17 subjects. Differences between the two tasks occurred in two phases of the PSEC: (1) in an early phase, around the R wave, there was significantly less EMG during jaw closing (mean EMG ratio between jaw-closing and clenching 0.71), and (2) in a late phase, around the transition between the S to the T wave, there was significantly more EMG during jaw closing (mean EMG ratio: 1.40). The decrease in EMG activity around the R wave during jaw closing may be due to a change in reflex sensitivity at an interneuron level. The increase in EMG activity around the transition between the S and T waves during jaw closing might, at least in part, be due to a proprioceptive stretch reflex. This reflex is mediated by muscles spindles that are activated by the deceleration of the jaw evoked by the lip stimulus. The finding of inhibitory reflex mechanisms that predominate more during rhythmic jaw movements than during clenching in an early phase of the PSEC might be related to protecting oral tissues from trauma when the jaw is closing with potentially a large muscle force. In contrast, when food is held between the teeth, a possible inhibitory influence of light noxious stimuli is diminished.