Effects of clenching on regulatory function of bite force in man-role of peridontal mechanoreceptor

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It has previously been reported that bruxism is noted as one of the important causes of a temporomandibular dysfunction. Various discussions have been done about a relation between muscular activity and bruxism. A negative view is proposed from the result of the resting EMG activity about a causal relationship between the pain and the hyperactivity of the masticatory muscle in patients with tempolomandibular dysfunction. However, the fundamental problem is not clarified how any influence the sutained clenching exerts to the masticatory system. We have investigated whether the clenching that is one of the form of bruxism influence for short term to the stomatognathic system. The conditioning clenching induced a significant error in reproducing a leamed criterion bite force under conditions of no visual feed-back of force, like the experiments of Hutton et al. These findings indicate the possibility that the facilitation of motoneurones of closing muscles might occur after clenching. Furthermore, we have investigated whether the periodontal mechanoreceopors are responsible for the effect after clenching. Following results were obtained. 1. The upper oentral incisors were stimulated by the mechanical stimulator under 5% maximum voluntary teeth clenching. A short-latency inhibitory response was induced in masseter muscle, followed by an exciatory response to brisk taps on the teeth, and mainly excitatory response was induced to slow pushes. 2. These responses were almost abolished by local anesthesia around the teeth stimulated. 3. An excitability response to slow pushes increased with background discharge level. 4. A significant change was not observed for response to slow pushes before and after the clenching for 25% m.v.c.-60 seconds. Consequently, although a reflex arc from the periodontal mechanoreceptor to the masseter motoneurone has relatively little role to the postclenching effect, it is obvious to have a facilitatory effect to slow pushes.