Upper-limb facilitation by teeth clenching and its brain activation pattern

Objective: In the field of dentistry and sports physiology, there are many reports that voluntary teeth clenching facilitates human motor function. The purpose of this study was to investigate the facilitatory effect of teeth clenching on upper-limb and its brain activation pattern by using functional magnetic resonance imaging (fMRI) technique. Methods: Nineteen healthy right-handed subjects with normal occlusion participated in this study. Experiment 1 (behavioral study): The subjects were asked to exert maximal grip force with their right hands under the two conditions outside the MR room; in the mandibular rest position and during voluntary teeth clenching. Experiment 2 (fMRI study): The data were acquired using a T2*-weighted gradient echo-planar imaging sequence with a 1.5T scanner. During fMRI scanning, subjects performed the following three tasks; i) clenching during gripping (CG), ii) non-clenching during gripping (NcG), iii) only clenching (C). The regions of interests were defined in the activated areas during only gripping and measured % signal change of these in each condition by using statistical parametric mapping (SPM) 5. Results: Experiment 1: The maximal grip force during voluntary teeth clenching was significantly larger than that in the mandibular rest position. Experiment 2: In the primary sensorimotor cortex (SMC), the supplementary motor area (SMA), and the cerebellum that related to the grip movement, % signal change in task-CG was significantly stronger than that in task-NcG. In the cerebellum, especially, a significant positive correlation was found between the increasing ratio of the maximal grip force and that of % signal change. Conclusions: The findings suggest that the SMC, SMA and the cerebellum are involved in the upper-limb facilitation by voluntary teeth clenching. Moreover, it is indicated that the cerebellum might regulate its facilitation.