Relationship between cortical motor functions and orofacial disease: the mirror neuron system and temporomandibular disorders

A group of neurons in the monkey premotor cortex discharges both when the monkey is performing a given action as well as when it is observing the experimenter performing a similar action (mirror neuron system; action execution/observation matching system). Several brain imaging studies indicate that this system also exists in humans. The present study aimed to compare activation patterns of the cortical areas activated during observation of jaw movements between healthy normal subjects and patients with temporomandibular disorders (TMD). Whole-scalp neuromagnetic responses were recorded from five healthy volunteers and six patients with TMD. The subjects were instructed to carefully observe a pre-recorded video of bilaterally symmetrical jaw-opening movements performed by another individual. During the observation task in healthy subjects, we found magnetic signals generated by the region of the occipitotemporal region near the inferior temporal sulcus and maxillofacial area of the primary motor cortex. In patients, however, we found the attenuation of magnetic response from primary motor cortex. These findings suggest that the maxillofacial area of the primary motor cortex is included in the jaw movement-related action execution/observation matching system in humans, and provide new neuropathological evidence that TMD patients exhibit dysfunction of the jaw movement-related action execution/observation matching system.