Mechanomyographic activity in the human lateral pterygoid muscle during mandibular movement.

The activity of the lateral pterygoid muscle has been regarded to be related to the pathological condition of the temporomandibular joint (TMJ) in the craniomandibular disorders. Because the lateral pterygoid muscle is a deep muscle, a needle electrode is necessary for EMG recordings. The purpose of this study was to establish a non-invasive method for the evaluation of muscle activity of the lateral pterygoid muscle using mechanomyogram (MMG). In three male subjects, surface electromyogram (EMG) in the left masseter muscle, left anterior and posterior belly of the temporal muscle, left anterior belly of the digastric muscle and needle EMG of the inferior head of the lateral pterygoid were recorded during mandibular movement tasks simultaneously with the MMG derived from a condenser microphone in the external ear canal. There were significant positive correlations between the needle EMG signal of the lateral pterygoid muscle and the MMG signal for the tasks of static jaw opened position of 30 mm of interincisal distance (p=0.000, R(2)=0.725), static jaw opened position of 40 mm of interincisal distance (p=0.000, R(2)=0.753), 5mm protruded mandibular position (p=0.000, R(2)=0.653), the most protruded mandibular position (p=0.000, R(2)=0803). On the contrary, for the task of maximal clenching, there was no significant correlation between the EMG signal of the lateral pterygoid muscle and the MMG signal. These results suggest that the activity of the lateral pterygoid muscle could be evaluated by the MMG signals recorded in the external ear canal, unless jaw closing major muscles show active contraction.

Influences of molar loss of rat on learning and memory.

PURPOSE: The maxillary unilateral or bilateral molars of rats were extracted, and the influences of the partial loss of occlusal support were evaluated using an 8-arm radial maze and a step-through type passive-avoidance apparatus. METHODS: Rats were randomly allocated to three groups not undergoing molar extraction or undergoing extraction of the maxillary unilateral or bilateral molars. Each group was further divided into two groups for maze or passive-avoidance experiments. Thus, a total of six groups were established. The maze experiment was conducted once daily for 10 days. The number of correct choices, number of errors, and the trial time were recorded. The passive-avoidance experiment consisted of an acquisition trial and retention trial. In the acquisition trial, rats were placed in a light room, and the response latency until their entry into a dark room was measured. After 24h, a similar procedure was performed as a retention trial. RESULTS: In the maze experiment, there was no significant difference by all the groups except on the day 1 in the number of correct choices. But bilateral molar loss group, the number of errors were significantly lower than no extraction group on days 1, 2, 3, 4, and 7. In the passive-avoidance experiment, though the response latency in the retention trial was longer than that in the acquisition trial in all three groups, according to the increase in the number of tooth extraction, it became significantly shorter between P1 and P3. CONCLUSION: These results suggested that molar loss may be a cause of learning/memory impairment.