Masseter Muscles In Man Research Articles

多極表面電極および多極針電極によるこう筋筋電位伝導速度の測定

Muscle fiber conduction velocity (MFCV) provides an index of muscular fatigue and muscular disease. The purpose of this study is to estimate MFCV of the masseter muscle in man. MFCV was measured by two different methods: surface multi-electrodes (Method1) and needle multi-electrodes (Method2).1. MFCV of the masseter muscle measured by Method1 (6.9±1.7m/s) was faster than that measured by Method2 (3.18±0.89m/s). The difference was statistically significant (p<0.01).2. MFCV of the biceps bracii muscle was estimated as a control. MFCV of the biceps bracii muscle measured by Method1 (3.6±0.8m/s) was in accordance that measured by Method2 (3.58±1.10m/s).3. In Method1, MFCV of the masseter muscle was faster than the biceps bracii muscle (p<0.01). To obtain a reliable estimate of MFCV with surface multi-electrodes, it is important to avoid the innervation zone in the measurement procedure. Innervation zones in the masseter muscle are not confined to a narrow band, but scattered over the entire muscle. Therefore, using the surface multi-electrodes, MFCV of the masseter muscle cannot be measured precisely.4. Using needle multi-electrodes, an accurate measurement of MFCV of the masseter muscle was obtained. The masseter MFCV was similar to those reported on limb muscles.

Functional property of the masseter muscle in man

The purpose of this paper is to investigate the functional property of the masseter muscle at the motor unit level in man. The responses of the surface EMG and the bite force were studied by averaging technique, with the following results.1. The higher threshold units with large twitch tensions were found in the superficial layer of the muscle, the lowers were throughout the muscle.2. Positive correlation was found between voluntary threshold force and peak twitch tension, linear correlation coefficient was 0.76.3. The motor units with a larger twitch tension tended to have shorter contraction time, but those of smaller had the wide range.4. There was a tendency for deeper-layer units to contribute less to the wave form of the surface EMG than did the superficial units.5. There was a significant tendency for the units recruited with larger forces to contribute extensively to the surface EMG. The linear correlation coefficient was 0.70.6. In 106 force records, DEM (the discrepancy between electrical activity and mechanical contraction) was observed. It seemed that DEM was similar phenomenon to the latency relaxation.

The influence of clenching level on the post-stimulus EMG complex, including silent periods, of the masseter muscles in man

Standardized mechanical taps were delivered on an upper central incisor, while the nine subjects investigated maintained constant clenching levels of 5, 10, 20, 40 and 60 per cent of their maximal EMG outputs. Using statistical criteria, a computer program enabled an objective determination of the interaction between the clenching level and the sequence of upward and downward going waves following the stimulus in full-wave rectified and superimposed EMG, called a post-stimulus EMG complex (PSEC). The morphology of the PSEC, including one or two silent periods, was subject-specific and reproducible over one year. The surfaces of the waves were greatly influenced by the clenching level. In general, their total surface decreased as a function of the clenching level. However, the individual waves decreased selectively. The results suggest that the sequence of waves results from overlapping of inhibitory and excitatory inputs. The inhibitory waves, constituting the silent periods, are largely cancelled by the excitatory reflexes of the PSEC.

Integrated electromyographic activity and biting force during rapid isometric contraction of fatigued masseter muscle in man

The slopes of the curves relating integrated electromyographic activities to biting forces during rapid isometric contractions ( voltage tension curve) were investigated before and after fatiguing contraction of the masseter muscle. The slope of the curves depended on the speed of isometric contraction in non-fatigued masseter muscle. When the muscle was fatigued, the speed became slower and the slope became steeper. This increased slope was not solely due to a decrease in the speed of isometric contraction, and it was shown that more electrical activity was associated with exertion of a tension in the fatigued state, as was evident from the voltage factor calculated in this experiment. This phenomenon could be due to fatigue in the intracellular mechanism of the muscle fibre contraction.

Vibration-induced discharge patterns of single motor units in the masseter muscle in man.

Single motor unit potentials were recorded with small bipolar wires from intact masseter muscles in the adult man and a detailed parametric analysis of the effects of muscle vibration on motor unit discharges was carried out. 2. When the vibration amplitude was kept constant, each unit started firing at a definite threshold of vibration frequency. With higher frequencies the rate of firing rapidly reached a maximum. Units recruited at higher frequencies presented a lower maximum rate of firing. 3. When the vibration frequency was kept constant, each masseter unit discharged at a definite threshold of vibration amplitude. With higher amplitudes the unit quickly reached a maximum rate of discharge. Units with a higher frequency threshold tended to also present a higher amplitude threshold. Motor unit "excitability" curves could be plotted using the combined threshold conditions for frequency and amplitude of applied vibrations. 4. With a given parametric set of vibration, the units only started firing at a given delay after the onset of vibration. The delay was quite different for different units and it increased considerably, sometimes by several seconds, when the vibration amplitude was made smaller. 5. In all the experimental conditions tested, and even when the unit discharge did not start until several seconds after vibration onset, the unit potential presented a close and highly consistent temporal relation to the vibration cycles. The slow recruitment process is thought to involve a polysynaptic excitatory mechanism which progressively depolarizes the masseter motoneurones close to their threshold, the actual firing being triggered by monosynaptic excitatory post-synaptic potentials from I(a) afferents, hence the small latency jitter recorded. This special pattern of tonic vibration reflex in jaw-closing muscles in man may result from the lack of reciprocal inhibition from the jaw-opening muscles.

A masseteric reflex eĺicited from the oral mucosa in man

A reflex response is described that can be recorded as a surface electromyogram over the masseter muscle in man. The response is elicited by a “rubbing” stimulus to oral mucosa and has a latency of about 20 msec. It is abolished by anaesthesia of the mucosa.

The standardization of electromyography of the masseter muscle in man

An electromyographic investigation of the masseter muscle was undertaken under standardized conditions. Analyses of variance showed no statistical differences in same-day recordings whereas between-day observations were significantly different. After consideration of these results and those of other workers, it is concluded that the source of between-day variation is probably lack of precision in re-locating the electrodes rather than in variable muscle activity.

The effect of jaw position on the excitability of two reflexes involving the masseter muscle in man

The excitatory monosyntic stretch reflex and the autogenic inhibitory reflex induced by Golgi tendon organ activation were both evoked in the masseter muscle bilaterally by tapping the chin with a reflex hammer. In each subject the reflexes were evoked in five static biting positions; (1) clenching in the intercuspal position, (2) biting on a 6 mm thick wooden stick placed between the anterior teeth, (3) between the right first molars, (4) betvveen the left first molars and, (5) between both first molars. The baseline EMG activity and the changes evoked in it by the chin tap were averaged in a PDP 12 computer and the amplitude of the stretch reflex, the ongoing level of activity, and the length of the inhibitory period were studied. The monosynaptic stretch reflex could not be evoked with the subjects clenching in the intercuspal position but could be evoked in all other positions. When the subjects were biting on one side the reflex on the opposite side was consistently increased by an average of over 100 per cent as compared to the levels obtained while biting bilaterally, while the reflex on the same side was abolished or markedly reduced. This was accompanied by a decrease in the ongoing level of activity on the biting side. The results indicate that, under the conditions of these experiments, static biting in a lateral excursion produces a decrease in the level of masseter muscle activity and stretch reflex excitability on the biting side while enhancing the stretch reflex excitability on the opposite side. The length of the inhibitory period was not markedly affected by changes in lower jaw position.