Introduction: Human bruxism is defined as the involuntary grinding, gnashing, or clenching of the teeth, and is non-functional in nature. Bruxism can happen either while awake or during sleep, with 8-16% and 20% of the population affected, respectively. There are two leading models on the etiology of bruxism: malocclusion and central disturbances in the basal ganglia. Currently, the only treatments for this disorder are night guards and botulinum toxin injections into the masseter muscle, with both having their downfalls. Central pattern generators (CPGs) are local circuits inside the brain and spinal cord that produce highly specific and rhythmic motor patterns. The mastication CPG exists in the pons of the brainstem. The rhythmic movements seen in bruxism either come from a separate brux-like CPG or a modular masticatory CPG that is capable of both rhythms, and can produce the brux-like movements seen in the bruxism. The current study compares brux-like activity in rats to mastication in order to further characterize the neuromuscular motor patterns of both behaviors.
Methods: Laboratory rats (Rattus norvegicus ) were used with electromyography (EMGs) and sonomicrometry measurements to characterize the motor patterns of both the anterior superficial masseter and the anterior digastric muscles comparing motor patterns for brux-like and masticatory behaviors (n=8). Bipolar silver wire electrodes (EMG) were inserted into both muscles, while 1 millimeter piezoelectric crystals (sonomicrometry) were sutured into the anterior superficial masseter muscle 5 mm apart parallel with the muscle fibers. Brux-like recordings were done while the animal was grinding its teeth and masticatory recordings were done when the animal was eating solid food. Recordings were collected and analyzed with Axoscope software. We determined the phase values, EMG burst duration, and cycle periods for both muscles during brux-like and masticatory motor patterns. Phase values are used to compare the timing of the two muscles. Cycle periods are used to compare the timings of complete cycles for one muscle. Sonomicrometry was analyzed qualitatively to visualize the contracting patterns of the anterior superficial masseter muscle in both behaviors, as well as, analyzed quantitatively for the sonomicrometry cycle periods and the electromechanical delay (EMD). The cycle periods here are to compare the muscle contracting cycles, while the EMD is the time from the onset of the muscle firing until the start of the contracting muscle. Preliminary experiments used laboratory rats ( Rattus norvegicus) to compare EMG amplitudes for both brux-like and masticatory behaviors in the masseter, anterior digastric, and temporalis muscles. EMG amplitudes were analyzed by Bioproc2 software.
Results: Preliminary results revealed lower EMG amplitudes during brux-like behaviors for the masseter and anterior digastric muscles, while the temporalis was non-significantly different (NS) between mastication and brux-like. EMG phase analysis revealed that the anterior superficial masseter and the anterior digastric muscles were antiphase during mastication (phase value = 0.633, circular standard deviation = 0.049, strength of coupling = 0.954) and in-phase during brux-like activity (phase value = 0.007, circular standard deviation = 0.018, strength of coupling = 0.994). EMG cycle periods, EMG digastric burst duration, and sonomicrometry cycle periods were all lower for brux-like activity compared to mastication (p<0.05). The EMD was larger during brux-like activity when compared to mastication (p<0.05). EMG anterior superficial masseter burst duration showed non-significance between brux-like and mastication. The sonomicrometry measurements also revealed a difference in the contracting pattern of the anterior superficial masseter muscle.
Conclusions: Our EMG measurements demonstrate that the anterior superficial masseter and the anterior digastric muscles are firing simultaneously during brux-like activity and are alternating during mastication. They also have lower amplitudes during brux-like behaviors. The muscles contract more rapidly and for shorter periods during brux-like activity, as revealed by the sonomicrometry measurements. These measurements also demonstrated qualitatively that the anterior superficial masseter muscle plays a large role in mandibular movement during brux-like activity, and produce little movement during mastication. The contraction patterns of the anterior superficial masseter muscle during mastication suggest that the anterior superficial masseter plays a large role in providing the correct bite during chewing behaviors. Overall, we have shown more evidence for the distinctly different motor patterns between brux-like and masticatory behaviors. We have also provided more evidence to support using both electromyography and sonomicrometry together during neurophysiology experiments.
|Commitee:||Essner, Richard, Jr, Kohn, Lucy|
|School:||Southern Illinois University at Edwardsville|
|School Location:||United States -- Illinois|
|Source:||MAI 56/01M(E), Masters Abstracts International|
|Subjects:||Biology, Neurosciences, Dentistry|
|Keywords:||Bruxism, EMG, Mastication, Sonomicrometry|
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