Optoelectronic analysis of mandibular movement and electromyography (EMG) of masticatory muscles in Cavia porcellus indicate bilateral, unilateral, and gnawing cycles. During bilateral and unilateral cycles, the mandibular tip moves forward, lateral, and down during the lingual phase of the power stroke to bring the teeth into occlusion. EMG activity is generally asymmetric, with the exception of activity of the temporalis muscle during bilateral cycles. During gnawing cycles, the mandible moves in an anteroposterior direction that is opposite that during bilateral and unilateral chew cycles. Bilateral and unilateral cycles of pellets were significantly longer than carrot. With the exception of the width of bilateral cycles, the magnitude of cycle width, length, and height during the mastication of carrots was greater than that during the mastication of pellets. Significant differences exist between EMG durations during mastication of pellets and carrots. The lateral pterygoid displays continuous activity during gnawing cycles. Significant differences also exist in the duration of EMG activity between the working and balancing side during all three cycle types. High level of activity of balancing side temporalis and anterior belly of digastric (ABD) during bilateral cycles occurs during rotation and depression of the mandible during the power stroke. The temporalis apparently provides a "braking" or compensatory role during closing and power strokes. Differences between Cavia masticatory patterns and those shown by Rattus and Mesocricetus are apparently due to differences in dental morphology, occlusal relationships, and, possibly, the poorly developed temporalis in Cavia. The large number and wide diversity of rodent groups afford students of mammalian mastication an opportunity to investigate and compare different masticatory specializations.

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Parafunctional masticatory activity, such as the tooth clenching and grinding that is associated with bruxism, is encountered by clinicians in many disciplines, including dentistry, neurology and psychiatry. Despite this, little is known about the neurological basis for these activities. To identify the brain network engaged in such complex oromotor activity, functional magnetic resonance imaging (fMRI) was used to elucidate the brain activation patterns of 20 individuals (10 males and 10 females, mean s.d. age of 26.3+/-4.1 years) with (parafunctional, PFx group, 5M/5F) and without (normal functional, NFx group, 5 M/5F) self-reported parafunctional grinding and clenching habits during clenching and grinding tasks. Subject group classification was based on: (i) self-reported history, (ii) clinical examination, (iii) evaluation of dental casts and (iv) positive responses to the temporomandibular disorder (TMD) History Questionnaire [Dworkinand LeResche, Journal of Craniomandibular Disorders, (1992) 6:301]. While subjects performed these oromotor tasks, each wore a custom-designed oral appliance minimizing head motion during imaging. Mean per cent signal changes showed significant between group differences in motor cortical (supplementary motor area, sensorimotor cortex and rolandic operculum) and subcortical (caudate) regions. Supplementary motor area data suggest that motor planning and initiation, particularly during the act of clenching, are less prominent in individuals with oromotor parafunctional behaviours. The overall extent of activated areas was reduced in subjects with self-reported parafunctional masticatory activity compared with the controls. This study's methodology and findings provide an initial step in understanding the neurological basis of parafunctional masticatory activities that are relevant for therapeutic research applications of temporomandibular joint and muscle disorders and associated comorbidities.

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