Comparison of mandibular movement trajectories and associated patterns of oral muscle electromyographic activity during spontaneous and apomorphine-induced rhythmic jaw movements in the guinea pig.

Vertical and horizontal movements of the lower jaw (mandible) of ketamine-anesthetized guinea pigs were recorded in association with electromyographic (EMG) activity in the anterior digastric, lateral pterygoid, medial pterygoid, and deep masseter muscles during spontaneously occurring rhythmic jaw movements (SRJMs) and during rhythmical jaw movements induced by the intravenous administration of apomorphine (ARJMs). Both ARJMs and SRJMs were near periodic and occurred at frequencies in the 2- to 5-Hz range. However, the profiles of the mandibular movements and associated patterns of jaw muscle EMG activity differed dramatically for SRJMs versus ARJMs. SRJMs were characterized by prominent lateral excursions of the mandible that occurred in association with both the jaw opening and closing movements. The lateral excursions were directed to the left side on some SRJM cycles and to the right side on others. The direction of the lateral component alternated irregularly, but no more than three consecutive cycles with horizontal movements to the same side were observed at any time. Each SRJM cycle was generated by the occurrence of one of two coordinated sequences of EMG activity. One sequence produced right-sided cycles, the other produced left-sided cycles. Each sequence was initiated by an EMG burst in the digastric muscle ipsilateral to the direction of the horizontal excursion of the mandible, followed by EMG bursts in the contralateral digastric, lateral pterygoid, and medial pterygoid muscles. The EMG bursts in the digastrics and contralateral lateral pterygoid muscles were associated with jaw opening and the initial stage of lateral movement. EMG activity in the contralateral medial pterygoid muscle was associated with the onset of closing and a second stage of lateral movement. Masseter muscle activity was also observed during SRJMs, but only in a subset of the animals tested (3 of 12). When present, the masseter activity began well after the onset of jaw closing. No significant horizontal mandibular movements were observed during ARJMs. The mandibular trajectories during opening and closing always remained close to the midline. The opening phase of ARJM cycles was associated with bilaterally synchronized activity in the digastric and lateral pterygoid muscles. The closing phase was associated with bilaterally symmetric activity in the masseter muscles. The medial pterygoid muscles displayed little or no EMG activity during ARJMs. The durations of the EMG bursts recorded in the masseter muscle were correlated with cycle time during SRJMs, as were the burst durations of the digastric and lateral pterygoid muscles during ARJMs.(ABSTRACT TRUNCATED AT 400 WORDS)