Effects of low occlusal loading on the neuromuscular behavioral development of cortically-elicited jaw movements in growing rats

The effect of altered occlusal force on masticatory-related neuromuscular control, which projects from the anterior part of the cortical masticatory area (A-CMA), during growth remains unclear. This study sought to evaluate the effect of occlusal hypofunction on neuromuscular development of jaw muscle activities and cortically-induced rhythmic jaw movements (RJMs) in growing rats. Sixty-four 2-week-old male albino Wistar rats were divided into the control (fed normal diet) and experimental (fed soft diet) groups soon after weaning. Electromyographic activity was recorded at 5, 7, 9, and 11 weeks from the right masseter and anterior digastric along with RJMs. We found a significantly longer onset latency and smaller peak-to-peak amplitude in the experimental group than that in the control group. The RJMs showed an increase in gape size and lateral excursion until up to 9 weeks in both groups. However, both the average gape size and lateral excursion were significantly smaller in the experimental group than that in the control group after 9 weeks. The jaw movement pattern also showed a significant decrease at the maximum opening period in the experimental group. Our findings indicate that inadequate occlusal function during growth alters neuromuscular control of masticatory behaviors and impairs the pattern of RJMs.

Functional Analysis of Rhythmic Jaw Movements Evoked by Electrical Stimulation of the Cortical Masticatory Area During Low Occlusal Loading in Growing Rats

The maturation of rhythmic jaw movements (RJMs) and related neuromuscular control has rarely been studied in animals, though this process is essential for regulating the development of stomatognathic functions. Previous studies have shown that occlusal hypofunction during growth alters masticatory performance. However, little is known about patterns of cortically-induced RJMs under conditions of soft-diet feeding during development. The aim of this study is to clarify the effect of low occlusal loading on the pattern of cortically induced RJMs and related neuromuscular responses in growing rats. Sixty-four 2-week-old male albino Wistar rats were randomly divided into two groups and fed on either a normal diet (control) or soft diet (experimental) soon after weaning. At 5, 7, 9, and 11 weeks of age, electromyographic (EMG) activity was recorded from the right masseter and anterior digastric muscles along with corresponding kinematic images in RJMs during repetitive intracortical microstimulation of the left cortical masticatory area (CMA). Rats in both groups showed an increase in gape size and lateral excursion until 9 weeks of age. The vertical jaw movement speed in both groups showed no significant difference between 5 and 7 weeks of age but increased with age from 9 to 11 weeks. Compared to the control group, the average gape size and vertical speed were significantly lower in the experimental group, and the pattern and rhythm of the jaw movement cycle were similar between both groups at each recording age. EMG recordings showed no age-related significant differences in onset latency, duration, and peak-to-peak amplitude. Moreover, we found significantly longer onset latency, smaller peak-to-peak amplitude, and greater drop-off mean and median frequencies in the experimental group than in the control group, while there was no significant difference in the duration between groups. These findings indicate that a lack of enough occlusal function in infancy impedes the development of patterns of RJMs and delays the neuromuscular response from specific stimulation of the CMA.

Effects of increased occlusal vertical dimension on the jaw-opening reflex in adult rats

OBJECTIVE

Malocclusion with deep overbite and facial esthetics improve when facial height is intentionally increased during orthodontic extrusion of the posterior teeth. Thus, a better understanding of post-treatment stability of increased occlusal vertical dimension (iOVD) in adult patients is important. We focused on the jaw-opening reflex (JOR), which plays an important role in the control of jaw movements during mastication, and investigated the effects of iOVD on the JOR in rats with an electrophysiological technique.

DESIGN

One hundred and twenty 13-week-old male Wistar rats were randomly divided into control and experimental groups. Rats in the experimental group received a 2-mm buildup of composite resin on the maxillary molars at 13 weeks of age. The JOR was induced by low-intensity electrical stimulation of the left inferior alveolar nerve. The electromyographic responses were recorded from the digastric muscle at 13, 14, 15, 17, 19, and 23 weeks of age. JOR properties including latency, duration, and peak-to-peak amplitude were measured and compared between the groups.

RESULTS

The latency of the JOR was significantly longer and the peak-to-peak amplitude was significantly smaller in the experimental group than in the control group from 14 to 19 weeks of age, while the reflex duration was not significantly different. Intra-group comparisons of the latency and peak-to-peak amplitudes among rats 14-19 weeks of age were significantly different between the experimental group and the control group.

CONCLUSIONS

iOVD affected the latency and amplitude of the JOR but not the duration. The JOR adapted after 10 weeks of iOVD.