Abstract
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.
Highlights
Mastication involves complex neuromuscular interactions between central and peripheral control mechanisms
rhythmic jaw movements (RJMs) are generated by interconnected neural systems termed the masticatory central pattern generator (CPG), which is located in the brainstem (Lund, 1991)
Body weights of the rats in both groups increased gradually throughout the experimental period, and there were no significant differences in mean body weight between groups at any time point (Figure 2)
Summary
Mastication involves complex neuromuscular interactions between central and peripheral control mechanisms. In order to produce the complex and variable patterns of mastication-like behavior, the CPG can be modulated by inputs from peripheral afferents as well as higher centers such as the cortical masticatory area (CMA) (Lund et al, 1981; Lund, 1991; Nakamura and Katakura, 1995). Several studies have reported that different areas of the CMA evoke different RJM patterns and jaw muscle EMG activity in rats (Sasamoto et al, 1990; Satoh et al, 2007; Maeda et al, 2014), guinea pigs (Isogai et al, 2012), rabbits (Lund et al, 1984), and monkeys (Huang et al, 1989). Stimulation of the postero-lateral region of the CMA induces grinding movements that resemble natural chewing patterns in rats (Sasamoto et al, 1990)
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