**Background:** Obesity is a global health concern associated with altered masticatory behaviors, such as faster eating rate and fewer chewing cycles. However, the effect of obesity on the coordinated motor function of masticatory components (jaw, tongue, muscles) is unclear. The cortical masticatory area (CMA) in the brain, comprising the anterior (A-area) and posterior (P-area) regions, controls rhythmic jaw movements (RJMs) via the brainstem central pattern generator. This study aimed to investigate whether obesity affects RJM patterns and masticatory muscle electromyographic (EMG) activity during electrical stimulation of the A-area and P-area in a genetic obesity model.
**Methods:** Seven male obese Zucker rats (OZRs, fa/fa) and seven lean Zucker rats (LZRs, Fa/Fa or Fa/fa) were studied at 10 weeks of age. Under ketamine anesthesia, bipolar EMG electrodes were inserted into the right anterior digastric (RAD) and masseter muscles. A glass-insulated tungsten microelectrode was inserted into the left A-area (3–4 mm anterior, 2–3.5 mm lateral to bregma, 2–3.5 mm deep) and P-area (0–2 mm rostral, 4–5.5 mm lateral to bregma, 4–5 mm ventral). Repetitive intracortical microstimulation (0.5 ms duration, 20 Hz, 120 µA for A-area, 180 µA for P-area, 8 s) was applied. Jaw movements were recorded with a high-speed camera and 2D motion analysis. EMG signals were amplified (1000× gain, bandpass 0.3–3 kHz), rectified, and analyzed offline. Parameters measured included gape size, lateral excursion, vertical jaw-opening speed, jaw-opening duration, jaw-closing duration, cycle duration, and EMG onset latency, peak-to-peak amplitude, duration, median frequency, and mean frequency. Three trials were performed per site. Statistical analysis used unpaired t-tests and two-way ANOVA with significance set at p < 0.05.
**Key Results:** Body weight and food intake were significantly higher in OZRs than LZRs (p < 0.05). During P-area stimulation, OZRs showed significantly shorter jaw-opening duration (24.3 ms vs. 27.9 ms, p < 0.01), faster vertical jaw-opening speed (67.5 mm/s vs. 50.8 mm/s, p < 0.05), and shorter RAD EMG duration (5.2 ms vs. 6.9 ms, p < 0.01) compared to LZRs. No significant differences were observed during A-area stimulation. Gape size and lateral excursion did not differ between groups. RAD EMG onset latency was significantly shorter in OZRs during both A-area and P-area stimulations (p < 0.05). Peak-to-peak amplitude, median frequency, and mean frequency of RAD EMG showed no significant differences between groups. Intragroup comparisons revealed that P-area stimulation elicited larger lateral excursion (p < 0.05 in LZRs), longer jaw-opening duration, and shorter jaw-closing duration than A-area stimulation in both groups.
**Clinical Implications:** This study demonstrates that obesity specifically affects RJM patterns and digastric muscle activity evoked by P-area stimulation, which is associated with masticatory-like behavior. The faster jaw-opening speed and shorter digastric contraction time in OZRs, despite unchanged muscle force and geometry, suggest that other masticatory components (e.g., tongue movement) contribute to altered coordination. These findings may explain the faster eating rate and fewer chewing cycles observed in obese individuals. Additionally, the results may provide insights into obesity-related conditions such as obstructive sleep apnea, where tongue and upper airway changes are implicated. The study highlights the need for further research on tongue movement and saliva secretion in obesity to fully understand the mechanisms underlying altered masticatory function.