Chewing modulates the human cortical swallowing motor pathways

Effects of different mylohyoid muscle stimulations on swallowing cortex excitability in healthy subjects

BACKGROUND

Dysphagia has been recognized by the World Health Organization as a medical disability. Improving mylohyoid muscle function plays an important role in pharyngeal dysphagia. The aim of this study was to evaluate the treatment of transcranial magnetic stimulation (TMS), peripheral magnetic stimulation (PMS), and electrical stimulation (ES) for dysphagia.

METHODS

Forty healthy subjects were randomly divided into four groups: TMS+PMS, TMS, PMS, and ES. TMS stimulated the cortical representative area of the mylohyoid muscle and the PMS was directly stimulating the mylohyoid muscle, both of them at a frequency of 10 Hz for a total of 1,800 pulses. The intensity of ES was based on the subject's tolerance level, usually 2-5 mA. Functional near infrared spectroscopy (fNIRS) and motor evoked potential (MEP) of the mylohyoid muscle were used to evaluate the immediate effects of stimulation on swallowing cortex excitability of healthy subjects before and after intervention.

RESULTS

The fNIRS results revealed notable activation across multiple channels in the four groups of healthy subjects both pre- and post- the intervention. Among these channels, the activation levels were most pronounced in the TMS+PMS group, followed by the TMS, PMS, and ES groups, respectively. Regarding the MEP results, post-intervention observations indicated a reduction in bilateral latency and an increase in bilateral amplitude in the TMS+PMS group. Additionally, the left amplitude exhibited an increase in the TMS group.

CONCLUSIONS

In fNIRS, all four stimulation methods significantly activated the swallowing cortex of healthy subjects, and the activation of TMS+PMS was the most obvious, followed by TMS, PMS, and ES.

Influence of restricted mastication on swallowing function

BACKGROUND

Oral food processing is an important part of daily food intake. A major part of this process is mastication, which prepares a bolus of food for swallowing by mechanically crushing and grinding ingested food between the teeth using rhythmic movements. Masticatory dysfunction is common in the elderly and in some neurological disorders and can have serious negative health consequences.

OBJECTIVE

This study investigated the effect of restricted mastication, achieved by experimentally reducing the duration of mastication, on masticatory patterns and subsequent swallowing function.

METHODS

Thirty healthy men (25 ± 3 years old) were instructed to chew gum jelly with a free mastication duration (G100), a half and a quarter duration of G100. Masseter and digastric electromyographic (EMG) activity was recorded to assess mastication and swallowing activity, respectively. In addition, the acceleration of the thyroid cartilage ridge was measured with an accelerometer. The root mean square (RMS) of muscle EMG activity in the masseter and digastric muscles, the number of masseter EMG bursts, time to peak and total duration of each masseter EMG burst, swallowing duration and laryngeal elevation latency were analysed.

RESULTS

Restricting masticatory duration reduced the number of mastication cycles (p < .001), prolonged the time to peak (p < .001) and total duration of masseter EMG bursts (p < .001) and resulted in an overall increased RMS score of masseter muscle activity (p = .017). Furthermore, restricted masticatory duration led to a decrease in both swallowing duration (p = .001) and laryngeal elevation latency (p = .012), with a significant increase in the RMS score of digastric muscle activity (p < .001).

CONCLUSIONS

Under the experimental conditions of restricted mastication, several adaptation features were observed, including changes in masticatory cycle characteristics and swallowing duration. Thus, although the overall masticatory efficiency was reduced, these adaptations allowed healthy individuals to still swallow safely.

Chewing and its influence on swallowing, gastrointestinal and nutrition-related factors: a systematic review

The study aimed to evaluate the hypothesis that chewing is a mechanical and physiological contributor to swallowing, physiologic/pathologic processes of the gastrointestinal tract (GIT), and nutrition-related factors. A search strategy was applied to three different databases to investigate if chewing function in adults affects the swallowing, physiologic/pathologic processes of the GIT, and nutrition-related factors compared to controls with no exposure. The included studies were evaluated for methodological quality and risk of bias and certainty of evidence. The results showed 71 eligible studies. Overall, the results showed that 46 studies supported the hypothesis while 25 refuted it. However, the GRADE analysis showed low to very low certainty of the evidence to support the hypothesis that chewing is an important contributor in the swallowing process, and physiologic/pathologic processes in the GIT. The GRADE analysis also showed a moderate to very low certainty of the evidence to suggest that chewing function contributes to nutrition-related parameters. The overall results of the current study showed that a majority (64.7%) of the studies (46 out of 71) supported the hypothesis. However, robust studies with proper design, adequate sample size, and well-defined outcome parameters are needed to establish conclusive evidence.