Modality of tracheal sensory inputs modulate swallowing reflexibility and discharge patterns of neurons of the dorsal swallowing group in rats

OBJECTIVE: Desensitization of the larynx significantly impairs swallowing function in patients with tracheostomies. Although it is clinically relevant that adequate airway clearance helps to improve swallowing responsiveness, a robust theoretical proposal has not been provided. Hence, we investigated the impact of prolonged fluid irritation of the trachea and larynx on the swallowing reflex. We additionally analyzed discharge activities of the swallowing-related neurons in presence and absence of airway fluid accumulation. METHODS: We analyzed the likelihood of swallowing evoked by electrical stimulation of the superior laryngeal nerve or oral water injection before and after removal of fluid accumulation in the trachea and larynx using air flow in perfused rats (n = 15). We then transected the superior laryngeal and vagus nerves which convey critical laryngotracheal sensory information. We recorded changes in the neuronal activities in the presence or absence of airway fluid contamination using multi-electrode arrays (n = 8). RESULTS: The frequency of swallows was increased by the removal of airway fluid accumulation and this change lasted at least 60 min. The effects were abolished following the section of either the superior laryngeal or vagus nerve. Specifically, we found that centrally recorded swallowing neurons showed changes in spontaneous discharge activities in the presence or absence of fluid-related laryngotracheal sensory stimulation. CONCLUSION: A diminished likelihood of swallowing caused by prolonged airway irritation can be cautiously improved through airway clearance with respiratory airflow via central and peripheral neuromodulation. Our findings provide a fundamental mechanism to recover impaired laryngotracheal susceptibility for severe dysphagia such as post-tracheostomy dysphagia.