Effect of Stimulation of the Laryngopharynx with Water and Salt Solutions on Voluntary Swallowing in Humans: Characteristics of Water Receptors in the Laryngopharyngeal Mucosa

Abstract

Stimulation of water receptors in the laryngopharynx (LP) with water facilitates voluntary swallowing in humans. Based on measures of swallowing intervals (SIs) in repetitive swallowing, we investigated characteristics of laryngopharyngeal water receptors in humans. Healthy adult volunteers were instructed to perform repetitive swallowing as quickly as possible during infusion of a solution into the LP. Infusion of water shortened SI, suggesting that water excites water receptors. Infusion of 0.3 M NaCl solution prolonged SI, suggesting that the NaCl solution inhibits activity of water receptors. SI increased with increasing concentration of NaCl. Anion or cation substitutions indicated that excitation of water receptors is due to absence or reduced concentration of Cl(-). With diminution of peripheral inputs, cortical inputs would play a dominant role in voluntary swallowing. With infusion of a nonstimulating solution (0.3 M NaCl at 0.2 mL/min), SI varied greatly from subject to subject, suggesting that the ability of central regulation of swallowing to initiate repetitive voluntary swallowing varies among subjects. Facilitation of swallowing by chemosensory inputs from water receptors appeared strongly in subjects with longer SI with infusion of the nonstimulating solution. It appears that chemosensory activation compensates for the difficulty in initiating swallowing via the central neural mechanism.

Contribution of Umami Substance to Swallowing

“…Such facilitatory effect of water stimulation to the pharyngeal region on swallow initiation has also been reported by others, showing that application of water to the laryngopharynx shortened swallowing intervals in repetitive voluntary swallowing in humans [13,14]. They also showed that application of 0.3 M NaCl solution prolonged swallowing intervals, and suggested that the action of NaCl is inhibition of water response [14]. The present result showing the suppression of swallow initiation by application of saline (0.15 M NaCl) may partly support their idea.…”

“…The finding strongly suggests that umami stimulation of the oropharynx facilitates the initiation of the swallow reflex. However, we cannot deny the possibility that the effect of umami solution was due to the vehicle (water), which is already known to be a major chemical stimulant of this area and an initiator of the swallow reflex [13,14]. To clarify this, we examined whether application of MSG solution dissolved with saline as well as distilled water to the oropharynx modulates the initiation of the swallow reflex.…”

Protective Effects of Ferulic Acid against Chronic Cerebral Hypoperfusion-Induced Swallowing Dysfunction in Rats

“…The swallowing reflex evoked by water is believed to depend on the activation of water receptors [34], whereas acid stimulation may depend on transient receptor potential vanilloid type 1 (TRPV1) channels expressed in laryngopharyngeal afferent nerve fibers [35]. Activation of TRPV1 by acidity, heat, or capsaicin is known to induce SP release from laryngopharyngeal afferent nerve endings, triggering the swallowing and cough reflex [36].…”

Peripheral and central control of swallowing initiation in healthy humans

“…ACCEPTED MANUSCRIPT 4 The potentials required to initiate voluntary swallowing have been found to vary greatly from subject to subject, even in healthy humans [10][11][12]. Human brain imaging has exposed the cortical representation of voluntary swallowing [13][14][15][16][17][18].…”