The Digastric Muscle is Less Involved in Pharyngeal Swallowing in Rabbits

Abstract

The swallowing reflex is centrally programmed by the lower brain stem, the so-called swallowing central pattern generator (CPG), and once the reflex is initiated, many muscles in the oral, pharyngeal, laryngeal, and esophageal regions are systematically activated. The mylohyoid (MH) muscle has been considered to be a "leading muscle" according to previous studies, but the functional role of the digastric (DIG) muscle in the swallowing reflex remains unclear. In the present study, therefore, the activities of single units of MH and DIG neurons were recorded extracellularly, and the functional involvement of these neurons in the swallowing reflex was investigated. The experiments were carried out on eight adult male Japanese white rabbits anesthetized with urethane. To identify DIG and MH neurons, the peripheral nerve (either DIG or MH) was stimulated to evoke action potentials of single motoneurons. Motoneurons were identified as such if they either (1) responded to antidromic nerve stimulation of DIG or MH in an all-or-none manner at threshold intensities and (2) followed stimulation frequencies of up to 0.5 kHz. As a result, all 11 MH neurons recorded were synchronously activated during the swallowing reflex, while there was no activity in any of the 7 DIG neurons recorded during the swallowing reflex. All neurons were anatomically localized ventromedially at the level of the caudal portion of the trigeminal motor nucleus, and there were no differences between the MH and DIG neuron sites. The present results strongly suggest that at least in the rabbit, DIG motoneurons are not tightly controlled by the swallowing CPG and, hence, the DIG muscle is less involved in the swallowing reflex.

Central nervous system integration of sensorimotor signals in oral and pharyngeal structures: oropharyngeal kinematics response to recurrent laryngeal nerve lesion

“…In particular, the superior laryngeal nerve (SLN), which innervates the valleculae and the upper portion of the larynx to the level of the vocal folds, triggers the swallow (4,21,50). Unilateral and bilateral lesion of the internal branch of the SLN results in significant aspiration (4,5,14), and electrical stimulation of the iSLN is sufficient to trigger a pharyngeal swallow (21,35,51).…”

“…pharyngeal swallow (35,43,51), and pharyngeal swallows can be elicited by injecting fluid into the valleculae in decerebrate animals (6,7,45). However, extensive investigation has shown that characteristics of the pharyngeal swallows can be modulated (21,27,32) by a variety of oral (17,42) and pharyngeal (44) sensory triggers.…”

Functional Role of Suprahyoid Muscles in Bolus Formation During Mastication

“…The suprahyoid muscles, particularly the anterior belly of the digastric and the mylohyoid, dominantly contribute to jaw-opening ( Khan and Bordoni, 2021 ). The digastric muscle helps in depressing and retracting the mandible functionally but is less involved in deglutition, at least in animals ( Doty and Bosma, 1956 ; Tsujimura et al, 2012 ). Conversely, the mylohyoid muscle also functions to elevate the floor of the mouth and the tongue during deglutition or speaking while the geniohyoid muscle contributes to upward and forward movements of the hyoid, and hence widening of the passage for the bolus during deglutition.…”

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“…The suprahyoid muscles, particularly the anterior belly of the digastric and the mylohyoid, dominantly contribute to jawopening (Khan and Bordoni, 2021). The digastric muscle helps in depressing and retracting the mandible functionally but is less involved in deglutition, at least in animals (Doty and Bosma, 1956;Tsujimura et al, 2012). Conversely, the mylohyoid muscle also functions to elevate the floor of the mouth and the tongue during deglutition or speaking while the geniohyoid muscle contributes to upward and forward movements of the hyoid, and hence widening of the passage for the bolus during deglutition.…”