Influence of tongue muscle contraction and dynamic airway pressure on velopharyngeal volume in the rat

Abstract

Fregosi RF. Influence of tongue muscle contraction and dynamic airway pressure on velopharyngeal volume in the rat. J Appl Physiol 104: 682-693, 2008. First published December 13, 2007 doi:10.1152/japplphysiol.01043.2007.-The mammalian pharynx is a collapsible tube that narrows during inspiration as transmural pressure becomes negative. The velopharynx (VP), which lies posterior to the soft palate, is considered to be one of the most collapsible pharyngeal regions. I tested the hypothesis that negative transmural pressure would narrow the VP, and that electrical stimulation of extrinsic tongue muscles would reverse this effect. Pressure (Ϫ6, Ϫ3, 3, and 6 cmH 2O) was applied to the isolated pharyngeal airway of anesthetized rats that were positioned in a 4.7-T MRI scanner. The volume of eight axial slices encompassing the length of the VP was computed at each level of pressure, with and without bilateral hypoglossal nerve stimulation (0.1-ms pulse, one-third maximum force, 80 Hz). Negative pressure narrowed the VP, and either whole hypoglossal nerve stimulation (coactivation of protrudor and retractor muscles) or medial nerve branch stimulation (independent activation of tongue protrudor muscles) reversed this effect, with the greatest impact in the caudal one-third of the VP. The dilating effects of medial branch stimulation were slightly larger than whole nerve stimulation. Positive pressure dilated the VP, but tongue muscle contraction did not cause further dilation under these conditions. I conclude that the narrowest and most collapsible segment of the rat pharynx is in the caudal VP, posterior to the tip of the soft palate. Either coactivation of protrudor and retractor muscles or independent contraction of protrudor muscles caused dilation of this region, but the latter was slightly more effective. apnea; functional electrical stimulation; hypoglossal nerve; magnetic resonance imaging; pharynx; upper airway THE PHARYNX IS A MUSCULAR tube with little skeletal support and is thus subject to collapse as its transmural pressure becomes negative during inspiratory contractions of the diaphragm and accessory inspiratory muscles. Various skeletal muscles residing within the pharyngeal walls (e.g., pharyngeal constrictor muscles), or lying within and adjacent to the pharynx (e.g., tongue and palatal muscles), are capable of dilating and/or stiffening it. Indeed, the posterior segment of the tongue comprises a large portion of the anterior pharyngeal wall, so movement and/or stiffening of the tongue can have a marked impact on the dimensions and compliance of the oropharynx and velopharynx (VP). For both anatomic and physiological reasons, neuromuscular activation is inadequate in at least some patients with primary snoring, upper airway resistance syndrome, or obstructive sleep apnea (41, 54). These patients cannot maintain a positive pharyngeal transmural pressure during sleep, leading to periodic pharyngeal collapse with associated hypoventilation and hypoxia throughout the night.At present, the favored treatme...

Neural Control of the Upper Airway: Integrative Physiological Mechanisms and Relevance for Sleep Disordered Breathing

“…Nevertheless, coactivation of the extrinsic and intrinsic muscles of the tongue (i.e., including both retractors and protruders) during eupnea, hypercapnia, and hypoxia in animals and humans contributes to enlarging and/or stiffening the upper airspace, including the retropalatal airspace, leading to increased stability and resistance to airway collapse (13, 14, 17, 18, 72, 146, 147, 152-154, 246, 316). As expected therefore, electrical activation of the tongue and hyoid muscles, or their motor nerves, in animals dilates the upper airspace and/or makes the upper airspace less vulnerable to suction collapse via increased stiffness (35,47,72,76,146,168,373,486,518,533,576). Similar effects are also observed in humans (236,310,(375)(376)(377).…”

“…What is known is that subatmospheric pressure applied to the upper airway in anesthetized rats increases activity in the medial and lateral branches of the hypoglossal nerve innervating the muscles of the tongue, the pharyngeal branch of the glossopharyngeal nerve innervating the pharyngeal constrictor muscles (452), as well as increasing activity of the hypoglossus muscle (455). As reviewed above (section Mechanical consequences of upper airway muscle activation during breathing) where coactivation of tongue protruder and retractor muscles during eupnea and in response to chemical respiratory stimulation contributes to enlarging and/or stiffening the upper airspace (14,17,18,72,146,152,154,316), such coactivation in response to subatmospheric airway pressures is also likely to be beneficial to the maintenance of upper airway patency. cricoarytenoid responses in conscious (426) and anesthetized dogs (579), and decerebrate cats (227).…”

Activation of the Hypoglossal to Tongue Musculature Motor Pathway by Remote Control

“…Some of the extrinsic muscles of the tongue have classically been designated as tongue “protruders” or “retractors” based on the original anatomical studies, with the intrinsic muscles being more classically associated with changes in tongue shape. However, co-activation of the intrinsic and extrinsic muscles of the tongue (i.e., including both “retractors” and “protruders”) contributes to increases in upper airway stability and resistance to airway closure in animals and humans4041424344. Accordingly, targeted activation of hypoglossal motoneurons and their innervated muscles en masse (i.e., rather than targeted activation of a particular or singular muscle group) would be expected to have net positive benefits to upper airway stability and OSA.…”

Peripharyngeal tissue deformation and stress distributions in response to caudal tracheal displacement: pivotal influence of the hyoid bone?

“…Image analysis. Analysis of upper airway lumen geometry was performed from axial CT images with all slices perpendicular to the horizontal axis, in a similar manner to several earlier studies (9,15,30,43). To the extent that the upper airway is curved, this approach runs the risk of obtaining cross-sectional data at varying angles to the curved upper airway centroid axis.…”