Association between Laryngeal Airway Aperture and the Discharge Rates of Genioglossus Motor Units.

Amy Lacross,E Fiona Bailey,Peter J Watson

doi:10.3389/fphys.2017.00027

Abstract

We know very little about how muscles and motor units in one region of the upper airway are impacted by adjustments in an adjacent airway region. In this case, the focus is on regulation of the expiratory airstream by the larynx and how changes in laryngeal aperture impact muscle motor unit activities downstream in the pharynx. We selected sound production as a framework for study as it requires (i) sustained expiratory airflow, (ii) laryngeal airway regulation for production of whisper and voice, and (iii) pharyngeal airway regulation for production of different vowel sounds. We used these features as the means of manipulating expiratory airflow, pharyngeal, and laryngeal airway opening to compare the effect of each on the activation of genioglossus (GG) muscle motor units in the pharynx. We show that some GG muscle motor units (a) discharge stably on expiration associated with production of vowel sounds, (b) are exquisitely sensitive to subtle alterations in laryngeal airflow, and (c) discharge at higher firing rates in high flow vs. low flow conditions even when producing the same vowel sound. Our results reveal subtle changes in GG motor unit discharge rates that correlate with changes imposed at the larynx, and which may contribute to the regulation of the expiratory airstream.

Highlights

Human tongue muscles participate in respiration-related and voluntary movements
We show increases in GG activation that occur against a background of changing airflow through the glottis
These findings are consistent with the notion of complimentary regulation of the laryngeal and pharyngeal airways (McClean and Tasko, 2002) and of the system wide regulation of airflow (Warren, 1986) but differ from previously published findings that show a diminution in expirationrelated GG activities during moderately heavy exercise (Walls et al., 2013)

Summary

INTRODUCTION

It is evident that the extrinsic tongue protrudor muscle genioglossus (GG) defends the airway against inspiratory narrowing at rest (Cheng et al, 2011), in exercise (Walls et al, 2013), and during sleep (Chuang et al, 2009). Most previous research has examined the GG’s role defending the airway on inspiration (Remmers et al, 1978; Mezzanotte et al, 1992, 1996; Fogel et al, 2001; Remmers, 2001), in this case we look for evidence of its expiration-related activity. We documented expiration-related GG activity during moderate and heavy exercise that suggested a role for the muscle in the regulation of the expiratory airstream (Walls et al, 2013). We exclude from our analysis inspiratory motor unit activity (Figure 1A) and sustained or tonic motor unit activity (Figure 1B), electing to focus in this case on GG motor unit activity associated exclusively with the production of sound on expiration (Figure 1C)

Participants

RESULTS

DISCUSSION

Experimental Method

Summary