Differential neuromotor control of the vertical and longitudinal genioglossus muscle fibers: An overlooked tongue retractor

Introduction

The genioglossus (GG) is known to be the main tongue protrusor, and therefore plays a major role in breathing. However, due to the fan shape of the GG fibers, it could be assumed that contraction of the anterior fibers of the GG do not cause tongue protrusion. In this study, we examined the effect of contraction of the anterior-vertical fibers of the GG (GG_V) on the tongue and their EMG activity during wakefulness and sleep. The findings were compared to those of the longitudinal fibers (GG_L), which, based on their orientation, are responsible for tongue protrusion.

Methods

Fine-wire electrode pairs were placed into the GG_V and GG_L in 11 patients with untreated OSA. Movement of the tongue during electrical stimulation at each site was videoed. The same electrodes were used to record EMG from both sites during respiratory stimulation by inspiratory loading and CO₂ rebreathing during wakefulness. During sleep, repetitive flow limitation events were induced with low-level CPAP to augment GG activity.

Results

In all participants, electrical stimulation of GG_L and GG_V protruded and retracted the tongue, respectively. Respiratory stimulation increased GG activity, but GG_V reached only 39 % and 23 % of peak GG_L activity during high resistive loading and PCO₂ of 65 mmHg, respectively. Flow limitation during sleep increased GG_L to levels that were considerably higher than awake baseline, but GG_V activity remained tonic or with minimal phasic activity, reaching on average 15 % of GG_L peak activity.

Conclusions

Our electrical stimulation findings indicate that GG_V is a tongue retractor and depressor. Tongue stimulation for OSA should avoid this area. The EMG results demonstrate that the anterior part of the GG is controlled very differently from the longitudinal protrusive fibers. The GG_V responses are similar to those previously found in tongue retractors and peri-pharyngeal muscles other than the GG, in which diminished activation during sleep is likely to be involved in the failure of increasing GG_L activity to alleviate flow limitation.