Activities of human genioglossus motor units

Abstract

Upper airway muscles play an important role in regulating airway lumen and in increasing the ability of the pharynx to remain patent in the face of subatmospheric intraluminal pressures produced during inspiration. Due to the considerable technical challenges associated with recording from muscles of the upper airway, much of the experimental work conducted in human subjects has centered on recording respiratory-related activities of the extrinsic tongue protudor muscle, the genioglossus (GG). The GG is one of eight muscles that invest the human tongue (Abd-El-Malek, 1939). All eight muscles are innervated by the hypoglossal nerve (cranial nerve XII) the cell bodies of which are located in the hypoglossal motor nucleus (HMN) of the caudal medulla. Much of the earlier work on the respiratory-related activity of XII motoneurons was based on recordings obtained from single motor axons dissected from the whole XII nerve or from whole muscle GG EMG recordings. Detailed information regarding respiratory-related GG motor unit activities was lacking until as recently as 2006. This paper examines key findings that have emerged from the last decade of work conducted in human subjects. Wherever appropriate, these results are compared with results obtained from in vitro and in vivo studies conducted in non-human mammals. The review is written with the objective of facilitating some discussion and some new thoughts regarding future research directions. The material is framed around four topics: (a) motor unit type, (b) rate coding and recruitment, (c) motor unit activity patterns, and (d) a compartment based view of pharyngeal airway control.

Figure 1

Parasagittal section through portion of the head approximately 3 mm from midline. The tongue is actively reflected. Per oral and per cutaneous needle insertion into the substance of the geniolgossus muscle. Abbreviations: Gh, geniohyoid; H, hyoid bone; W, insulated wire of bipolar recording electrode; Ps, lica sublingualis with underlying submandibular duct. (From Sauerland et al. 1981, Electromyography and Clinical Neurophysiology).

Figure 2

Raw data from 1 subject demonstrating decrement from wakefulness to non-rapid-eye-movement (NREM) sleep in tonic tensor palatini moving time average (MTA) electromyogram (EMG) and masseter MTA EMG. Peak phasic and tonic genioglossus and diaphragm MTA EMG are well maintained. EEG, electroencephalogram. (From Tangel et al., 1993 J. App. Physiol., used with permission of the American Physiologic Society).

Figure 3

Alterations in discharge patterns on a hypoglossal fiber from inspiratory-expiratory to tonic by hypoxia. Phr, phrenic activity; Int Phr, integral of phrenic activity. FETCO₂ and FETO₂, end-tidal fractional concentrations of CO₂ and O_2. (From Hwang et al., 1983 J. App. Physiol., used with permission of the American Physiologic Society).

Figure 4

Representative raw recordings of a MU with a phasic–tonic discharge pattern. In wakefulness, these MUs discharged maximally in inspiration falling silent in late expiration. In NREM sleep discharge persisted throughout the respiratory cycle. Top trace: instantaneous firing rate (dots). Middle trace: discriminated motor unit potentials,and insets show overlay of 10 consecutive potentials of the identified unit from each trial. Bottom traces: intramuscular whole muscle GG electromyogram (EMG), central electroencephalogram (EEG) recording site (C4/A1), right outer canthus (ROC) electrooculogram (EOG), and lung volume (sum of ribcage and abdomen volumes) signals. (From Bailey et al., 2007 J. Neurophysiology, used with permission of the American Physiologic Society).

Figure 5

Frequency histograms of the population of GG motor units (MUs) as a function of the consistency and strength of their respiratory activity as estimated by η², in wakefulness (n-81) and NREM sleep (n=64). (From Bailey et al., 2007 J. Neurophysiology, used with permission of the American Physiologic Society).