Motor unit territories in human genioglossus estimated with multichannel intramuscular electrodes

Abstract

The discharge patterns of genioglossus motor units during breathing have been well-characterized in previous studies, but their localization and territories are not known. In this study, we used two newly developed intramuscular multichannel electrodes to estimate the territories of genioglossus motor units in the anterior and posterior regions of the muscle. Seven healthy men participated. Each electrode contained fifteen bipolar channels, separated by 1 mm, and was inserted percutaneously below the chin, perpendicular to the skin, to a depth of 36 mm. Single motor unit activity was recorded with subjects awake, supine, and breathing quietly through a nasal mask for 180 s. Motor unit territories were estimated from the spike-triggered averages of the electromyographic signal from each channel. A total of 30 motor units were identified: 22 expiratory tonic, 1 expiratory phasic, 2 tonic, 3 inspiratory tonic, and 2 inspiratory phasic. Motor units appeared to be clustered based on unit type, with peak activities for expiratory units predominantly located in the anterior and superficial fibers of genioglossus and inspiratory units in the posterior region. Of these motor unit types, expiratory tonic units had the largest estimated territory, a mean 11.3 mm (SD 1.9). Estimated territories of inspiratory motor units ranged from 3 to 6 mm. In accordance with the distribution of motor unit types, the estimated territory of genioglossus motor units varied along the sagittal plane, decreasing from anterior to posterior. Our findings suggest that genioglossus motor units have large territories relative to the cross-sectional size of the muscle. NEW & NOTEWORTHY In this study, we used a new multichannel intramuscular electrode to address a fundamental property of human genioglossus motor units. We describe the territory of genioglossus motor units in the anterior and posterior regions of the muscle and show a decrease in territory size from anterior to posterior and that expiratory-related motor units have larger estimated territories than inspiratory-related motor units.

Regional respiratory movement of the tongue is coordinated during wakefulness and is larger in severe obstructive sleep apnoea

“…This compartmentalisation of the genioglossus has been supported by evidence of regional variation in neural drive in awake healthy adults during breathing (Vranish & Bailey, 2015;Luu et al 2018). Luu et al (2018) showed that peak inspiratory motor unit activity predominantly occurs in the posterior region of the genioglossus, although only a few inspiratory units were recorded. Vranish & Bailey (2015) reported a larger electromyographic signal (EMG) in the posterior muscle regions than the anterior compartments without differential activation between the regions.…”

“…Morphologically, these compartments have muscle fascicles that course posteriorly from their insertion into the mandible either horizontally or fan out obliquely (Sanders & Mu, 2013), and we adopt this terminology in our study. Furthermore, the posterior and anterior compartments of the genioglossus are also innervated by different branches of the hypoglossal nerve (Mu & Sanders, 2010) and they may be composed of muscle fibres arranged in series (Sanders & Mu, 2013;Luu et al 2018). This compartmentalisation of the genioglossus has been supported by evidence of regional variation in neural drive in awake healthy adults during breathing (Vranish & Bailey, 2015;Luu et al 2018).…”

“…Furthermore, the posterior and anterior compartments of the genioglossus are also innervated by different branches of the hypoglossal nerve (Mu & Sanders, 2010) and they may be composed of muscle fibres arranged in series (Sanders & Mu, 2013;Luu et al 2018). This compartmentalisation of the genioglossus has been supported by evidence of regional variation in neural drive in awake healthy adults during breathing (Vranish & Bailey, 2015;Luu et al 2018). Luu et al (2018) showed that peak inspiratory motor unit activity predominantly occurs in the posterior region of the genioglossus, although only a few inspiratory units were recorded.…”

Regional activation in the human longissimus thoracis pars lumborum muscle

“…human muscles. Héroux et al (2015) and Luu et al (2018) showed a mostly uniform distribution of muscle fibres belonging to a MU in the medial gastrocnemius and in the genioglossus, respectively. The defined MU territories we observed in the human longissimus pars lumborum are in line with findings in the cat where stimulation of a nerve branch innervating the longissimus generated a localized action potential spanning mainly adjacent segments (Wada et al 2003;Durbaba et al 2007).…”

“…MUs with a minimum of 300 action potentials during the lumbar isometric contractions were considered to compute spike-triggered averages in the fine-wire multi-unit EMG (Farina et al 2008;Héroux et al 2015). We determined the spatial localization of a single MU by evaluating the fine-wire recording sites where the spike-triggered fine-wire EMG average crossed a threshold value of ±4 standard deviations (SD) of the baseline activity computed in a 25 ms window (−35 ms to −10 ms) prior to the target MU spike time (Héroux et al 2015;Luu et al 2018) (Fig. 2).…”