Functional divergence of human genioglossus motor units with respiratory-related activity

The genioglossus muscle has at least two types of motor unit with respirato ry-related activity. Inspiratory motor units show phasic activity during in spiration, whereas inspiratory/expiratory motor units show phasic inspirato ry activity superimposed on tonic activity, The purpose of this study was t o investigate the physiological roles of these different genioglossus motor units. The unitary activities of 12 inspiratory and 12 inspiratory/expiratory moto r units were recorded using fine-wire electrodes during quiet nasal breathi ng in eight normal adult males. The mean interspike interval and the so of successive spikes were calculated for inspiratory and inspiratory/expirator y motor units, respectively. Scattergrams of the mean interspike interval v ersus so were constructed for the two groups of motor units. The effects of changes in head position on the firing activity and the patt erns of distribution of the mean interspike interval versus its so were sig nificantly different for inspiratory and inspiratory/expiratory motor units . These results suggest that the inspiratory and inspiratory/expiratory motor units have different functional roles in respiration; inspiratory motor un its may be phasically active to counteract intraluminal negative pressure d uring inspiration, whereas inspiratory/expiratory motor units may be tonica lly active to maintain tongue posture.