The genioglossus (GG) is considered the principle protrudor muscle of the human tongue. Unlike most skeletal muscles, GG electromyographic (EMG) activities are robustly preserved in sleep and thus may fulfill a critical role in preserving airway patency. Previous studies in human subjects also confirm that the GG EMG increases in response to chemoreceptor and mechanoreceptor stimulation. This increase occurs secondary to the recruitment of previously inactive motor units (MUs) and/or an increase in firing rate of already active MUs. Which strategy the nervous system uses when the synaptic drive onto GG motoneurons increases is not known. Here we report on GG whole muscle and tonic MU activities under conditions that mimic sleep, i.e., mild-moderate elevations in CO(2) (3% inspired CO(2) or the addition of a 1.0 l dead space) and elevated airway resistance. Based on previous work in rat, we hypothesized that mild hypercapnia would increase the firing rates of tonic MUs and that these effects would be further potentiated by a modest increase in airway resistance. Fine wire and tungsten microelectrodes were inserted into the GG to record whole muscle and single MU activities in 21 subjects (13 women, 8 men; 20-55 yr). Either 3% inspired CO(2) or added dead space resulted in a 200-300% increase in the amplitude of both tonic and phasic components of the whole muscle GG EMG and a doubling of minute ventilation. Despite these changes, recordings obtained from a total of 84 tonically discharging GG single MUs provide no evidence of a change in firing rate under any of the conditions. On this basis we conclude that in healthy adults, the increase in the tonic component of the whole muscle GG EMG secondary to mild hypercapnia is due almost exclusively to the recruitment of previously inactive MUs.
The muscular-hydrostat model of tongue function proposes a constant interaction of extrinsic (external bony attachment, insertion into base of tongue) and intrinsic (origin and insertion within the tongue) tongue muscles in all tongue movements (Kier WM and Smith KK. Zool J Linn Soc 83: 207-324, 1985). Yet, research that examines the respiratory-related effects of tongue function in mammals continues to focus almost exclusively on the respiratory control and function of the extrinsic tongue protrusor muscle, the genioglossus muscle. The respiratory control and function of the intrinsic tongue muscles are unknown. Our purpose was to determine whether intrinsic tongue muscles have a respiration-related activity pattern and whether intrinsic tongue muscles are coactivated with extrinsic tongue muscles in response to respiratory-related sensory stimuli. Esophageal pressure and electromyographic (EMG) activity of an extrinsic tongue muscle (hyoglossus), an intrinsic tongue muscle (superior longitudinal), and an external intercostal muscle were studied in anesthetized, tracheotomized, spontaneously breathing rats. Mean inspiratory EMG activity was compared at five levels of inspired CO2. Intrinsic tongue muscles were often quiescent during eupnea but active during hypercapnia, whereas extrinsic tongue muscles were active in both eupnea and hypercapnia. During hypercapnia, the activities of the airway muscles were largely coincident, although the onset of extrinsic muscle activity generally preceded the onset of intrinsic muscle activation. Our findings provide evidence, in an in vivo rodent preparation, of respiratory modulation of motoneurons supplying intrinsic tongue muscles. Distinctions noted between intrinsic and extrinsic activities could be due to differences in motoneuron properties or the central, respiration-related control of each motoneuron population.
The genioglossus (GG) muscle is considered the principal protruder muscle of the tongue that dilates and stiffens the pharyngeal airway. We recorded whole muscle and single motor unit (MU) activities in healthy adults performing progressive intensity exercise on a cycle ergometer. Tungsten microelectrodes were inserted percutaneously into the GG of 11 subjects (20-40 years) to record electromyographic (EMG) activities and pulmonary ventilation (VI) at rest and at workload increments up to 300 W. Increases in respiratory drive were associated with increases in VI, mean inspiratory flow (Vt/Ti) and tonic and phasic components of the GG EMG activity. In contrast, individual MUs typically showed expiration-related decreases in firing as exercise intensity increased. We suggest the decrease in MU activity may occur secondary to afferent feedback from lungs/chest wall and that compensation for more negative inspiratory airway pressures generated during heavy exercise occurs primarily via recruitment of previously silent MUs.
Inspiratory muscle training holds promise as a non-pharmacologic treatment that can improve respiratory muscle strength, reduce blood pressure, and improve autonomic balance in hypertensive patients. There is a gap in knowledge regarding the specific respiratory stimulus that gives rise to these favorable outcomes. We implemented five respiratory training protocols that differed in the magnitude and direction of the lung volumes and/or intrathoracic pressures generated by subjects in training. Normotensive adults were randomly assigned to each group and trained daily for 6 weeks. Pre-post and weekly measures of blood pressure showed significant declines in systolic [-8.96 mmHg (95% CI, 7.39-10.53)] and diastolic [-5.25 mmHg (95% CI, 3.67-6.83)] blood pressures for subjects who trained with large positive or negative intrathoracic pressures. Subjects who trained with modest intrathoracic pressures or large lung volumes saw no improvement in blood pressure (P > 0.3). Large intra-thoracic pressures are the specific respiratory stimulus underpinning breathing training related improvements in blood pressure.
What is the central question of this study? What impact does inspiratory muscle training have on systemic vascular resistance, cardiac output and baroreflex sensitivity in adult men and women? What is the main finding and its importance? Inspiratory muscle training exerts favorable effects on blood pressure, vascular resistance and perception of stress. This exercise format is well-tolerated and equally effective whether implemented in men or women.