Human hypoglossal motor unit activities in exercise

Authors


E. F. Bailey: Department of Physiology, College of Medicine, The University of Arizona, Tucson, AZ 85721-0093, USA.  Email: ebailey@u.arizona.edu

Key points

  • • Motor units (MUs) are the fundamental element of force generation. Here we document the MU activities of the airway dilator muscle genioglossus (GG) during breathing at rest and in heavy exercise.
  • • Based on our observations we suggest that tonic MU activity, albeit with varying degrees of respiratory modulation, is the preponderant activity in the GG motoneuron pool of healthy young adults at rest in the upright position.
  • • Second, we show that exercise-driven increases in respiratory drive are associated with increased GG electromyographic activity principally due to recruitment of previously silent MUs and to a shift in the firing behaviour of already active MUs to inspiratory phase-dependent bursting.
  • • Such expiration-related inhibition may arise secondary to an exercise-related central enhancement of motoneuron excitability that amplifies the response to afferent input.

Abstract  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.

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