Abstract
There is little information available regarding how respiratory-related activity of the genioglossus motor unit is modulated in various head positions. The present study was undertaken to investigate what type of genioglossus motor unit contributes to the respiratory-related activity of the genioglossus muscle and how its activity is modulated with various head positions in 8 awake adult males. Single genioglossus motor unit activity and nasal airflow were simultaneously recorded in the natural head position during nasal breathing. Forty-five genioglossus motor units were classified into three types: inspiratory motor unit, inspiratory-expiratory motor unit and non-respiratory motor unit. The duration and instantaneous firing frequency of the inspiratory motor unit during inspiration significantly increased in the 30° headup position while it decreased in the 30° head-down position (p<0.05). The intantaneous firing frequency of the inspiratory-expiratory motor unit also significantly increased in the 30° head-up position while it decreased in the 30° head-down position (p<0.05), however, there was no significant difference between respiratory phases. These findings suggest that there are two types of respiratory-related genioglossus motor unit, which show different responses to various head positions. These genioglossus motor units may alleviate tongue collapse in altered head positions by different mechanisms.
