Breath-to-breath variability in hypoglossal motor unit firing

Instability in the magnitude and timing of motor output to pharyngeal dilator muscles occurs during breathing. This contributes to alterations in upper airway resistance, and is one of several factors that play a role in the pathophysiology of obstructive apneas. To define the motor unit mechanisms accounting for such variability, geniohyoid motor unit activity was recorded simultaneously with diaphragm EMG in anesthetized cats spontaneously breathing 7% CO2 in O2. Variability was quantified with the coefficient of variation [CV = (SD/mean) x 100%]. In this preparation, we confirmed greater breath-to-breath variability of geniohyoid compared to diaphragm peak moving average EMGs. During recordings of geniohyoid motor unit activity, average CV of other respiratory parameters were as follows: peak diaphragm EMG 5.8%, inspiratory time 3.5%, expiratory time 3.8%. The average CV for geniohyoid motor unit activity patterns were substantially higher: spikes per breath 15.6%, mean firing frequency 13.3%, peak firing frequency 19.0%, minimal firing frequency 26.3%, onset time 40.9%, offset time 10.0% and duration of firing 12.8%. Values differed considerably among motor units, even when activity was recorded simultaneously. These findings suggest that variability is present in both intensity and timing of geniohyoid motor unit firing during breathing, and that different geniohyoid motor units appear to have varying degrees of stability during breathing.