Inhibition of expiratory muscle EMG and motor unit activity during augmented breaths in cats

To test the hypothesis that expiratory muscle activity is reduced during augmented breaths, electromyographic activity (EMG) of the triangularis sterni (TS) was recorded from eight pentobarbital anesthetized cats. Augmented breaths significantly increased tidal volume and peak diaphragm EMG, and prolonged inspiratory time and the first phase of expiration. However, the duration of the second phase of expiration was unchanged. Peak TS EMG was reduced during sighs in all animals, from 25 +/- 5 to 12 +/- 2 arbitrary units (P less than 0.005). Furthermore, the onset of TS activity during expiration was significantly delayed during augmented breaths (P less than 0.002), whereas the duration of expiratory firing tended to decrease but not significantly. Electrical activity was recorded from eight motor units of the TS in five cats. During resting breathing the motor units had a mean relative expiratory onset time of 46 +/- 4% of expiration, and a mean firing frequency of 19 +/- 2 impulses/sec. Two motor units became quiescent during augmented breaths. Of the remaining six motor units, three minimally shortened their duration of activity (by less than 15%) while three substantially abbreviated their period of firing (by 50% or more). In addition, all TS motor units reduced their mean firing frequency (P less than 0.05) and number of impulses per breath (P less than 0.002) during sighs. We conclude that expiratory activity of the triangularis sterni muscle is reduced during augmented breaths, due to a combination of motor unit derecruitment and a slowing of motor unit firing frequency.