Fig. 5.

The time and frequency plots. A–H: each individual unit in each condition. Each horizontal line represents the firing of a genioglossus single motor unit (591 in total). Vertical dotted lines represent the onset (0%) and end (100%) of inspiration. For each motor unit, the onset time (small colored circle on left) of firing is joined by a thick horizontal line to its end discharge time (small colored circle on right). The color of the circles indicates the onset and end firing frequencies (see inset). The mean peak firing frequency is indicated by the color of the thick horizontal line, and the time of the peak firing frequency is indicated by a black circle. For units that discharged throughout both phases of the respiratory cycle, a thin horizontal line is colored to code the tonic firing frequency. Units are ordered in each category (phasic or tonic), according to their onset discharge time. With hypercapnic stimulation, the peak discharge rate for the population is increased, and more single motor units were activated (63 vs. 93; both P < 0.01). A greater percentage of the inspiratory phasic motor units was activated before the onset of flow 0% Ti (20%, P = 0.02) and continued to discharge after 75% Ti (60%, P < 0.001). The stepwise application of CPAP led to reduced discharge frequencies for the population at 6, 8, and 10 cmH₂O (P < 0.01). The firing times were also markedly altered under CPAP with a 22% Ti decrease in the duration of discharge at 8 cmH₂O. The onset firing times of these units that were preinspiratory with CO₂ (−3.7% Ti) were delayed with the application of 8-cmH₂O CPAP to postinspiratory activation (7.7% Ti).