Effects of increased ventilatory drive on motor unit firing rates in humaninspiratory muscles

This study was designed to determine whether increased neural drive increas es firing rates of inspiratory motoneurons uniformly in humans. The dischar ge of single motor units in the diaphragm, parasternal intercostal and scal ene muscles was recorded with monopolar electrodes. Ventilation was increas ed threefold with an external dead space. The discharge of 516 motor units was sampled in four subjects. All but 4 units increased their discharge rat e during inspiration with only 46 discharging tonically during expiration. With increased dead space, discharge frequencies of diaphragmatic motor uni ts increased from 11.0 +/- 2.7 to 17.7 +/- 3.3 Hz (mean +/- SD; p < 0.001). However, firing rates increased for parasternal intercostals from 10.0 rt 1.6 to only 11.9 +/- 1.9 Hz (p < 0.001), and for scalenes from 8.7 +/- 1.8 to only 9.5 +/- 1.2 Hz (p < 0.05). Proportionate increases in rib cage and abdominal expansion accompanied the increased ventilation with added dead s pace. These results suggest that previously reported predominant increase i n firing rates of diaphragmatic motor units in patients with chronic airflo w limitation reflects the normal response of respiratory motor output to in creased neural drive. The motoneuron pools of the parasternal intercostals and scalenes may show more prominent recruitment than frequency modulation.