Am. J. Respir. Crit. Care Med., Volume 160, Number 5, November 1999, 1598-1603

Effects of Increased Ventilatory Drive on Motor Unit Firing Rates in Human Inspiratory Muscles

SIMON C. GANDEVIA, ROBERT B. GORMAN, DAVID K. MCKENZIE, and ANDRÉ DE TROYER

Prince of Wales Medical Research Institute, University of New South Wales, Sydney, Australia; and Laboratory of Cardiorespiratory Physiology, Chest Service, Erasme University Hospital, Brussels, Belgium

This study was designed to determine whether increased neural drive increases firing rates of inspiratory motoneurons uniformly in humans. The discharge of single motor units in the diaphragm, parasternal intercostal and scalene muscles was recorded with monopolar electrodes. Ventilation was increased threefold with an external dead space. The discharge of 516 motor units was sampled in four subjects. All but 4 units increased their discharge rate during inspiration with only 46 discharging tonically during expiration. With increased dead space, discharge frequencies of diaphragmatic motor units 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 ± 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 space. These results suggest that previously reported predominant increase in firing rates of diaphragmatic motor units in patients with chronic airflow limitation reflects the normal response of respiratory motor output to increased neural drive. The motoneuron pools of the parasternal intercostals and scalenes may show more prominent recruitment than frequency modulation. Gandevia SC, Gorman RB, McKenzie DK, De Troyer A. Effects of increased ventilatory drive on motor unit firing rates in human inspiratory muscles.

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