Diaphragm motor unit recruitment during ventilatory and nonventilatory behaviors

The forces generated by the cat diaphragm (DIA) during different ventilatory and nonventilatory behaviors were determined by measuring transdiaphragmatic pressures (Pdi). The Pdi generated during eupnea was only ~12% of the maximum Pdi (Pdi(max)) generated by bilateral phrenic nerve stimulation. When the animals breathed a gas mixture of 10% O₂ and 5% CO₂, the Pdi increased to ~28% of Pdi(max). During total airway occlusion, the Pdi generated by the diaphragm increased to ~49% of Pdi(max). Only during the gag reflex and sneezing did Pdi reach maximal levels. A model for diaphragm motor unit recruitment during these different behaviors was presented based on the proportion of different motor unit types within the diaphragm, the relative tetanic tensions produced by each unit type, and the assumption of an orderly pattern of motor unit recruitment.