The graded response to stimulation of medullary respiratory neurons

The discharge activity of medullary respiratory neurons was measured concurrently with pulmonary ventilation in decerebrated and anesthetized cats. Resting values in animals breathing room air were compared with responses evoked by hypercapnia, hypoxia and hyperoxia. During CO 2 stimulation, changes in total spike output of both inspiratory and expiratory neurons directly correlated with changes in minute ventilation and the impulse frequency of the spike discharge correlated with tidal volume. Recruitment was a minor component of graded medullary respiratory activity, and the number of active respiratory units remained nearly constant over wide ranges of ventilation from near apnea to substantial hyperpnea. In individual neurons, the frequency response to different levels of stimulation was graded but often nonlinear and its magnitude differed from cell to cell. Some neurons discharged with maximal frequency at relatively low levels of ventilation whereas others augmented their frequency only as much larger tidal volumes were produced. Hypoxia induced less consistent neuronal responses than did either hypercapnia or hyperoxia, and depressed expiratory units. The vagus influenced equally the number of impulses discharged by both inspiratory an expiratory neurons and modified the graded response chiefly by limiting the duration of each burst of impulses and the number of impulses in the burst. It appears that the medullary respiratory neuron population regulates the magnitude of ventilation primarily by changes in the impulse frequency and total number of discharges of already active cells rather than by increasing or decreasing the number of cells actively discharging during each breath.