Expiratory bulbospinal neurons of dogs. I. Control of discharge patterns by pulmonary stretch receptors.

To better understand expiratory bulbospinal (EBS) neuronal processing of afferent patterns from slowly adapting pulmonary stretch receptors (PSRs), the responses of these caudal medullary neurons to various inflation patterns were analyzed in thiopental sodium-anesthetized paralyzed dogs. Normocapnic ventilation was obtained from lung inflations delivered during the neural inspiratory phase of control respiratory cycles by a solenoid ventilator triggered by the onset of phrenic nerve activity. Test inflation patterns, delivered during the neural expiratory (E) phase of specific cycles, were separated by 6-10 control cycles and consisted of slow augmenting and decrementing ramps and steps. Transpulmonary pressure (Pt) was used as an index of PSR activity, and the discharge patterns of single EBS neurons were quantified using cycle-triggered histograms. Based on the time course of the discharge pattern during no-inflation cycles, two types of neurons were identified. Type D, the most common, exhibited a decrementing-like pattern, while type A exhibited an augmenting-plateau pattern. Plots of discharge frequency vs. Pt for type D neurons consisted of a linear excitatory phase (1 less than Pt less than 4.6 mmHg) and a linear inhibitory phase (4.6 less than Pt less than 12 mmHg). Type A neurons exhibited only a linear inhibitory phase for 3.3 less than Pt less than 12 mmHg. Because the responses to step inflations (4 s duration) exhibited only a small amount of time dependency, and the activation Pt of PSRs, but not of the other pulmonary receptors with vagal fibers, is in the proper range, it appears that PSRs may mediate both excitatory and inhibitory components of the neuronal response. These data suggest that discharge patterns of the caudal EBS neurons in dogs are highly dependent on the time course of Pt in a predictable manner, and implicate the importance of instantaneous lung volume in the control of the E musculature and the optimization of the breathing pattern.