Processing of pulmonary afferent input patterns by respiratory I-beta neurons.

To characterize the dynamics of the control of respiratory I-beta neurons by slowly adapting pulmonary stretch receptors, the neuronal discharge responses to lung inflation and electrically induced vagal input patterns were analyzed. Unitary recordings from single medullary I-beta neurons and whole phrenic nerve activity were recorded in chloralose-urethan-anesthetized paralyzed cats. Neuronal discharge patterns were quantified in terms of cycle-triggered histograms. The net response to a test afferent input pattern generated during neural inspiration was expressed as the difference between the central component of I-beta activity and the total response. The central component was obtained during control respiratory cycles in which lung inflation occurred during neural expiration and no vagal feedback occurred during neural inspiration. For a set of test inflations with different ramp rates, the net responses, measured at fixed times with respect to the onset of neural inspiration, were linearly related to transpulmonary pressure. However, the slopes of these relationships increased as a function of time during neural inspiration. Neuronal responses to electrically induced ramp vagal input patterns were similar to those produced by ramp inflation. The net response due to electrically induced ipsilateral step patterns consisted of a rapid excitatory and a slow inhibitory component, whereas only the slow inhibitory component was observed for contralateral patterns. The implications of these findings with respect to the modes of neural processing and effects on phrenic output patterns are discussed.