Central Mechanisms of Interaction Between Sympathetic Preganglionic Neurons and the Respiratory Oscillator

A great deal of experimental evidence suggests that the activities of the central nervous system (CNS) neurones controlling the cardiovascular system must be closely interrelated with the activity of neurones controlling respiration. Some of these interactions have been shown to occur entirely within the CNS (1 – 3). As a result of our studies on the firing of sympathetic preganglionic neurones (SPNs), simple neurophysi-ological mechanisms can be proposed to account for the central interaction between these two control systems. Our studies have been carried out in anaesthetized, paralyzed, artificially ventilated cats. We have observed the properties of firing patterns of single cervical nerve SPNs, their temporal relation to central inspiratory activity (CIA) and their modifications by procedures which alter CIA in a predictable manner. Monitoring of CIA was achieved, indirectly, by recording phrenic nerve activity. This work led to the identification of a large subpopulation of SPNs (approximately half of the total) composed of units which fire either exclusively in inspiration or throughout the respiratory cycle, but reach their peak firing frequency during inspiration. The units constituting this subpopulation can therefore be labelled “inspiratory” SPNs (4).