Central Coordination of Respiratory and Cardiovascular Control in Mammals

Cardiorespiratory homeostasis in mammals requires the regulation of O2, CO2, and pH and is accomplished via central nervous system (CNS) control of two exquisite, constantly active pumping systems. The bidirectional res­ piratory pump (lung and associated skeletal musculature) moves air in and out of the alveoli, where gas exchange takes place. The unidirectional cardiovas­ cular pump (heart and vasculature) moves 02-enriched blood from the pul­ monary circulation to the systemic capillaries, where O2 diffuses into tissue and CO2 moves into blood; the systemic venous blood returns to the heart, then is pumped through the pulmonary circulation for gas exchange. The CNS controls all aspects of the respiratory pump, since skeletal muscle contracts only in response to motoneuronal activity, whereas the eNS controls the cardiovascular pump by modulating the pattern of cardiac and vascular smooth muscle contraction. The CNS also coordinates these pumps and participates in the optimization, adaptation, and adjustment of their perform­ ance. Understanding the CNS role in cardiorespiratory homeostasis requires knowledge of how (i) respiratory rhythm and sympathetic and parasympathet­ ic tone are generated; (ii) the spatiotemporal patterns of motor outflow are determined; (iii) these systems are coordinated, and; (iv) responses to changes in behavioral state or afferent signals are mediated. We focus on point (iii) and concentrate on brain stem and spinal mechanisms (for discussion of other points see 11,21,32,39,69).