Central nervous mechanisms responsible for cardio-respiratory homeostasis.

Abstract

There is a body of data that indicates that the generation of respiratory activity involves the interplay of a limited number of distinct classes of respiratory neurones in the ventrolateral medulla [1]. Similarly the cardiovascular system is regulated by the activity of premotor and preganglionic neurones localised in an equivalent region of the CNS [2]. Cardio-respiratory homeostasis depends on the integration of the activity of the two systems and it could be envisaged that this was achieved by the action of two independent control systems that are loosely dependent on one another. Studies over the last decade have drawn attention, however, to the fact that the two systems are tightly coupled being sensitive to the same reflex inputs and participating in the expression of behavioral activities in a totally co-ordinated manner [3]. This has led to a recognition that rather than operating through independent control systems the CNS acts to maintain cardio-respiratory homeostasis by means of a single group of‘ cardio-respiratory’ neurones located in the ventrolateral medulla which regulate the activity of the two systems. This is achieved by generating complementary patterns of discharge of bulbospinal neurones that control respiratory motoneuronal and sympathetic preganglionic neuronal firing and through brainstem interneurones influencing the activity of vagal motoneurones that supply the heart, lungs and airways (and subsidiary muscles of respiration). This notion is not novel being firmly based in the 19th century literature but the experimental justification is only now emerging [4]. This results from an extensive electrophysiological analysis of the properties of the various motoneurones and interneurones in these networks, the nature of the reflex inputs impinging on them and the action of regions of the forebrain in generating changes in their activity.