Central nervous integration of cardiovascular control.

SUMMARY In this account an attempt has been made to identify integrative interactions in the control of the cardiovascular system. Three main sites of such interaction have been considered, the nucleus of the tractus solitarius (NTS), the vagal preganglionic supply to the heart and sympathetic pre-ganglionic neurones. In the case of the NTS the extent and range of afferent inputs from cardiovascular and respiratory receptors have been reviewed. In addition the interactions of these inputs on the activity of NTS neurones have been indicated although the details are as yet vague. With respect to the baroreceptor reflex, it is clear that its relay through the NTS permits the action of intrinsic drives, such as inspiratory activity, and extrinsic drives, for example the defence reaction, to modify to a greater, or lesser, extent the efficacy of the reflex. In the case of respiratory activity, changes in transmission of baroreceptor activity through the NTS are minimal, since a baroreceptor effect can be shown to exert itself on CVM activity throughout the respiratory cycle under appropriate experimental conditions (Mc-Allen & Spyer, 19786). The effectiveness of this excitatory input is, however, 'gated' by a direct inspiratory control of CVM activity. This indicates an essentially integrative role of vagal preganglionic neurones. The role of inputs from the lungs themselves evoked during inspiration, which also contribute to the respiratory modifications of the baroreceptor reflex, in the control of CVM activity is as yet uncertain although evidence is accumulating tosuggest that they act by a mechanism different from that of inspiratory drive. The influence of the defence reaction on transmission through the NTS has yet to be fully documented, but it appears as if there may be the potential for a marked modification. However, it is now certain that an inhibitory control of CVM activity can be evoked from the hypothalamus which is independent of any modification of the baroreceptor input at that level of the NTS and acts rather through an inhibitory synaptic influence directly onto CVMs. In much the same way the baroreceptor influence on sympathetic pre-ganglionic neurones is determined by their excitability, an excitability which is dependent on the balance of activity in several bulbospinal inhibitory and excitatory pathways, as well as segmental inputs. The role, if any, of direct hypothalamo-spinal pathways and the nature and organization of the descending pathways involved in the defence reaction requires eluci-dation. Existing data, however, make it clear …