1.4.2 Autonomic control of the cardiovascular system
The cardiovascular system is under control of both neuronal and humoral components of the ANS. The system acts both on the heart itself and on the peripheral vasculature. The principal aim of the autonomic control of the carsystem is to maintain blood supply to vital organs, particularly the brain. This requires adequate blood pressure, which can be maintained in the short term by altering systemic resistance and cardiac output. In the longer term, blood pressure is controlled by adjusting blood volume via a complex homeomechanism involving the hypothalamus, pituitary gland, adrenal gland and kidney.
The heart is innervated by the sympathetic and the parasympathetic nervous system. Parasympathetic fibres are carried in the vagus nerve and their discharge results in a decreased heart rate and, to a lesser extent, reduced contractility. The sympathetic nervous system acts on the heart via direct neuronal control and also via the release of adrenaline and noradrenaline, mediated by beta receptors. This results in an increased heart rate and increased myocardial contractility. The cardiac centre in the medulla controls both the central and peripheral circulatory system via the ANS. Information on blood pressure is fed to the brain from baroreceptors in the ventricles, aortic arch and carotid bodies. The pattern of autonomic control of the heart is shown in Figure 1.6.
Fig 1.5 The phases of the cardiac cycle, A-F (see text)