Effects on the Intact Cardiovascular System

An increase in sympathetic neuronal activity causes an increase in heart rate (positive chronotropic effect, or tachycardia) and an increase in cardiac contractile force (positive inotropic effect) such that the stroke output is increased. Cardiac output, which is a function of rate and stroke output, is thus increased. A physiological increase in sympathetic tone is almost always accompanied by a diminution of parasympathetic vagal tone; this allows full expression of the effects of increased sympathetic tone on the activity of the heart.

An increase in sympathetic tone constricts blood vessels in most vascular beds and therefore causes a net increase in total peripheral resistance. Increased sympathetic tone increases neural release of norepinephrine and its interaction both with p-adrenoceptors on cardiac cells and with a-adrenoceptors on vascular smooth muscle cells. As a consequence, the systolic and diastolic blood pressures are elevated. It follows that the mean arterial blood pressure must also be increased.

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