Cardiac receptors: normal and disturbed function.

Widely distributed throughout the heart is a network of fibers connected to the medullary cardiovascular centers by nonmedullated vagal afferent fibers. When the traffic in these fibers is interrupted by vagal cooling, and the input from the arterial baroreceptors is prevented, the arterial blood pressure increases. Thus, these receptors act to inhibit tonically the vasomotor center. The receptors in the atria alter their rate of discharge with changes in atrial transmural pressure and contractility and are most active during end-inspiration and early expiration when the transmural pressure is maximal. The receptors in the ventricles respond to changes in ventricular end-diastolic pressure (preload), to the pressure generated during systole (afterload) and to changes in ventricular contractility. The cardiac mechanoreceptors have an equal or greater effect on the renal bed than the arterial mechanoreceptors and this effect is enhanced by hypercapnia. In animals, the cardiac mechanoreceptors have less control of the muscle vessels than the arterial mechanoreceptors, but the reverse is true in man. Both the cardiac and arterial mechanoreceptors can modulate the output of renin from the kidney, but the cardiac mechanoreceptors are more sensitive to small changes in blood volume. During coronary occlusion, in association with the bulging of the ischemic myocardium, the rate of discharge of these cardiac receptors is greatly increased.