Three groups of receptors in the heart are activated by changes in pressure in the cardiac chambers. Those at the venous-atrial junctions with myelinated vagal afferent nerves indicate changes in heart rate and degree of atrial filling. A second group, present in all the cardiac chambers, served by unmyelinated vagal afferent nerves, signals changes in ventricular preload, afterload and cardiac contractility. A third group, also present in all the cardiac chambers, has both myelinated and unmyelinated afferent nerves that pass to the spinal cord. Their normal function is unknown. Abnormal activation of the cardiac mechanoreceptors during myocardial ischemia may be important in the genesis of life-threatening arrhythmias.