Modulation of parasympathetic and baroreceptor control of heart rate.

The heart rate is modulated from beat to beat by efferent vagal and sympathetic fibers, the former being the predominant mediators of the chronotropic influence of arterial baroreceptors and respiration and the latter being important in the cardiac responses to physical and mental stress. Cardiac vagal influences are modulated by a number of factors. These can be grouped as: 1) neural factors, such as the wakefulness-sleep cycle, the alerting reaction, and exercise; 2) humoral-pharmacological factors, such as angiotensin II, atrial natriuretic factor, cardiac glycosides; 3) normal aging; 4) a number of cardiovascular and other diseases, such as arterial hypertension, coronary artery disease, congestive heart failure and diabetes mellitus. The mechanisms underlying modulation of cardiac vagal control are not completely understood, the range of the possibilities including structural or functional alterations in baroreceptor afferents, in central and efferent vagal pathways and in cardiac responsiveness to neural stimuli. Irrespective of the mechanisms involved, the modulation of cardiac vagal control may have important implications for normal cardiovascular homeostasis, as well as for the pathophysiology, diagnosis and prognosis of various diseases.