heart rate, resulting in a tendency to lower blood pressure. By coupling sympathetic inhibition and parasympathetic activation, the baroreflex maximizes blood pressure reduction. Sympathetic inhibition leads to a drop in peripheral resistance, while parasympathetic activation leads to a depressed heart rate (reflex bradycardia) and contractility. The combined effects will dramatically decrease blood pressure. In a similar manner, sympathetic activation with parasympathetic inhibition allows the baroreflex to elevate blood pressure. Set point and tonic activation Baroreceptor firing has an inhibitory effect on sympathetic outflow. The sympathetic neurons fire at different rates which determines the release of norepinephrine onto cardiovascular targets. Norepinephrine constricts blood vessels to increase blood pressure. When baroreceptors are stretched (due to an increased blood pressure) their firing rate increases which in turn decreases the sympathetic outflow resulting in reduced norepinephrine and thus blood pressure. When the blood pressure is low, baroreceptor firing is reduced and this in turn results in augmented sympathetic outflow and increased norepinephrine release on the heart and blood vessels, increasing blood pressure. Effect on heart rate variability The baroreflex may be responsible for a part of the low-frequency component of heart rate variability, the so-called Mayer waves, at 0.1 Hz. Baroreflex activation therapy High blood pressure The baroreflex can be used to treat resistant hypertension. This stimulation is provided by a pacemaker-like device. While the devices appears to lower blood pressure, evidence remains very limited as of 2018. Heart failure The ability of baroreflex activation therapy to reduce sympathetic nerve activity suggests a potential in the treatment of chronic heart failure, because in this condition there is often intense sympathetic activation and patients with such sympathetic activation show a markedly increased risk of fatal arrhythmias and death. One trial has already shown that baroreflex activation therapy improves functional status, quality of life, exercise capacity and N-terminal pro-brain natriuretic peptide. See also Heart rate turbulence Valsalva maneuver References Autonomic nervous system Reflexes Cardiovascular physiology Vagus nerve Blood pressure Human homeostasis