(USMLE topics) How heart rate is controlled by the parasympathetic and sympathetic divisions of the autonomic nervous system, with overview of baroreceptor resetting.

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Voice by: Sue Stern

Baroreflex, or baroreceptor reflex, is one of the mechanisms the body uses to maintain stable blood pressure levels or homeostasis. Baroreflex is a rapid negative feedback loop in which an elevated blood pressure causes heart rate and blood pressure to decrease. Reversely, a decrease in blood pressure leads to an increased heart rate, returning blood pressure to normal levels. The reflex starts with specialized neurons called baroreceptors. These are stretch receptors located in the wall of the aortic arch and carotid sinus. Increased blood pressure stretches the wall of the aorta and carotid arteries causing baroreceptors to fire action potentials at a higher than normal rate. These increased activities are sent via the vagus and glossopharyngeal nerves to the nucleus of the tractus solitarius – the NTS - in the brainstem. In response to increased baroreceptor impulses, the NTS activates the parasympathetic system – the PSNS - and inhibits the sympathetic system – the SNS. As the PSNS and SNS have opposing effects on blood pressures, PSNS activation and SNS inhibition work together in the same direction to maximize blood pressure reduction. Parasympathetic stimulation decreases heart rate by releasing acetylcholine which acts on the pacemaker cells of the SA node. Inhibition of the sympathetic division decreases heart rate, stroke volume and at the same time causes vasodilation of blood vessels. Together, these events rapidly bring DOWN blood pressure levels back to normal. When a person has a sudden drop in blood pressure, for example when standing up, the decreased blood pressure is sensed by baroreceptors as a decrease in tension. Baroreceptors fire at a lower than normal rate and the information is again transmitted to the NTS. The NTS reacts by inhibiting parasympathetic and activating sympathetic activities. The sympathetic system releases norepinephrine which acts on the SA node to increase heart rate; on cardiac myocytes to increase stroke volume and on smooth muscle cells of blood vessels to cause vasoconstriction. Together, these events rapidly bring UP blood pressure levels back to normal. Baroreflex is a short-term response to sudden changes of blood pressure resulted from everyday activities and emotional states. If hypertension or hypotension persists for a long period of time, the baroreceptors will reset to the “new normal” levels. In hypertensive patients for example, baroreflex mechanism is adjusted to a higher “normal” pressure and therefore MAINTAINS hypertension rather than suppresses it.

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