Baroreceptors are specialized sensory neurons that enable the central nervous system (CNS) to maintain short-term control of blood pressure. These mechanoreceptors participate in a reflex (baroreceptor reflex, carotid sinus reflex) that regulates the mean arterial pressure, relatively constant at a preset value, usually around 100 mm Hg. In this negative feedback loop, a rise in blood pressure from baseline results in rapid signals from the baroreceptors to the CNS which then reduces MAP back down to normal level through the autonomic nervous system. A slight change in pressure causes a strong change in the baroreflex signal to readjust arterial pressure back toward normal. The arterial baroreceptor reflex serves as short-term blood pressure buffering system in response to relatively abrupt changes in blood volume, cardiac output, or peripheral resistance, such as during daily activities (posture changes, exercise) and during surgery (anesthesia, hemorrhage).
COMPONENTS OF THE BARORECEPTOR REFLEX
Baroreceptors are sensory neurons that can be divided into two types. High-pressure arterial baroreceptors are found clustered in abundance within the adventitia of the carotid sinus and in the aortic arch. The carotid sinus is the dilated root of the internal carotid artery, typically found where the common carotid artery bifurcates into the internal and external carotid arteries. These receptors participate in the classically described negative feedback reflex. In contrast, low-pressure cardiopulmonary baroreceptors are located in the right atrium (near the entrance of superior and inferior vena cavae) and left atrium (near the entrance of pulmonary veins). Unlike their counterparts in the carotid sinus, volume distension near these nerve endings will yield an increase in neuronal discharge.
The response rate of baroreceptors to changes in arterial blood pressure is rapid. They are mechanoreceptors with specialized nerve endings that get excited by stretch. Carotid sinus baroreceptors are not at all stimulated by pressures between 0 and 60 mm Hg. An increase in blood pressure causes stretching and distortion of the vascular wall, which is sensed by the baroreceptor’s specialized nerve endings. This will increase axonal depolarization and the frequency of action potential firing. As arterial pressure rises, impulse transmission progressively increases to a ceiling of around 180 mm Hg. Aortic arch baroreceptors are less sensitive than those in the carotid sinus and respond in a similar manner but function at pressure levels of about 30 mm Hg higher. Both types of baroreceptors can detect not only the rise of arterial pressure, but also the rate of change in pressure with each beat. Baroreceptors have higher impulse discharge rates when blood pressure increases rapidly as opposed to a simply stationary higher MAP (Figure 164-1).
Baroreceptor activity versus arterial blood pressure. (Reproduced with permission from Hall JE, Guyton AC. Guyton and Hall Textbook of Medical Physiology, 12th ed. Philadelphia, PA: Saunders/Elsevier; 2011.)