The Central Nervous System Feedback System and Blood Pressure
Monday, Dec 10 2007
The brain and circulatory system also make use of important feedback systems that assist in directing which elements of the neuroendocrine, renal, or autonomic nervous systems should be activated (see baroreceptor feedback system in Figure 1.2). Located along portions of the carotid artery and the aorta are several hundred pressure detectors called baroreceptors. These specialized cells operate like ‘stretch’ detectors that respond to alterations in blood pressure in the arteries and send neural signals to the brainstem to inform the brain which component of the autonomic nervous system should fire (Kezdi, 1977). For a properly functioning baroreceptor, increased blood pressure should lead to firing the parasympathetic nervous system and an immediate drop in heart rate. To the contrary, inadequate blood pressure should result in a signal to increase either heart rate or vasoconstriction via the sympathetic nervous system.
There are various elements of the central nervous system involved in this feedback system, including the brainstem, the hypothalamus, and other components of the limbic system. It is known that activation of baroreceptors results in activity in the upper medulla called the nucleus of tractus solitarius, which is a portion of the brain that regulates vagal tone (Lovallo, 1997).
Stimulation of this group of neurons leads to increased parasympathetic activity, and deactivation results in labile blood pressure (DeJong et al., 1977). Blood pressure is also regulated by cortical mechanisms other than those in the brainstem. For example, stimulation of parts of the hypothalamus and other portions of the limbic system has been shown to result in blood pressure responses (Folkow and Van Euler, 1954; Gebber and Klevans, 1972).
Figure 1.2. Major physiological systems involved in the regulation of blood pressure (dotted arrows represent local blood cell autoregulation; solid arrows represent neural influences; dashed arrows represent neuroendocrine influences; SNS = sympathetic nervous system).
In contrast to the relatively few mechanisms influencing water pressure in a hydraulic water-pumping system, it should be evident that a multitude of factors interact with one another to influence blood pressure. Not only do the factors outlined above contribute to an organism’s current blood pressure, but they also influence one another (for example, the hypothalamus affects kidney function via the neuroendocrine system), and these interactive effects further complicate the identification of mechanisms causing a specific elevation in blood pressure. Given the integrated system outlined, it is possible that no single mechanism is responsible for conditions of high blood pressure for every hypertensive patient. Perhaps different physiological mechanisms involved in the regulatory system result in dysfunctions in blood pressure regulation among different patients. Elevated blood pressure, then, may represent the final common pathway that results from a variety of underlying physiological disturbances.
Larkin, K. T., and Zayfert, C.
Published with assistance from the foundation established in memory of Amasa Stone Mather of the Class of 1907, Yale College.
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Last revised: by Dr. J. Gregory Frits, , M.D.
Provided by Armina Hypertension Association
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