The neural mechanisms in controlling blood pressure include baroreceptor and chemoreceptor reflex arcs and influence from hypothalamus and other areas of the limbic system. The blood pressure regulatory reflex arcs include peripheral receptors -- baroreceptors and chemoreceptors; afferent fibers from receptors to integrating centers in the medulla oblongata of the rain stem; medullary centers cardiovascular and vasomotor centers an sympathetic and parasympathetic fibers to cardiac and vascular smooth muscle. higher centers for instance anterior and posterior hypothalamic nuclei also influence blood pressure by inputs into the medullary centers. The influence of these mechanisms on blood pressure is mediated largely changing vasomotor tone , peripheral resistance, cardiac output and heart rate.
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The reflex arcs.; The baroreceptor reflex arc. The baroreceptor reflex arc is faster and more important than the chemoreceptor reflex arc Components i. Baroreceptors. These are pressure or stretch receptors that are located in the carotid sinuses of the carotid arteries that supply blood to the brain, in the aortic arch, and in other large arteries of the neck and thorax.. When arterial walls are stretched these mechanoreceptors respond by firing off streams of impulses that are carried to medullary cardiovascular center in the brain stem( in medulla oblongata).. ii. Medullary centers. The cardiovascular center includes the vasomotor center and the cardiac centers. "The cardiac centers include the cardiac acceleratory center and the cardiac inhibitory center. Vasomotor center responses: Stimuli from the baroreceptors stimulate the cardioinhibitory center and inhibit the vasomotor center and the cardioacceleratory center. The consequence is that blood vessels relax and the blood pressure decreases. This happens because inhibition of the vasomotor constriction allows the blood vessels to relax and resistance to decrease . There is also an effect on veins. Relaxation of venous smooth muscle permits veins to dilate , and more blood to accumulate in venous reservoirs, This last effect decreases venous return. The result is that cardiac output (CO) drops and so blood pressure decreases. Cardiac center responses Impulses sent from baroreceptors to the cardiac centers also affect blood pressure. These impulse affect both sympathetic and parasympathetic responses: sympathetic nerve activity increases and parasympathetic activity increases.. The consequences are that the heart rate falls and the contractile force of the heart beat decreases ; also CO falls and blood pressure (MAP) decreases.. Regulation in the opposite direction operates in the following way. If mean arterial pressure(MAP) decreases, reflex vasoconstriction follows.This has the effect of increasing CO and restoring blood pressure to normal level. The baroreceptors of the cardiac sinus are of special importance because they protect the blood supply to the brain. The aortic baroreceptors are responsible for assuring adequate systemic blood flow to the rest of the body Chemoreceptors are f less importance in short-term control of blood pressure than baroreceptors.. The former respond to increases in blood carbon dioxide (CO2)levels, decreases in oxygen levels or the lowering of blood pH (increased H+ concentration). Chemoreceptors are located on the aortic arch and in large arteries of the neck ( including the carotids). Chemoreceptor impulses sent to the cardioacceleratory center have the effect of increasing cardiac output. Increased carbon dioxides sets of reflex vasoconstriction; this raises blood pressure and increases the speed of return of blood to the heart and lungs. This naturally increases the speed of gaseous exchange in the lungs. This is abetted by the effect of chemoreceptors on respiratory the rate(RR). Higher Neural Centers While medullary centers of the brain stem are primarily responsible for integrating impulses from the blood pressure regulatory reflex arc receptors, higher neural centers can influence blood pressure . The the influence of hypothalamic nuclei and limbic system nuclei ( amygdala ) are not routine but can be profound, especially in emergencies. The effect of the hypothalamus is evident in threat situations in mediating blood flow changes associated with the flight or fight response. Higher center influences also affect blood pressure changes in circumstances of anxiety. In addition, the hypothalamus influences transfer and redirection of cardiac flow in vigorous exercise ( to skeletal muscles) or in body temperature changes to cool or warm the body by greater peripheral ( skin) flow or greater deep internal circulation to combat a fall in core temperature.