Ans. is C Nucleus of tractus solitarius o Baroreceptors are mechanoreceptors that are located in the adventia of carotid artery and aorta, at specialized locations called sinuses. Carotid sinus is a little bulge at the root of internal carotid artery, located just above the bifurcation of the common carotid artery. It is innerv ated by the sinus nerve, a branch of glossopharyngeal (IX cranial) nerve. Aortic arch (aortic sinus) also contains mechenoreceptors (stretch receptors) which are similar to carotid sinus receptors. However, their afferent nerve fibers travel in the aortic nerve, a branch of Vagus (X cranial) nerve. o The sinus nerve (from carotid sinus) and aortic nerve/vagal fibers (from aortic sinus) are together called 'Sino- aortic nerves'. They, together, are also refered to as 'Buffer nerves' because they are the afferents of cardiovascular reflexes that buffer abrupt changes in blood pressure. o Baroreceptors are highly sensitive to any change in mean blood pressure. Sinoaortic nerves (buffer nerves) normally discharge rhythmically, synchronous with the pressure fluctuation during systole and diastole. They respond to BP changes between 70 mm Hg and 150 mm Hg. When BP rises, baroreceptors are stimulated and their afferents (through sinoaortic nerves) stimulate nucleus of tractus solitarus (NTS) which inturn inhibits the pressor area of VMC, i.e., Rostral ventrolateral medula (RVLM). This results in decreased sympathetic outflow and therefore decreases in vasomotor tone and vasodilation. Vasodilation brings down the BP. Thereby helping hemostasis. Activated NTS also stimulates nucleus ambiguous (cardioinhibitory center) of medulla, which increases parasympathetic (vagal) output, through vagus, that decreases heart rate. Reduction in heart rate reduces the cardiac output, which also reduces BP. Baroreceptor stimulation also weekly inhibits respiration, o When BP falls, for instance while changing the posture from lying down to standing, reverse change takes place. When a person stands up, his blood is pooled in the veins of lowrer limbs by the effect of gravity. Central venous pressure and venous return decrease, which causes a fall in stroke volume. Hence the systolic BP falls. As a result, the discharge rate of baroreceptors decreases leading to a decrease in the inhibitory' influence on the pressor area of VMC. Hence vasomotor tone increases, leading to vasoconstriction, and consequently an increase in BP. Simultaneously, the nucleus ambiguous of the vagus is also inhibited, increasing the heart rate and consequently stroke volume and eventually BP. Thus fall in BP due to change of posture is very brief (Transient).