parasympathetic stimulation, it becomes more gradual. blockers like propranofol causes fall in HR

mulated, the gradience of diastolic depolarization (ab) becomes seeper and with parasympathetic stimulation, it becomes more gradual. blockers like propranofol causes fall in HR (as well as cardiac contractility). Thus propranolol causes reduction of cardiac metabolism reduction on the demands on coronary arteries. .'.In coronary heart disease (CHD), pro-pranol is a popular drug. Propranolol, obviously can reduce BP also. The Reflexes The reflexes regulating heart rale can be classified as follows: (1) Reflexes arising from the systemic arteries. (2) Those arising from the heart itself (3) Miscellaneous reflexes. (a) Reflexes arising from the systemic arteries are of two kinds - (l) baroreceptor reflexes, and (n) chemoreceptor reflexes. (b) Reflexes arising from the heart are also of two kinds, viz. (i) those arising from the atria] receptors, and (ii) those arising from the ventricles. Efferent limbs of all These reflexes are either the vagus (parasympathetic) or sympathetic nerves. These reflexes are catted cardiac reflexes (or better cardio vascular reflexes). Some of them have also been discussed elsewhere in this book (chap.8, sac: V) again, and the reader may consult them as the situation arises. Reflexes arising from the systemic arteries: Baroreceptor Reflex (fig. 5.6.3) : Two reflexes, in this connection, are most important. They are - (i) carotid sinus reflex, and (ii) Ihe reflex arising from The aortic arch. The mode of action and the characteristics of both The reflexes are same, Therefore, the physiologists include bolh of them under the same group, namely, 'smo aorlic mechanism' (or reflex). However, belween them, Ithe carotid sinus refles appears to be more important for the human beings, the aortic arch reflex enjoying only a secondary importance. Further, because of the fad tha! the carolid sinus is readily accessible, the carotid sinus reflex has been studied most extensively, the pioneers of the studies being such celebrated people like C.Heymans, J.Bouckaert and Eric Neil" in the 1930s. Receptors of this type of reflex are situated in the walls of arteries. When the blood pressure (henceforth abbreviated as, BP} within the artery rises, these receptors are compressed and thus stimulated, and the reflex is set up. As a result of initiation of Ihe reflex - (i) bradycardia and (ii) vasodilatatian develop, reducing the BP and Ihe horneoslasis is regained. As the receptors are sensitive to the pressure (the blood pressure), they are usually called 'barareceptors' or 'pressure receptors'. They are also catted 'strelch receptors'. Some aulhors call them 'meccano receptors.' Stnu aortic mechanism (see also chap 8 sec V, and 'baroreceptors' in chap 3 sec IV) Baroreceptors are present in the carotid sinus and aortic arch (fig 4.3.5). When the 6P rises, these baroreceptors are stimulated afferent traffic set up → these afferent impulses ultimately terminate in three places, (i) VMC (ii) nucleus ambiguus (ie, the origin of cardiac parasympathetic of vagus, fig.10A. 1. 2), (iii) respiratory center. When these baroreceptors are stimulated, (i) the VMC is depressed, (ii) nucleus amluguus is excited, and (iii) respiratory center depressed (this however is a weak action). Result is, if there is elevation of BP, (i) the sympathetic system is inhibited (ii) vagus stimulated and (in) respiration depressed.. They cause, ultimately, bratfycardia and vasodilatation and the elevated BP is corrected (BP homeostasis). Respiratory depression leads to reduction of 'pumping action of respiration' reduction of cardiac inflow reduction of cardiac outflow (via Frank Starling's mechanisms) fall of BP. However respiratory effect is seen only when there is sharp and acute rise of BP as in 'ad-renal") apnea' (chap 3, sec IV). ' Heymans subsequently received a Nobel Prize and Neil_ became an editor of Samson Wrights' renowned book on Physiology. The anatomical details are as follows : 1. Carotid sinus is a dilatalion present at the beginning of the internal carolid artery (fig.4.3.5). The baroreceplors are situated m the adventitia. Similar baroreceptors, called aortic arch baroreceptors, are situated in the arch of aorta (similar baroreceptors have been found also in subdavian and innominate arteries). 2 Afferent imposes from carotid sinus form sinus nerve sinus nerve joins IXth nerve (fig 4.3.5) IXth nerve nucleus tractus solilarius (NTS) to nucleus ambiguus and VMC. From nucleus ambiguus, the cardiac vagal fibers arise (fig. 10A. 1.2) and from VMC fibers arise to terminate on the spinal centers (lateral horns of the spinal cord) From the baroreceplors of the aortic arch, afferent impulses arise travel by aortic nerve join vagus nerve and travel up rest is as in carotid sinus. The paths of Ihese two reflexes may be shown in a campsite diagram as below (fig 5.4.3): Fig. 5. 4.3 Following points may be noted : (1) Nucleus tractus solrtanus (NTS) is a mass of gray matter, one on either side, present deep in the medulla (fig. 10A. 1.2). It is also an intermediate relay stalion of taste sensation (fig. IOB2. 5.8). (2) As slated above, the VMC has an intrinsic activity, ie. left to itself, it discharges excitatory impulses to the spinal center. .'. stimulation of bamreceptors causes depression of this intrinsic activity of VMC (3) From the above, it is apparent that stimulation of baro receptor causes stimulation of efferent vagal libers (parasympalhetic) (fig.5.4.3). Our normal BP is such that the sinuaortic mechanism remains active. Also, normally, our vagal fibers always exert a tonic effect (recall, atropmization causes tachvcardia unto 150 beats/min, see earlier). This reslmg vagal tone is almost entirely due to. activity of the sinuaortic ref