Regulation of heart rate

How heart rate is related with other physiological factors

1. REGULATION OF HEART RATE GOVERDHAN.R

2. HEART Normal heart rate is 72/minute. It range between 60 and 80 per minutes. It is subjected for variation during normal physiological condition such as exercise, emotion , etc.

3. Regulation of heart rate: Heart rate is regulated by nervous mechanism. Nervous mechanism consists of three components: 1.Vasomotor center. 2.Motor nerve fibers to the heart (efferent). 3.Sensory nerve fibers from the heart (afferent). 3

4. Vasomotor center: It is the nervous center that regulates the heart rate. It is situated in the reticular formation of medulla oblongata and the lower part of the pons. It has three areas: Vasoconstrictor area. Vasodilator area. Sensory area. 4

5. Vasomotor centers: Vasomotor center Situation Function Vasoconstrictor area Reticular formation of medulla in the floor of the IV ventricle and forms the lateral portion of Vasomotor center It increases the heart rate by sending accelerator impulses to heart through sympathetic nerves Vasodilator area Reticular formation of medulla in the floor of the IV ventricle and forms the medial portion of Vasomotor center It decreases the heart rate by sending inhibitor impulses to the heart through vagus nerve Sensory area Posterior part of vasomotor center, which lies in nucleus of tractus solitaries in medulla oblongata and pons It controls the vasoconstrictor and vasodilator area 5

6. Motor (efferent) nerve fibers to heart Heart receives efferent nerves from both the divisions of autonomic nervous system. Parasympathetic nerve fibers arise from the medulla oblongata and pass through vagus. Sympathetic nerve fibers arise from upper thoracic (T1-T4) segments of spinal cord. 6

7. Nerve supply to heart: 7

8. Brain and spinal cord: 8

9. Parasympathetic nerve fibers: Origin: They arise from the dorsal nucleus of vagus. The nucleus is situated in the IV ventricle in medulla oblongata. Function: The vagus nerve fibers are cardioinhibitory in function and carry inhibitory impulses from vasodilator area to heart. Vagal tone: It is the continuous stream of inhibitory impulses from vascodilatory area to heart.HR is inversely proportional to vagal tone. 9

10. Parasympathetic nerve fibers(contd…) Effects of stimulation of vagus nerve: Vagal escape: Stimulation of right vagus stops the heart beat due to inhibition of SA node and atria. Because of this, when stoppage of heart beat is continued for some time, a part of ventricular musculature becomes a pacemaker and starts producing impulses. Heart block: Stimulation of l eft vagus inhibition of Av node. Because of this, some of the impulses from SA node are not conducted to ventricles. This is called the partial heart block. Stimulation with strong stimulus causes stoppage of ventricular contraction, which causes Complete heart block 10

11. Effect of vagal stimulation on frog heart 11

12. Sympathetic nerve fibers: Origin: The preganglionic fibers of sympathetic nerves to the heart arise from lateral grey horns of the first four thoracic(T1-T4) segments of the spinal cord. Function: The sympathetic nerves are cardioaccelarators in function and carry cardioaccelarator impulses from vasoconstrictor area to the heart. 12

13. Sensory Nerve fibers from heart: The afferent nerve fibers from the heart pass through the inferior cervical sympathetic nerve. These nerve fibers carry sensations of stretch and pain from the heart to the brain via spinal cord. 13

14. Factors affecting vasomotor center: Impulses from higher centers. Impulse from respiratory centers. Impulses from Baroreceptors-Mary’s Reflex Impulses from Chemoreceptors Impulses from Right atrium-Bainbridge reflex 14

15. 1. Impulses from the higher centers: Cerebral cortex: During emotional conditions, this area sends inhibitory impulses to the vasodilator area. This causes reduction in vagal tone leading to cardioaccelaration. Hypothalamus: It influences heart rate via vasomotor center. Stimulation of posterior and lateral hypothalamic nuclei causes Tachycardia 15

16. 2. Impulses from Respiratory centers: In forced breathing, HR increases during inspiration and decreases during expiration and this variation is called respiratory sinus arrhythmia. Sinus arrhythmia is due to alternation of vagal tone because of impulses arising from respiratory center during inspiration. These impulses inhibits the vasodilator area, result in increase of heart rate. During expiration ,respiratory center stop sending impulses to vasodilator center. Now, vagal tone increases, leading to decrease in heart rate. 16

17. Nerve supply to baroreceptors and chemoreceptors 17

18. 3.Impulses from baroreceptors - Mareys Reflex It is a cardionhibitory reflex that decreases heart rate when blood pressure increases. Whenever the blood pressure increases the aortic and carotid baroreceptors are stimulated. The stimulatory receptors are sent to nucleus of tractus solitarius. Nucleus of tractus solitarius stimulates the vasodilator area, which in turn increases the vagal tone leading to decrease in heart rate. 18

19. Marey reflex 19

20. Marey’s Law: Pulse rate is inversely proportional to blood pressure. 20

21. 4. Impulses from chemoreceptors Chemoreceptors give response to change in chemical constituents of blood, particularly oxygen , carbon dioxide and hydrogen ion concentration . Peripheral chemoreceptors are situated in the carotid body and aortic body, adjacent to baroreceptors. Structure. Chemoreceptors in the carotid body are supplied by Hering nerve. Chemoreceptors in the aortic body are supplied by aortic nerve. 21

22. 4. Impulses from chemoreceptors( contd …) Whenever there is hypoxia, hypercapnea and increased hydrogen ions concentration in the blood, the chemoreceptors are stimulated. Inhibitory impulses are sent to vasodilator area. Vagal tone decreases and heart rate increases. The nerves supplying these receptors are called buffer nerves. 22

23. 5. Impulses from right atrium – bainbridge reflex Bainbridge reflex is a cardioaccelerator reflex that increases the heart rate when venous return is increased . Since this reflex arises from right atrium, it is also called right atrial reflex. 23

24. Bainbridge ( cardioaccelerator ) reflex 24

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