CONTROL OF HEART RATE.pdf

REGULATION OF HEART RATE The heart rate is constantly regulated within the normal range. Although the heart rate is subjected to variations during physiological conditions such as emotion and exercise, it is brought back to normal due to regulatory mechanism in the body...

REGULATION OF HEART RATE The heart rate is constantly regulated within the normal range. Although the heart rate is subjected to variations during physiological conditions such as emotion and exercise, it is brought back to normal due to regulatory mechanism in the body. Heart rate is regulated by the nervous mechanism, which consists of three components: A. Vasomotor center B. Motor (efferent) nerve fibers to the heart C. Sensory (afferent) nerve fibers from the heart. VASOMOTOR CENTER–CARDIAC CENTER The vasomotor center is the nervous center that controls the heart rate and the blood pressure. It is also referred to as the cardiac center. It is bilaterally located in the reticular formation of medulla oblongata and the lower part of the pons. The vasomotor center is made up of three major areas: 1. Vasoconstrictor area 2. Vasodilator area 3. Sensory area VASOCONSTRICTOR AREA –CARDIOACCELERATOR CENTER Situation Vasoconstrictor area is situated in the reticular formation of medulla in floor of IV ventricle and it forms the lateral portion of vasomotor center. It is otherwise known as pressor area or cardioaccelerator center. Function Vasoconstrictor area increases the heart rate by sending accelerator impulses to heart, through sympathetic nerves. It also causes constriction of blood vessels. Control Vasoconstrictor area is under the control of hypothalamus and cerebral cortex. VASODILATOR AREA –CARDIOINHIBITORY CENTER Situation Vasodilator area is also situated in the reticular formation of medulla oblongata in the floor of IV ventricle. It forms the medial portion of vasomotor center. It is also called depressor area or cardioinhibitory center. Function Vasodilator area decreases the heart rate by sending inhibitory impulses to heart through vagus nerve. It also causes dilatation of blood vessels. Control Vasodilator area is under the control of cerebral cortex and hypothalamus. SENSORY AREA Situation Sensory area forms the posterior part of vasomotor center, which lies in nucleus of tractus solitarius in medulla and pons. Function Sensory area receives sensory impulse via glossopharyngeal nerve and vagus nerve from periphery, particularly, from the baroreceptors. In turn, this area controls the vasoconstrictor and vasodilator areas. MOTOR (EFFERENT) NERVE FIBERS TO HEART The Heart is innervated by both Parasympathetic and sympathetic nervous system. Parasympathetic fibers originates from the medulla oblongata and pass through vagus nerve. Sympathetic fibers originates from upper thoracic (T1 to T4) segments of spinal cord. PARASYMPATHETIC NERVE FIBERS Parasympathetic nerve fibers are the cardioinhibitory nerve fibers. These nerve fibers reach the heart through the cardiac branch of vagus nerve. Function Vagus nerve is cardioinhibitory in function and carries inhibitory impulses from vasodilator area to the heart SYMPATHETIC NERVE FIBERS Sympathetic nerve fibers supplying the heart have cardioacceleratory function. Function Sympathetic nerves are cardioaccelerators in function and carry cardioaccelerator impulses from vasoconstrictor area to the heart. SENSORY (AFFERENT) NERVE FIBERS FROM HEART Afferent (sensory) nerve fibers from the heart pass through inferior cervical sympathetic nerve. These nerve fibers carry sensations of stretch and pain from the heart to brain via spinal cord. FACTORS AFFECTING VASOMOTOR CENTER – REGULATION OF VAGAL TONE 1. IMPULSES FROM HIGHER CENTERS Vasomotor center is under the control of impulses from higher centers in cerebral cortex and hypothalamus. Cerebral Cortex Emotional response is under the control of Area 13 in cerebral cortex. During emotional situations, inhibitory impulses are sent to the vasodilator area which cause a reduction in vagal tone and consequential increase in heart rate. Hypothalamus Stimulation of posterior and lateral hypothalamic nuclei causes tachycardia. Stimulation of preoptic and anterior nuclei causes bradycardia. 2. IMPULSES FROM RESPIRATORY CENTERS In forced breathing, the heart rate increases during forced inspiration and decreases during expiration. This variation is called respiratory sinus arrhythmia. 3. IMPULSES FROM BARORECEPTORS –MAREY REFLEX Baroreceptors give response to change in blood pressure. 4. IMPULSES FROM CHEMORECEPTORS Chemoreceptors respond to change in chemical constituents of blood, particularly oxygen, carbon dioxide and hydrogen ion concentration. Whenever there is hypoxia, hypercapnea and increased hydrogen ions concentration in the blood, the chemoreceptors are stimulated and inhibitory impulses are sent to vasodilator area. Vagal tone decreases and heart rate increases. 5. IMPULSES FROM RIGHT ATRIUM –BAINBRIDGE REFLEX Bainbridge reflex is a cardioaccelerator reflex that increases the heart rate when venous return is increased. Increase in venous return causes distention of right atrium and stimulation of stretch receptors, situated in the wall of right atrium. Stretch receptors, in turn, send inhibitory impulses through inferior cervical sympathetic nerve to vasodilator area of vasomotor center. Vasodilator area is inhibited, resulting in decrease in vagal tone and increase in heart rate. Factors Affecting Vagal Tone and Heart Rate