Preksha Meditation & Human Health: 1.7.1 Regulation of Heart Rate

The heart rate can be changed to adapt to different situations. The nervous system can and does bring about necessary changes in heart rate as well as in force of contraction. The medulla of the brain contains the two cardiac centers, the accelerator center and the inhibitory center. These centers send impulses to the heart along autonomic nerves. The autonomic nervous system has two divisions: sympathetic and parasympathetic. Sympathetic impulses from the accelerator center along sympathetic nerves increase heart rate and force of contraction during exercise and stressful situations. Parasympathetic impulses from the inhibitory center along the vagous nerve decrease the heart rate. At rest these impulses slow down the depolarization of the SA node to what we consider a normal resting rate, and they also slow the heart after exercise is over. Basically changes in blood pressure and oxygen level of the blood are stimuli for changes in heart rate. Press receptors and chemoreceptors are located in the carotid arteries and aortic arch. Press receptors in the carotid sinuses and aortic sinus detect changes in blood pressure. Chemoreceptors in the carotid bodies and aortic body detect changes in the oxygen content of the blood. The sensory nerves for the carotid receptors are the glossopharyngeal (9th cranial) nerves; the sensory nerves for the aortic arch receptors are the vagous (10th cranial) nerves. The sudden drop in blood pressure causes fewer impulses to be generated by the press receptors. These impulses travel along the glossopharyngeal nerves to the medulla, and the decrease in the frequency of impulses stimulates the accelerator center. The accelerator center generates impulses that are carried by sympathetic nerves to the SA node, AV node, and ventricular myocardium. As heart rate and force increase, blood pressure to the brain is raised to normal, and the sensation of light-headedness passes. When blood pressure to the brain is restored to normal, the heart receives more parasympathetic impulses from the inhibitory center along the vagous nerves to the SA node and AV node. These parasympathetic impulses slow the heart rate to a normal resting pace.