Heart rate Definition.
Heart rate is the frequency of Contraction of heart Musculature During completing 1beat in an appropriate time interval and acquiring efficient number of beat in 1 min to complete pumping effect mechanism of heart to recover and transfer blood in parts of the body and towards the heart. Normal heart rate is 72/minute. It ranges between 60 and 80 per minute. The Heart rate is based for contraction of Heart Musculature to beat per min followed by pumping activity of the heart and drainage activity throughout the body.
Tachycardia.
Tachycardia is the increase in heart rate above 100/ minute. Tachycardia term refers to that your heart is beating faster than normal resting heart rate ; which signifies some medical condition related to heart but sometimes it changes due to abnormal body mechanism Which includes lifestyle habits , food styles and lack of alertness towards yoga and exercises . few tachycardia are normal ; other might be life-threatening.
When you are engage in yoga and exercise activity during this time if tachycardia appeared . Then, this might be normal if you are healthy individual or abnormal if you are already disease individual. During exercise activity , tachycardia appeared because of compensating the oxygen requirement of the body. Tachycardia also known as Tachyarrhythmia . Tachycardia classifies as Sinus tachycardia , Supraventricular tachycardia and ventricular tachycardia.
Physiological Conditions when Tachycardia Occurs.
1. Childhood .
2. Exercise.
3. Pregnancy.
4. Emotional conditions such as anxiety.
Pathological Conditions when Tachycardia Occurs.
1. Fever.
2. Anemia.
3. Hypoxia.
4. Hyperthyroidism.
5. Hypersecretion of catecholamines.
6. Cardiomyopathy.
7. Diseases of heart valves.
Bradycardia.
Bradycardia is the decrease in heart rate below 60/minute. Bradycardia term refers to that heart is beating slower than normal resting heart beat. Bradycardia is slower in heart rate due to this ; there is lack of oxygen availability in the body and leads to directly or indirectly several complication as in forms of tiredness, weakness, breathlessness. Bradycardia is a condition where there is alternation in Rhythm of heart beating at a normal level ; which signifies that there is climatic changes or abnormal heart beat which means providing alert to serious disease of heart .
Bradycardia is quite common in few individual ; who are healthy young adults. trained athletes and during sleep time it happen because body requirement of oxygen and nutrient is less and overall functionality are on fully relaxed condition . only essential biological and physiological mechanism are on active state and complete necessary steps in maintaining normal function of the body ;remaining are on resting or relaxed condition . Bradycardia are observed in condition where there is an alternation of homeostasis mechanism of the body and on any heart disease .
Physiological Conditions when Bradycardia Occurs.
1. Sleep.
2. Athletes.
Pathological Conditions when Bradycardia Occurs.
1. Hypothermia.
2. Hypothyroidism.
3. Heart attack.
4. Congenital heart disease.
5. Degenerative process of aging.
6. Obstructive jaundice.
7. Increased intracranial pressure.
Drugs which Induce Bradycardia .
1. Beta blockers.
2. Channel blockers.
3. Digitalis and other antiarrhythmic drugs.
Regulation of Heart rate.
Heart rate is maintained within normal range constantly. It is subjected for variation during normal physiological conditions such as exercise, emotion, etc. However, under physiological conditions, the altered heart rate is quickly brought back to normal. It is because of the perfectly tuned 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 Centre.
Vasomotor center is the nervous center that regulates the heart rate. It is the same center in brain, which regulates the blood pressure. It is also called the cardiac center. Vasomotor center is bilaterally situated in the reticular formation of medulla oblongata and lower part of pons.
Areas of Vasomotor Center.
Vasomotor center is formed by three areas:
1. Vasoconstrictor area.
2. Vasodilator area.
3. Sensory area.
Vasoconstrictor area- Cardioaccelerator center.
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. Vasoconstrictor area increases the heart rate by sending accelerator impulses to heart, through sympathetic nerves. It also causes constriction of blood vessels. Stimulation of this center in animals increases the heart rate and its removal or destruction decreases the heart rate. Vasoconstrictor area is under the control of hypothalamus and cerebral cortex.
Vasodilator area- cardioinhibitory center.
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. Vasodilator area decreases the heart rate by sending inhibitory impulses to heart through vagus nerve. It also causes dilatation of blood vessels.
Stimulation of this area in animals with weak electric stimulus decreases the heart rate and stimulation with a strong stimulus stops the heartbeat. When this area is removed or destroyed, heart rate increases. Vasodilator area is under the control of cerebral cortex and hypothalamus. It is also controlled by the impulses from baroreceptors, chemoreceptors and other sensory impulses via afferent nerves.
Sensory area.
Sensory area is in the posterior part of vasomotor center, which lies in nucleus of tractus solitarius in medulla and pons. 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 .
Heart receives efferent nerves from both the divisions of autonomic nervous system. Parasympathetic fibers arise from the medulla oblongata and pass through vagus nerve. Sympathetic fibers arise 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. Parasympathetic nerve fibers supplying heart arise from the dorsal nucleus of vagus. This nucleus is situated in the floor of fourth ventricle in medulla oblongata and is in close contact with vasodilator area.
Preganglionic parasympathetic nerve fibers from dorsal nucleus of vagus reach the heart by passing through the main trunk of vagus and cardiac branch of vagus. After reaching the heart, preganglionic fibers terminate on postganglionic neurons. Postganglionic fibers from these neurons innervate heart muscle. Most of the fibers from right vagus terminate in sinoatrial (SA) node. Remaining fibers supply the atrial muscles and atrioventricular (AV) node.
Most of the fibers from left vagus supply AV node and some fibers supply the atrial muscle and SA node. Ventricles do not receive the vagus nerve supply. Few fibers are located in the bases of ventricles, but the functions of these nerve fibers are not known. Vagus nerve is cardioinhibitory in function and carries inhibitory impulses from vasodilator area to the heart.
Vagal Tone.
Vagal tone is the continuous stream of inhibitory impulses from vasodilator area to heart via vagus nerve. Heart rate is kept under control because of vagal tone. These impulses reach the heart and exert inhibitory effect on heart. Heart rate is inversely proportional to vagal tone.
In experimental animals (dog), removal of vagal input (by sectioning vagus) increases the heart rate. This proves the existence of vagal tone. Under resting conditions, vagal tone dominates sympathetic tone . Impulses from different parts of the body regulate the heart rate through vasomotor center, by altering the vagal tone. Vagal tone is also called cardioinhibitory tone or parasympathetic tone.
Effect of Stimulation of Vagus Nerve.
Effect of stimulation of right vagus nerve – Vagal escape.
Right vagus supplies mainly SA node. Stimulation of right vagus in experimental animals such as dog, with a weak stimulus causes reduction in heart rate and force of contraction. Stimulation with strong stimulus causes stoppage of heart due to inhibition of SA node.
If the stimulus is continued for some time, the ventricle starts beating; but the rate of contraction is slower than before. This is because of vagal escape. Vagal escape refers to escape of ventricle from inhibitory effect of vagal stimulation. If stimulation of vagus nerve is stopped, heart starts beating normally.
Cause for vagal escape.
Stimulation of right vagus stops the heartbeat due to inhibition of SA node and atria. However, ventricles are not supplied by vagus. So, the ventricles are not inhibited by vagal stimulation. Because of this, when stoppage of heart beat is continued for some time (by vagal stimulation), a part of ventricular musculature becomes pacemaker and starts producing impulses. It results in contraction of ventricles, which is called vagal escape. Thus, vagal escape includes only ventricular contractions. However, the rhythmicity of ventricular muscle is less and it is about 20/minute.
Effect of stimulation of left vagus nerve – heart block.
Left vagus supplies mainly the AV node. Stimulation of left vagus in dog with a weak stimulus causes a slight reduction in rate of ventricular contraction. Stimulation of left vagus causes inhibition of AV node. Because of inhibition of AV node, some of the impulses from SA node are not conducted to ventricles. This is called the partial heart block.
The ratio between atrial contraction and ventricular contraction is 2 : 1, 3 : 1 or 4 : 1, depending upon the strength of stimulus. Stimulation of left vagus with strong stimulus causes stoppage of ventricular contraction, which is called complete heart block. This is because of the complete inhibition of AV node. The prolongation of stimulation causes idioventricular rhythm, which is different from the rhythm of atrial contraction.
Mode of Action of Vagus Nerve.
Vagus nerve inhibits the heart by secreting the neurotransmitter substance known as acetylcholine.
Sympathetic nerve fibers.
Sympathetic nerve fibers supplying the heart have cardioacceleratory function. Preganglionic fibers of the sympathetic nerves to heart arise from lateral gray horns of the first 4 thoracic (T1 to T4) segments of the spinal cord. These segments of the spinal cord receive fibers from vasoconstrictor area of vasomotor center.
Preganglionic fibers reach the superior, middle and inferior cervical sympathetic ganglia situated in the sympathetic chain. Inferior cervical sympathetic ganglion fuses with first thoracic sympathetic ganglion, forming stellate ganglion. From these ganglia, the postganglionic fibers arise. Sympathetic nerves are cardioaccelerator in function and carry cardioaccelerator impulses from vasoconstrictor area to the heart.
Postganglionic fibers form three nerves:
1. Superior cervical sympathetic nerve, which innervates larger arteries and base of the heart.
2. Middle cervical sympathetic nerve, which supplies the rest of the heart.
3. Inferior cervical sympathetic nerve, which serves as sensory (afferent) nerve from the heart.
Sympathetic Tone.
Sympathetic tone or cardioaccelerator tone is the continuous stream of impulses produced by the vasoconstrictor area. Impulses pass through sympathetic nerves and accelerate the heart rate. Under normal conditions, the vagal tone is dominant over sympathetic tone. Whenever vagal tone is reduced or abolished, the sympathetic tone becomes powerful. It is generally believed that the sympathetic tone does not play an important role in the regulation of cardiac function under resting physiological conditions.
However, it plays a definite role in increasing the heart rate during emergency conditions. Rate of contraction of a completely denervated heart of dog is higher than the rate of an innervated heart in resting conditions. This shows that under resting conditions, the vagal tone is dominant over sympathetic tone.
Effect of Stimulation of Sympathetic Nerves.
Stimulation of sympathetic nerves increases the rate and force of contraction of heart. The effect depends upon the strength of stimulus.
Mode of Action of Sympathetic Nerves.
Cardioacceleration by sympathetic stimulation is due to the release of neurotransmitter substance, noradrenaline.
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.
Vasomotor center regulates the cardiac activity by receiving impulses from different sources in the body. After receiving the impulses from different sources, the vasodilator area alters the vagal tone and modulates the activities of the heart. Various sources from which the impulses reach the vasomotor center:
Impulses From Higher Centers.
Vasomotor center is mainly controlled by the impulses from higher centers in cerebral cortex and hypothalamus.
Cerebral Cortex.
Area 13 in cerebral cortex is concerned with emotional reactions of the body. During emotional conditions, this area sends inhibitory impulses to the vasodilator area. This causes reduction in vagal tone, leading to increase in heart rate.
Hypothalamus.
Hypothalamus influences the heart rate via vasomotor center. Stimulation of posterior and lateral hypothalamic nuclei causes tachycardia. Stimulation of preoptic and anterior nuclei causes bradycardia.
Impulses from Respiration Centers.
In forced breathing, heart rate increases during inspiration and decreases during expiration. This variation is called respiratory sinus arrhythmia. This is common in some children and in some adults even during quiet breathing. Sinus arrhythmia is due to the alteration of vagal tone because of impulses arising from respiratory centers during inspiration.
These impulses inhibit the vasodilator area, resulting in decreased vagal tone and increased heart rate. During expiration, the respiratory center stops sending impulses to vasodilator center. Now, vagal tone increases, leading to decrease in heart rate.
Impulses from Baroreceptors-Marey Reflex.
Baroreceptors.
Baroreceptors are the receptors which give response to change in blood pressure. These receptors are also called pressoreceptors . Depending upon the situation, baroreceptors are divided into two types:
1. Carotid baroreceptors, situated in carotid sinus, which is present in the wall of internal carotid artery near the bifurcation of common carotid artery.
2. Aortic baroreceptors, situated in the wall of arch of aorta.
Carotid baroreceptors are supplied by Hering nerve, which is the branch of glossopharyngeal (IX cranial) nerve. Aortic baroreceptors are supplied by aortic nerve, which is a branch of vagus (X cranial) nerve. Nerve fibers from the baroreceptors reach the nucleus of tractus solitarius, which is situated adjacent to vasomotor center in medulla oblongata.
Function – Marey Reflex.
Baroreceptors regulate the heart rate through Marey reflex. Stimulus for this reflex is increase in blood pressure. Marey reflex is a cardioinhibitory reflex that decreases heart rate when blood pressure increases. Whenever blood pressure increases, the aortic and carotid baroreceptors are stimulated and stimulatory impulses are sent to nucleus of tractus solitarius via Hering nerve and aortic nerve (afferent nerves).
Now, the nucleus of tractus solitarius stimulates vasodilator area, which in turn increases the vagal tone, leading to decrease in heart rate . Marey reflex includes aortic reflex and carotid sinus reflex. When pressure is less, the baroreceptors are not stimulated. So, no impulses go to nucleus of tractus solitarius. There are no inhibitory impulses to the heart and heart rate is not decreased. Thus, the heart rate is
inversely proportional to blood pressure.
Marey law.
According to Marey law, the pulse rate (which represents heart rate) is inversely proportional to blood pressure. Baroreceptors induce the Marey reflex only during resting conditions. So, in many conditions such as exercise, there is an increase in both blood pressure and heart rate.
Impulses from Chemoreceptors.
Chemoreceptors.
Chemoreceptors are receptors giving 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. Chemoreceptors are made up of two types of cells, type I or glomus cells and type II or sustentacular cells.
Glomus cells have afferent nerve endings, which are stimulated by hypoxia. Type II cells are glial cells and provide support for type I cells. Chemoreceptors in the carotid body are supplied by Hering nerve, which is the branch of glossopharyngeal nerve. Chemoreceptors in the aortic body are supplied by aortic nerve which is the branch of vagus nerve.
Function.
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. Chemoreceptors play a major role in maintaining respiration than the heart rate.
Sinoaortic Mechanism and Buffer Nerves.
Sinoaortic mechanism is the mechanism of baroreceptors and chemoreceptors in carotid and aortic regions, that regulates heart rate, blood pressure and respiration. The nerves supplying these receptors are called buffer nerves.
Impulses from right atrium – Brain-bridge 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. 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 .
Impulses from other Afferent Nerves.
Stimulation of sensory nerves produces varying effects.
Examples:
a. Stimulation of receptors in nasal mucous membrane causes bradycardia. Impulses from nasal mucous membrane pass via the branches of Vth cranial nerve and decrease the heart rate.
b. Most of the painful stimuli cause tachycardia and some cause bradycardia. Impulses are transmitted via pain nerve fibers .
Bezold-Jarisch Reflex .
Bezold-Jarisch reflex is the reflex characterized by bradycardia and hypotension, caused by stimulation of chemoreceptors present in the wall of left ventricles by substances such as alkaloids. It is also called coronary chemoreflex. Vagal fibers form the afferent and efferent pathways of this reflex.
Conditions when Bezold-Jarisch Reflex Occurs.
Bezold-Jarisch reflex is a pathological reflex and it does not occur in physiological conditions. Conditions when this reflex occurs:
1. Myocardial infarction.
2. Administration of thrombolytic agents.
3. Hemorrhage.
4. Aortic stenosis.
5. Syncope.
Thanks for Visiting us.
