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Electro Physiology of the Human Heart

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Published: Fri, 23 Feb 2018


This chapter is the first chapter in the thesis which gives introduction of the present study. The chapter defines electro physiology of human heart, blood circulation in both pulmonary and systemic in detail, the components in cardiovascular system and heart sounds. It explains in detail the generation of potential due to mechanical activity of human heart and sounds produced due to closure of valves during blood pumping from atrias to ventricles and to respective parts of the body. This chapter presents a detailed survey on literature focusing on different methods to measure and analyse ECG and PCG.

Electricity plays an important role in medicine. The control and operation of nerves, muscles and organs are functioning by the electricity generated inside the body. The forces of muscles, the action of brain and all nerve signals to and from the brain are caused by the attraction and repulsion of electrical charges. Many electrical signals are generated to carry out the special functions of the body. These signals are the result of electrochemical action of certain type of cells. The best known signals are electrical potentials of nerve transmission and the electrical signals observed in electromyogram (EMG) of the muscle, the electrocardiogram (ECG) of the heart and the electroencephalogram (EEG) of the brain.

One means of obtaining diagnostic information about muscles, heart and brain are to measure their electrical activity. The record of the potential from muscles during movement of is called the electromyogram (EMG). The rhythmical action of the heart is controlled by an electrical signal initiated by spontaneous stimulation of pacemaker cells located at apex of the right atrium i.e. sinoatrial node (SA node). The recording of heart’s potentials on skin is called electrocardiogram (ECG). The recording of the electric signals due to electrical activity of neurons in the cortex of the brain is called electroencephalogram (EEG). The present study is to study the electrical activity of heart during its mechanical vibrations.

The primary step in investigations of physiological systems requires the appropriate sensors to transducer the phenomenon of interest into a measurable electric signal. The field of biomedical has advanced to the stage of practical application of signal processing and pattern analysis techniques for efficient and improved non- invasive diagnosis.

1.1 Physiology of Heart and Vascular System

The analysis of variability in cardiovascular signals is applied widely and many experimental setups were put forward. Spontaneous fluctuations can be observed in cardiovascular function, such as heart rate and blood pressure, even when the environmental parameters are maintained at a constant level as possible and no perturbations influences can be identified.

The observations of heart rate fluctuations is related to various cardiovascular disorders, the analysis of heart rate variability has become widely used tool in the assessment of the regulation of heart rate behavior (Timo Makikallo 1998). The study of cyclic variations of heart rate plays an important role in the assessment of both physiological and clinical aspects (Narayana Dutt & Krishnan 2000).

The heart is actually two separate pumps. A right heart that pumps the blood through the lungs and left heart pumps the blood through the peripheral organs. Each of these composed of ‘atrium’ and ‘ventricle’. Atrium receives the blood and pumps into ventricles. Ventricles supply the main force that circulates the blood either through pulmonary circulation by the right ventricle or through the systemic circulation by the left ventricle(Fig 1.1)

The blood, blood vessels and heart make up the cardiovascular system (CVS). The blood and its supply of oxygen are so important to the body that the heart is the first major organ to develop in the embryo.

The mechanism in the heart provides cardiac rhythmcity and transmits action potentials through the heart muscle to cause the heart’s rhythmical beat. The cardiac event that occurs from the beginning of the next are called the cardiac cycle. Each cycle is initiated by spontaneous generation of an action potential in the’ Sino atrious node’ or ‘Sinus node’.

The cardiac cycle consists of a period of relaxation called ‘diastole’, during which the heart fills with blood fallowed by a period of contraction called ‘systole’ together is known as a ‘beat’.

The heart is composed of three major types of cardiac muscle; atrial muscle, ventricular muscle and specialized excitatory and conductive muscle fibers. Cardiac muscle is a syncytium of many heart muscle cells which are interconnected with “intercalated discs” which are of actually cell membranes separates cardiac muscle cells from one another and offers low resistance to ions to diffuse through cells. If one of these cells is excited, the action potential spreads to all of them.

The heart is composed of two syncytiums the atrial syncytium that consists of walls of two atria and ventricular syncytium consists of the walls of two ventricles. The atria are separated from the ventricles by tissue that surrounds the atrio-ventricular valvular openings. Potentials are conducted from atrial syncytium into ventricular syncytium through the specialized conductive system called A-V bundle a bundle of conductive fibers.

The division of the muscle of the heart into two functional syncytiums allows the atria to contract a short time ahead of ventricular contraction, which is important for effective heart pumping through lungs and peripheral organs. Another importance of the system is that it allows all portions of the ventricles to contract almost simultaneously, which is essential for most effective pressure generation in the ventricular chambers

The cardiac cells present in the heart tissue are individually surrounded with an insulating membrane (supporting a potential mV) containing selective permeable ionic channels. The currents through these channels interact with the membrane potential to regulate the activity of the cell. The flow of various ions (Na,K,Ca …etc) through out the cardiac tissue is responsible for the propagation of the electrical waves through tissue in turn provides the driving force behind the heart’s mechanical contraction and its ability to pump blood through the body.

1.2. Components of Heart

The heart is a conical, hollow muscular organ placed obliquely behind the body of the sternum and adjoining parts of the body of the costal cartilages, so that 1/3 rd of it lies right and 2/3 rd to the left of the median plane. The heart measures about 12x9cm and weighs 300 gm in males and 250 gm in females.

The human heart has four chambers as shown in fig 1.2. The upper two chambers, the right and left atria are receiving chambers of blood. Atria collects venous blood from the body and about 75% of the blood flows directly into the ventricle even before atrial contraction. The atrial contraction causes an additional 25% filling the ventricles. The heart’s lower chambers right and left ventricles are the powerful pumping chambers. The right and left sides of the heart are separated from each other by a wall of tissue .each side pumps blood through a different circuit of blood vessels.

1.2.1. The Right Atrium

It is the right upper chamber of the heart receives venous blood from the whole body and pumps it to the right ventricle through right atrioventricular (tricuspid) opening. The chamber is elongated vertically, receiving the superior vena cava at the upper end and the inferior vena cava at the lower end. Deoxygenated blood from the whole body feeds into two large veins, the superior vena cava and inferior venecava, which empty into the right atrium of the heart and the same pumps to the right ventricle.

1.2.2. The Right Ventricle

The right ventricle is a triangular chamber which receives blood from the right atrium and pumps it to the lungs through the pulmonary trunk and pulmonary arteries.

Externally, the right ventricle has two surfaces anterior and inferior. The cavity of the right ventricle is crescent in section because of the forward bulge of inter ventricular septum. The wall of the right ventricle is thinner than that of left ventricle in a ratio 1:3.

1.2.3. The left atrium

The left atrium forms the left 2/3 of the base of the heart and is a quadrangular chamber. It receives oxygenated blood from the lungs through four pulmonary veins and pumps it to the left ventricle through Mitral valve.

1.2.4. The Left Ventricle

The left ventricle receives oxygenated blood from the left atrium and pumps it into the aorta, the body’s largest artery. Smaller arteries that branch off the aorta distribute blood to the various parts of the body. It forms the apex of the heart .The cavity of the left ventricle is circular in cross section and has the thickest walls nearly half an inch in an adult because it must work the hardest to propel blood to the farthest reaches of the body.

1.2.5. Valves of the Heart

The valves of the heart maintain unidirectional flow of the blood and prevent blood from flowing backward in the heart i.e. the valves open easily in the direction of blood flow, but when blood pushes against the valves in the opposite direction the valves close.

There are two pairs of valves in the heart i) atrio ventricular valves ii)Semilunar valves. Atrio-ventricular valves are located between the atria andventricles as shown figure. The right atrio-ventricular valve is formed from three cusps of tissue and is called “Tricuspid valve”. While the left atrio- ventricular valve has two cusps and is called “Bicuspid or Mitral valve”. Both valves are made up of a fibrous ring to which the cusps are connected .The cusps are flat and project into the ventricular cavity. The atrio- ventricular valves kept competent by active contraction of the papillary muscles.

Semi lunar valves are located between the ventricles and arteries and each of them consist of three half moon shaped flaps of tissue. They are not attached to fibrous ring but are to the blood vessel .The right semi lunar valve between right ventricle and pulmonary artery is “pulmonary valve ” and the valve between left ventricle and aorta is “aortic valve “.These valves are closed during ventricular diastole.

1.2.6. Superior Vena Cava

It is about 7 cm long venous channel which receives blood from the upper half of the body and empties it to the right atrium like other large veins. It has no valves.

1.2.7. The Aorta

The aorta is the great arterial trunk which receives oxygenated blood from the left ventricle and distribute it all parts of the body.

1.2.8. Myocardium

It is the muscle tissue wraps around a scaffolding of tough connective tissue to form the walls of the heart chamber. The atria the receiving chambers of the heart have relatively thin walls than the ventricles, the pumping chambers.

1.2.9. Pericardium

It is a tough, double layered sac which surrounds the heart. The inner layer of the pericardium is known as epicardium rests on top of the heart muscle. The outer layer is attached to the breast bone and other structures in the chest cavity and helps hold the heart in place. The space between the two layers of the pericardium filled with watery fluid which prevents these layers from rubbing against each other during heart beat.

1.2.10. Endocardium

It is the inner surface of the heart’s chambers lined with a thin white sheet of shiny tissue. The same type of tissue also lines the blood vessels forming continuous lining throughout the circulatory system. The lining helps blood to flow smoothly and prevents clotting of blood in the circulatory system.

The heart is nourished not by blood passing through, but by the blood vessels also known as “coronary arteries” which encircle the heart like a crown.

About 5% of the blood pumped to the body enters the coronary arteries, which branch from the left ventricle .Three main coronary arteries the right , the left circumflex and the left anterior descending nourish different regions of the heart muscle. From these three arteries small branches arise to provide a constant supply of oxygen.

1.3. A Detailed Description of Vascular System

The cardio vascular system is concerned with the transport of blood and lymph through the body. It may be divided into four major components, the heart, the macro circular i.e. blood vessels arteries and veins, micro circular i.e. capillary and lymph vascular system i.e. water and other components of blood plasma. The cardio vascular system (CVS) controls the blood pressure by altering the heart rate and compliance i.e. elasticity of blood vessels. (Isla Gilmour 1995).

1.3.1. Arteries

Arteries transport blood from high pressure to body tissues as their structure permits them to expand and contract under different pressures due to the presence of elastic fibers. The main artery of the heart is “aorta”, which starts from the left ventricle transporting oxygen and nutrients to all body tissues. The presence of elastic fiber enables the arteries to expand when each pulse of blood pumped by the heart and regains its original shape when tension is released.

Like all blood vessels the inner layer of arteries is known as “tunica intima”, composed of a single layer of flattened endothelial cells fitted together to form a smooth, continuous tube. In large arteries the same layer is supported by thick band of elastic fibers. The middle layer is known as “tunica media” consisting of smooth muscle and elastic fibers. In very large arteries the outer layer is known as “tunica adventitia” also contains elastic fibers and connective tissue.

1.3.2. Veins

Veins transport deoxygenated blood at low pressure toward the heart and act as reservoirs of different capacities to maintain a steady return of blood to heart. The veins of systemic circulation terminate at body’s largest veins superior and inferior vena cava which empty into the right atrium of the heart.

The walls of the veins are thinner and contain little elastic fiber with greater internal diameter. These structural properties help them to stretch and store the blood. Since the pressure in veins is low some structural changes is needed to prevent blood from downward pull of gravity. The veins in the lower body contain special one-way valves prevent the accumulation of blood in the legs and feet.

During exercise the muscles are in extremities, relaxing and contracting alternately squeezing the veins to force the blood upward towards the heart. The tunica media of veins is thinner and contain less elastic fiber and smooth muscle to function at low pressure and serving as reservoirs to maintain a steady return of blood to the heart.

1.3.3. Arterioles

The functions of arterioles are to distribute the blood and pressure reducing valves. They play an important role in determining the blood pressure. The arterioles have smooth muscle in their walls and do not stretch rather act as pressure reducing valves between the arteries and capillaries. They prevent delicate capillaries from high pressure of blood in the arterial system.

The degree of muscular tension in the walls of arterioles decides their internal diameter in turn changes the resistance of blood flow in arterioles. As they affect the blood pressure because they account for a large component of the peripheral resistance to blood flow. Blood pressure is the product of total peripheral resistance and cardiac output.

1.3.4. Venules

The function of venules is to drain blood from the capillary bed into the venous system.

1.3.5. Capillaries

Capillaries are very small blood vessels their diameter ranges from 4-15 μm.

The sum of the diameters of all capillaries is significantly larger than that of the aorta which results in decrease of blood pressure and flow rate. Capillaries are composed of a single layer of flattened endothelial cells fitted together to form a continuous tube. This results in a very large surface to volume ratio. The low rate of blood and large surface area facilitate the functions are

* Providing nutrients and oxygen to the surrounding tissue.

* The absorption of nutrients, waste products and carbon dioxide and

* The execution of waste products from the body.

1.3.6. Lymphatic Vessels

Parts of the blood plasma will execute from the blood vessels into the surrounding tissues because of transport across the endothelium. The fluid entering tissues from capillaries adds to the interstitial fluid normally found in the tissue. The surplus of liquid will return to the circulation .Lymph vessels are dedicated to this unidirectional flow of liquid, the lymph. The lymph vessels can be divided into three types depending on their shape and size.

Lymph Capillaries

These are larger than blood capillaries and very irregular on shape. They begin as blind ending tubes in connective tissues.

Lymph Collecting Vessels

They appear almost similar to lymph capillaries but a bit large and form valves. The lymph is moved by the compression of the lymph vessels by surrounding tissues. The direction of lymph flow is determined by the valves

Lymph Ducts

They contain one or two layers of smooth muscle cells in their wall and form valves. The walls of lymph ducts are less elastic and during contractions contribute to the movement of lymph towards the heart in addition to the compression of the ducts by surrounding tissues.

1.3.7. Relations to Other Systems and Organs

The heart and vascular system perform almost the same function to provide oxygen, nutrients and harmonic to the cells of the body tissue. They can be considered as one unit rather than two, because each is equipped to carry out half of that function.

The vascular system is also closely related to the adrenergic receptors and the autonomic nervous system, which together control important aspects of its function.

The alpha adrenergic receptors are the smooth muscle cells in arteries, veins, arterioles and venules. These receptors bind molecules released by cells of the autonomic nervous system and respond by contracting.

1.4. Blood Circulation -Systemic and Pulmonary

The heart basically a double pump provides the force to circulate the blood through two major circulatory systems, the pulmonary circulation in the lungs and the systemic circulation is in organ system that transports substances to and fro from cells. The blood in normal individual circulates through one system into before being pumped by the other part of the heart to the second system.

The heart is a muscle composed by cells containing small filaments of actin and myosin. These proteins interact in the sense of forming actomyosin during muscle contraction, thus leading to the main purpose of the heart: pumping the blood through the circulatory system (Manuel Duarte Ortigueiva 1959). The synchronous nature of contraction of heart results in the efficient pumping of blood through the pulmonic and systemic circulation (J.Olansen et al 2000).

The circulatory system can be thought of as a closed loop circulation system with two pumps. One way valves keep the flow downward through the pumps.

1.4.1. Systemic Circulation

The heart ejects oxygen rich blood under a pressure about 125 mm Hg from main pumping chamber left ventricle, through the largest artery the aorta. Subdivided into smaller arteries in turn divided into even smaller arteries called arterioles and finally into a very fine meshwork of vessels called the capillary bed.

Capillaries permit to dissolve oxygen and nutrients from the blood to diffuse across the fluid, known as “interstitial fluid” that fills the gaps between the cells of tissues of organs. The dissolved oxygen and nutrients enter cells through interstitial fluid by diffusion across the cell membranes.

Mean while carbon dioxide and other wastes leave the cell diffuse through the interstitial fluid, cross the capillary bed and enter the blood. The blood collects in small veins called venules gradually join together to form progressively larger veins. Finally the veins converge into two large veins, the superior vena cava and the inferior vena cava bringing blood from upper half and lower half of the body respectively. Both of these main veins join at the right atrium of the heart.

1.4.2. Pulmonary Circulation

The deoxygenated blood returning from the organs and tissues of the body stored momentarily in the reservoir i.e. right atrium, during weak contraction (5 to 6 mm Hg) the blood pushed into the right ventricle. On the next ventricular contraction this blood is pumped at a pressure of about 25 mm Hg through pulmonary arteries to the capillary system in the lungs.

At this site microscopic vessels pass adjacent to the “alveoli” or air sacs of the lung where it exchanges oxygen from the membrane to the blood and leaves carbon dioxide from blood to the same membrane. The freshly oxygenated blood then travels through the main veins from the lungs into the left reservoir i.e. left atrium of the heart. During weak arterial contraction (7 to 8 mm Hg) blood enters the left ventricle.

On the next contraction of the left ventricle sends blood to the aorta and then to general circulation. On average a typical adult has about 4.5 lts of blood and each section of the heart pumps about 80 ml in each contraction. About 30 sec to 1 min is needed for the average red blood cell to complete a full circuit through both the pulmonary and systemic circulation.

The blood volume is not uniformly divided between the pulmonary and systemic circulation. At any one time 80% of the blood is in the systemic circulation and 20% is in the pulmonary circulation. Of the blood in the systemic circulation about 15% is in the arteries, 10% is in the capillaries and 75% is in the veins. In the pulmonary circulation about 7% of the blood is in the pulmonary capillaries and the remaining is almost equally distributed between the pulmonary arteries and pulmonary veins.

1.4.3. Additional Functions

In addition to oxygen, the circulatory system also transports nutrients derived from digested food to the body. These nutrients enter the blood from the walls of the intestine carries the nutrients to the liver for farther metabolic processing.

The liver stores variety of substances such as sugar, fats and vitamins and releases glucose to the blood as needed. The liver also cleans the blood by removing waste products and toxins. After the blood is cleaned, enter the veins converge to form the large vein that joins the vena cava at right atrium.

The circulatory system plays an important role

* In regulating body temperature

* To collect chemical messengers called hormones from hormone producing glands and transports to specific organs and tissues to regulate body’s rate of metabolism, growth, sexual development and other functions.

* With immune system and coagulation system, the immune system is a complex system of many disease fighting white blood cells and anti bodies circulate in the blood and are transported to sites of infection. The coagulation system is composed of special proteins called clotting factors which circulate in the blood. When ever blood vessels are cut to torn, the coagulation system works rapidly to stop the bleeding by forming clots.

Other organs support the circulatory system are the brain and the parts of nervous system constantly monitor blood circulation, sending signals to the heart or blood vessels to maintain constant blood pressure.

New blood cells are produced in the bone marrow and old blood cells are broken down in the spleen, where iron and other minerals are recycled. Metabolic waste products are removed from the blood by kidneys which also screen the blood for excess salt and maintain blood pressure and to maintain blood pressure and to balance minerals and fluids of the body.

1.5. Heart Diseases

Heart disease has become very common nowadays due to changes in life style. Many of these diseases are due to either increase the work load of heart or reduce the ability to work at normal rate.

1.5.1. Tachycardia

There are many factors that are responsible for development of heart disease. One such factor is “High blood pressure” (Hypertension) which causes the muscle tension to increase in proportion to the pressure. A fast heart rate (Tachycardia) increases the work load.

1.5.2. Heart Attack

The heart disease that causes most deaths is “heart attack”. A Heart attack is caused by blockage of one or more arteries to the heart muscle. During and after heart attack the ability of the heart is seriously impaired. Bed rest and giving oxygen reduces the work load on heart which increases the oxygen content in the blood so that blood pumped by the heart will be less. Alternate method to reduce risk of heart attack is the regular exercise program which opens alternate routes in cardiovascular system.

1.5.3. Congestive Heart Failure

Another common disease is congestive heart failure which is due to enlarge in size of the heart reduce the ability for adequate blood circulation.

Applying law of Laplace, if the radius of the heart is doubled, the tension of the heart muscle should be doubled which in turn reduces the efficiency of the heart muscle to maintain the same blood pressure.

Since the heart is stretched it may not be able to produce sufficient force to maintain normal circulation. Stretched heart muscle is less efficient than the normal. It consumes much more O2 for the same amount of work.

1.5.4. Bradycardia

Patients with inadequate electrical signal in the heart muscle will affect the work load of heart. The artrioventricular node i.e. between Atria and Ventricles is fatty and does not conduct electric signal and ventricle receive no signal from Atria, but being natural pacing centers which provide a pulse. The resulting heart rate is 30 beat/min i.e. Bradycardia results semi invalidism.

1.5.5. Pace Makers

If heart’s electrical signals are inadequate to stimulate heart muscles, artificial pace makers are available. To improve the quality of life of faulty atrioventricular nodes, artificial pacemakers are developed.

The pacemaker contains a pulse generator that put out 72 beats / min. The pace maker is put just below the right collarbone. It lasts for 2 years and impervious to body fluids and do not cause tissue reaction.

1.5.6. Valve Defects

Another heart disease is defective heart valves. These are of two types.

1) The valve either does or opens wide enough (stenosis). In stenosis large amount of work is to be done by heart to obstruct the narrow opening.

2) It does not close well enough (insufficiency).In insufficiency some of the pumped blood flows back and the amount of blood in circulation is reduced .Both types can be replaced by artificial valves.

1.5.7. Cardiovascular Diseases


Some cardiovascular diseases involve the blood vessels. An aneurysm is a weakening of the wall of an artery which results increase in its diameter in turn increases the tension in the wall proportionately. If it is ruptured in brain, a type called Cerebrovascular accident (CVA).

A more common blood vessel problem is the formation of sclerotic plaques on the walls the artery which causes turbulence in blood flow increases the blood velocity at that point with a decrease in wall pressure due to Bernoulli’s theorem.

A Disease in Varicose Vein

Veins with defective valves which allow the blood to flow backward become enlarged or dilated to form the varicose veins. During walking or other exercise, the contraction of the muscle forces the venous blood toward the heart called venous pump. At various points along the veins there are one way flaps or valves that prevent the blood from going back. If these valves become defective blood run backward and pool up in the vein becomes “varicose”. The standard treatment for varicose veins is surgical removal of the offending vessels. There are sufficient parallel veins to carry the blood back to the heart.

Stiffness of RBC Membrane

In some cases, mainly in smoking, the membrane of RBC s becomes stiff. There may not be normal flow of blood in the vascular system. Blood may become viscous leading to Thrombosis.

1.6. Electrophysiology of Heart

The rhythmical action of the heart is considered by an electrical signal initiated by spontaneous stimulation of special muscle cells located in the upper right hand corner of the right atrium near the superior vena cava. This area is known as “sino atrial node”

(Fig 1.5).

Cardiac electro physiology is dedicated to the study of the electro chemical activity of the heart. Studies include electrical activation of individual cells as well as the system- level activation, which results in normal or abnormal heart rhythm. (J.Olansen

et al 2000).

The complex system found by the Autonomous Nervous System (ANS) and the heart is modeled as if it was a modulation system, where the first generates a signal that modulates a sequence of pulses which excite the heart (Manuel Duarte Ortigueira et al 1959 ).

The sinus rhythm fluctuates around the mean heart rate, which is due to continuous alteration in the autonomous neural regulation i.e. sympathetic and parasympathetic balance. Periodic fluctuations found in heart rate originate from regulation related to respiration, blood pressure (baroreflex) and thermoregulation (Pauli Tikkanen 1999).

Cells in the SA node generate their electrical signal more frequently than cells else where in the heart. These impulses spread rapidly through inter nodal pathways to Atrioventricular node (AV node). At this node the signal is delayed so that all muscle cells of the atria contract virtually in unison. Now the impulse conducts through fibrous connective tissue between atria and ventricles known as “Atrio ventricular bundle “(AV bundle). AV bundle conducts the signal through left and right bundles of “Purkinje fibers” which conduct the cardiac signal to all parts of the ventricles.

1.5.Fig. Electrophysiology of the heart.

1.6.1. Sinoatrial Node

The sinoatrial node is a small, flattened ellipsoid strip of specialized muscle about

3 mm wide, 15 mm long and 1mm long located at the upper right hand corner of the right atrium immediately below and slightly lateral to the opening of the superior vena cava.

The Sinoatrial (SA node), the atrioventricular (AV node) and the Purkinje system can be regarded as potential pacemaker tissues in heart. As the fastest depolarization impulse spreads through the conduction system to other pacemakers before they spontaneously depolarize, the sinoatrial node usually defines heart rate (Pali Tikkanen 1999 12).

The sinus nodal fibers connect directly with the atrial muscle fibers, so that any action potential generates at the sinus node spreads immediately to the atrial muscle wall. For this reason Sinoatrial node is also known as “pace maker” of the heart. It generates the impulse at the rate of about 70/min and initiates the heart beat. However this rate may increase or decrease by the demand of blood supply to the body.

Three types of membrane ion channels play an important role in causing the voltage charges the action potential. They are 1) fast sodium channels 2) slow calcium-sodium channels 3) potassium channels. As the ions move in muscle cells in fractions of second creates action potential at the Sinoatrial node. This can be observe

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