The adult cardiac is a hollow muscular organ which located in the thoracic cavity between the sternum and the spinal column. Heart Consists from four chambers (right atrium, right ventricle, left ventricle and left atrium), ventricles are in the lower side and have thicker walls than atria from top side. The heart is also divided in to left and right part by muscular wall called septum. Tricuspid valve is between right atrium and right ventricle, Mitral valve is between the left atrium and left atrium, pulmonic valve is between right ventricle and the pulmonary artery and Aortic valve is between the left ventricle and the aorta. The contraction of the right ventricle will pump out the deoxygenated blood to the lungs which this occurring in a rhythmatic beats and the contraction of the left ventricle will pump out the oxygenated blood from the heart in to the rest part of the body.
When cardiac cells in the resting state the concentration of the Na+ ions is higher from out side of the cells than inside the cells and the concentration of the negatively charged ions (organic phosphate, organic sulphate ions and protein ions) and small portion of positively charged potassium ions (K+) is higher inside the cells which makes the internal cellular negatively charged. The cell membrane is impermeable for free flow ions to cross the membrane from out side to inside and vice versa, the resting cell is called polarized. The electrical potential across the cardiac cell membrane called resting membrane potential which is (-90mV) in atrial and ventricular myocardial cells and (-70mV) in the SA and AV nodes.
When electrical impulses activates myocardial cells the membrane of polarized myocardial cells will turn to open the Na+ channels leading to flow in the sodium ions from out side of the cells in to the cells with their ionic concentration gradient, thus the interior cell become less negative with respect to it is exterior and the cell membrane potential decreases to (-65mV to -70mV) this causes the huge number of the Na channels open and these channels facilitate free flow of sodium ions across the membrane in to the cells and this results in change of the interior charge in to positive and the moment in which the positively charged ions inside the cells equal to the positively charged ions out side the cells the membrane potential becomes (0mV) and the myocardial is depolarized and when reaches (+20mV to +30mV) is called (overshoot).
After when myocardial cells depolarized potassium (K+) ions flow out from the cells and cells return to resting state and this process called repolarisation. Cardiac action potential (AP) It is representation of the changes in the membrane potential of a cardiac cell during depolarization and repolarisation, action potential of the heart is divided in to four phases (figer1): Phase 0: which is depolarization phase which is tall upstroke of action potential ,when it triggering the fast sodium channels to open and sodium ions enter the cells and the intercellular charge becomes positive (+20mV to + 30mV).
Phase 1: which is early depolarization phase in which the fast sodium channels getting closed and this causes the reduction of the flow of sodium ions in to the cells and the loss of potassium concentration in the cells and the membrane potential decreases to around (0mV). Phase 2: this is slow repolarisation phase, sodium ions (Na+) enter the cells also calcium ions (Ca+) enters the cell through the calcium channels slowly and potassium keep leaving the cells.
Phase 3: end of the rapid repolarisation and the membrane potential returns to (-90mV) resting state. Phase 4: after when the membrane potential returned to resting state there will be still some extra sodium inside the cells and extra potassium ions outside the cells, sodium-potassium pump is stimulated to transport the potassium in to the cells and sodium out side the cells. (Figure1) shows the cardiac action potential phases The cardiac cells have important characteristics which are Automaticity (which is the ability of the cardiac pacemaker cells to produce electrical impulses), Excitability-Irritability (response to the electrical impulses), Conductivity (the ability of cardiac cells to receive the electrical impulses and transmitting them) and Contractility (ability of cardiac cells to contract).
As the heart is able to generate it is own electrical impulses. The conduction system which consists of two areas called nodes that contain conduction cells and special pathways that transmit the impulse transmits signals throughout the (atria) and (ventricles) chambers of the heart to make it beat in a regular coordinated rhythm. Electrical conduction pathway The normal heartbeat starts when an electrical impulse is fired from the sinoaterial node (SA node) from the right atrium. The sinus node is controlling the rate and rhythm of the heart and therefore it is the heart’s “pacemaker”. (SA node) which initiate 60 to 100 electrical impulses per minute, impulses transmit down via the muscle of both atria through (intra-arterial conduction pathways) leading to the depolarization of the atrium.
Atrioventricular node (AV node), which can function as a secondary pacemaker if AV node stopped from working, located in the lower right atrium and this transmits the impulses from atria to the bundle of His. This makes 40 to 60 impulses per minute. Bundle of His, it is located under AV node and transmits electrical impulses to it is branches.