The Action Potential PDF

THE ACTION POTENTIAL Dr. Ahmed AL-twegree INTRODUCTION Cardiac action potential is a brief changes in voltage (membrane potential) across the cell membrane of the heart cells. This is caused by movements of charged ions between the inside and the outside of the cell through...

THE ACTION POTENTIAL Dr. Ahmed AL-twegree INTRODUCTION Cardiac action potential is a brief changes in voltage (membrane potential) across the cell membrane of the heart cells. This is caused by movements of charged ions between the inside and the outside of the cell through protein called ion channels. Action potential in heart initiated by group of specialized cells called SA node. SA node produce roughly about 60-100 action potential per minute. Action potential activity with in the heart can be recorded to produce an ECG. Fast response action potential  Phases of Cardiac action potential - Phase 0 (Depolarization) - Phase 1 (initial repolarization) - Phase 2 (plateau) - Phase 3 (rapid repolarization) - Phase 4 (RMP) PHASE 0 (DEPOLARIZATION) - Voltage gated sodium channels (fast sodium channels) open and permit sodium to rapidly flow into the cell and depolarize it. - The membrane potential reaches about +20 millivolts before the sodium channels close PHASE 1 (INITIAL REPOLARIZATION) - Fast sodium channels close and the cells begins to repolarize, and potassium ions leave the cell through open potassium channels. PHASE 2(PLATEAU) - Is due to (A) increased calcium ion permeability and (B) decreased potassium ion permeability - The combination of decreased potassium ion efflux and increased calcium ion influx causes the action potential plateau. PHASE 3(RAPID REPOLARIZATION) - The closure of calcium ion channels and increased potassium ion permeability, permitting potassium ions to rapidly exit the cell ending the plateau phase PHASE 4(RMP) - Averages about -90 millivolts. COMPARISON WITH SKELETAL MUSCLE: - The action potential of skeletal muscle is caused by the sudden opening of fast sodium channels that allow sodium ions to enter the skeletal muscle fiber. - In cardiac muscle, (1) voltage activated fast sodium channels and (2) L-type calcium channels (slow calcium channels), which are also called calcium- sodium channels. - This second population of channels differs from the fast sodium channels in that they are slower to open and remain open for several tenths of a second. - During this time, a large quantity of both calcium and sodium ions flows through these channels to the interior of the cardiac muscle fiber, and this activity maintains a prolonged period of depolarization, causing the plateau in the action potential. - Immediately after the onset of the action potential, the permeability of the cardiac muscle membrane for potassium ions decreases about fivefold, an effect that does not occur in skeletal muscle. - This decreased potassium permeability may result from the excess calcium influx through the calcium channels just noted. - The decreased potassium permeability greatly decreases the outflux of positively charged potassium ions during the action potential plateau and thereby prevents early return of the action potential voltage to its resting level. Slow response action potential VELOCITY OF SIGNAL CONDUCTION IN CARDIAC MUSCLE - The velocity of conduction of action potential signal along both atrial and ventricular muscle fibers is about 0.3 to 0.5 m/sec, or about 1/250th of the velocity in very large nerve fibers and about 1/10 the velocity in skeletal muscle fibers. - The velocity of conduction in the specialized heart conductive system—in the Purkinje fibers—is as great as 4 m/sec. EFFECTIVE REFRACTORY PERIOD (ERP), ARP RRP & SNP - The effective refractory period is the amount of time in which the cell can not respond to a new conducted stimuli. - The normal refractory period of the ventricle is 0.25 to 0.30 second. - During this period heart stays in rhythm and prevents arrhythmia. - It is composed of phase 0, 1, 2 and 3. - The cell also will not respond to a stimulus that comes from a neighboring cell.