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Download EXCITATION AND CONDUCTING SYSTEM OF HEART LEARNING
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EXCITATION AND CONDUCTING SYSTEM OF HEART • • • • • LEARNING OBJECTIVES At the end of this lecture the students should be able to know: Generation of an action potential of heart. Phases of action potential. Components of conducting system of heart. Conducting pathway. Rate of conduction. ACTION POTENTIAL • An action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a stereotyped trajectory. Action potential in cardiac muscle STEP 1 • The conduction cells stimulate the myocardial cells • Events of depolarization Sodium pumps in Pottasium pumps out Sharp spike of initial action potential Action potential in cardiac muscle STEP 2 • Sodium channel close • Calcium channel open slowly Calcium trickle into the cell allow for the stronger contraction All cells contract together • Potassium movement is minimal Action potential in cardiac muscle STEP 3 • Calcium channel close up • Potassium channel widen Rush of potassium back into cell Slow return to complete resting state Contraction finish Ventricles start filling with blood Phases of action potential PHASES OF ACTION POTENTIAL • Phase 0 is the immediate that sends the voltage past the zero millivolt level, making it positive. This is due to the sudden increase in membrane permeability to sodium ions and decrease in potassium permeability PHASES OF ACTION POTENTIAL • Phase 1. The membrane potential then reaches a steady point at around zero millivolts.This is called the plateau of the action potential, PHASES OF ACTION POTENTIAL • Phase 2 There is a steadiness in the voltage. The inward flow of calcium ions is equal to that of the outward flow of potassium ions PHASES OF ACTION POTENTIAL • Phase 3 initiates repolarization • Phase 4 The voltage decreases to its original value where it will remain steady until the next action potential is generated Action potential: • • • • • • Phase 0, Na+ influx. The threshold potential, -70 mv Phase 1, K+ outward flow Phase 2, Ca2+ inward flow and K+ outward flow Phase 3, K+ outward flow Phase 4, Na+ - K+ pump and Na + - Ca2 + exchange (primary and secondary active transport) GENERAL DESCRIPTION • • • • • • • Resting potential: -90mv (-85-95mv) Action Potential Phase 0: rapid depolarization, 1-2ms Phase 1: early rapid repolarization, 10 ms Phase 2: plateau, slow repolarization, the potential is around 0 mv. 100 – 150ms Phase 3, late rapid repolarization. 100 – 150 ms Phase 4 resting potentials CONDUCTING SYSTEM OF HEART Components of conducting system • SA Node • AV Node • AV Bundle • Right and left bundle branches • Purkinje fibers Conducting tissue of the heart CONDUCTING SYSTEM OF HEART SA NODE ACTION POTENTIAL • • • • • • • • • SA node: The pacemaker. Specialized cardiac muscle cells. Generate spontaneous action potentials (autorhythmic tissue). At rest the SA Node usually produces 60-70 signals a minute Action potentials pass to atrial muscle cells and to the AV node ACTION POTENTIAL AT AV NODE AV node: atrioventricular node. The resting rate of the AV Node is slower, generating 40-60 beats a minute. Action potentials conducted more slowly here than in any other part of system. Ensures ventricles receive signal to contract after atria have contracted ACTION POTENTIAL AT AV BUNDLE • AV bundle: passes through hole in cardiac skeleton to reach interventricular septum ACTION POTENTIAL OF RIGHT AND LEFT BUNDLE BRANCHES • Right and left bundle branches: extend beneath endocardium to apices of right and left ventricles • The Bundle of HIS can generate 30-40 signals a minute. ACTION POTENTIAL OF PURKINJE FIBERS • Purkinje fibers: – – – – Large diameter cardiac muscle cells with few myofibrils. Many gap junctions. Conduct action potential to ventricular muscle cells (myocardium) Ventricular muscle cells may generate 20-30 signals a minute CONDUCTING PATHWAY OF HEART • CONDUCTING SYSTEM OF HEART All heart cells, muscle and conducting tissue, are capable of generating electrical impulses that can trigger the heart to beat. Under normal circumstances all parts of the heart conducting system can conduct over 140-200 signals (and corresponding heart beats) per minute. REFERENCES • GUYTON AND HALL text book of medical physiology • GANONG’s review of medical physiology **********************************^^^^^^^^^^^^^^^^^^^*************************
