Lecture 5 – Physiology Of The Heart Specialized Conduction System Of the Mammalian Heart (Slide 1, Fig 14.9) 1. Myocardium refers to heart muscle. 2. The two atria and the two ventricles have a total of four valves: one between the two atria and ventricles, one between the right ventricle and the pulmonary artery, and one between the left ventricle and the aorta. 3. The yellow lines in the drawing, the Bundle of His and Purkinje fibers, are modified muscle cells, not neurons. 4. The coronary osteum is the point in the right atrium where all venous blood drains into the heart from the superior and inferior vena cavas (coronary sinus veins). 5. Just to the left of the coronary osteum is the SA node, or pacemaker. The SA node displays automaticity, meaning it is a non-nervous group of cells that generate their own rhythm. 6. An action potential is initiated at the SA node. A wave of depolarization beginning at the SA node, sweeps over the two atria simultaneously. This wave converges at the AV node, located at the bottom of the right atrium. 7. There is a wall between the atria and ventricles called the AV ring. This ring of cells does not depolarize, which is why the AV node is necessary. The depolarization wave stops at the AV node, giving the ventricles enough time to fill with blood. 8. The AV node displays automaticity as well. From the AV node, a wave of depolarization travels down to the Bundle of His located in the wall between the two ventricles. From there, it bifurcates. Two sets of bundle branches run down the divide, one for the left ventricle and one for the right. At the base of the heart, the fibers divide into Purkinje fibers, which run along the outside of the heart. These Purkinje fibers run from the base to the apex of the ventricles. 9. Because the AV node has automaticity, the ventricles can pump even if the SA node is not working. 10. Parasympathetic innervation of the heart can slow it down. Cardiac Muscle Behaves Like A Syncytium (Slide 2, Fig 14.8) 1. Syncytium means “same tissue.” 2. The heart behaves as if it were just one cell. 3. Each cell is connected to the next using intercalated discs. The disc is a space between two adjacent sarcomeres with gap junctions. These junctions are low-resistance channels that carry Na+ current to the next cell. This helps conduct the action potential very quickly. Pathway Of Action Potential (Slide 3, Fig 14.10) 1. An action potential is initiated at the SA node. 2. Action potentials are conducted from the SA node to the atrial muscle. 3. Action potentials spread through the atria to the AV node, where conduction slows. The conduction slows to allow the ventricles to fill with blood as well as to give the ventricles a break during their resting stage. Because there is a transition of cells from atrial to ventricle cells at the AV node, the conduction slows. The conduction rates for the two cells are different, with ventricular cells conducting slower than atrial ones. 4. Action potentials travel rapidly through the conduction system to the apex of the heart. 5. Action potentials spread upward through the ventricular muscle. 6. Eventually, the entire heart returns to the resting state, remaining there until another action potential is generated at the SA node. 7. The rates of depolarization are as follows: 1 m/s for the SA node, 0.5 m/s for the AV node and 5 m/s for the Purkinje fibers. 8. The septum acts as an anchor to stabilize the ventricles as they contract upward. 9. The AV node is highly susceptible to rhythm disturbances because of its morphology and because it has a high degree of sympathetic/parasympathetic innervation from the central nervous system. Summary And Conclusions (Slide 4) 1. Syncytium and intercalated discs 2. Origin of the action potential 3. Specialized conduction system (not nerves) 4. Spread of conduction of action potential Comparative Morphology Of Action Potentials (Slide 5, Fig 14.11, 14.12) 1. In this figure, the left side represents SA and AV nodes. The right represents atria, ventricles and Purkinje fibers. There are many differences between the two action potentials. SA and AV Nodes Atria, Ventricles and Purkinje Fibers Response Slow response Fast response Phases 3 phases (1, 2, 3) 5 phases (4, 0, 1, 2, 3) Resting potential Higher Lower Repolarization Does not go above 0 and falls straight down Plateaus at the top, then drops slowly 2. The action potential of the SA/AV node is a normal action potential depolarization/repolarization. 3. The action potential of the atria, ventricles and Purkinje fibers is different. a. Phase 4 – During this phase, PK, PCa and PNa are at their resting potentials. The cell is at -90mV. b. Phase 0 – This is the depolarization phase. Potential rises to +30mV to +40mV. Na+ flows into the cell. c. Phase 1 – Repolarization begins. Na+ channels begin to close. However, the depolarization that began in phase 0 causes two other events to occur: the closing of voltage-gated K+ channels and the opening of voltage-gated Ca2+ channels. This depolarization counteracts the repolarization. d. Phase 2 – During this plateau phase, the settings of last phase remain in place, keeping this depolarization-repolarization in check. e. Phase 3 – During this full repolarization phase, voltage-gated K+ channels open. These channels are different from those in phase 1. So the flow of K+ out of the cell increases, making the internal cellular environment more negative. f. Phase 4 – Everything returns to back to equilibrium at -90mV. Characteristics Of Fast And Slow Cardiac Myocytes (Tables 14.1, 14.2) Table 14.1 Autorhythmic Cell Potential Change Ion Channel Gating Ion Movement Pacemaker potential: initial period of spontaneous depolarization to subthreshold Latter period of spontaneous depolarization to threshold Funny channels open T-type calcium channels open Sodium moves in, potassium moves out Calcium moves in Rapid depolarization phase of action potential L-type calcium channels open Calcium moves in Repolarization phase of action potential Potassium channels open Potassium moves out 1. There is no true resting potential during phase 0 in these cells. They go through diastolic depolarization (pacemaker potential, automaticity, slow conduction velocities, especially in AV node). Table 14.2 Phase Of Contractile Cell Action Potential Ion Channel Gating Ion Movement 0 Rapid depolarization Sodium channels open Sodium moves in 1 Small depolarization Sodium channels inactivate Sodium movement in decreases 2 Plateau Potassium inward rectifier channels close Calcium L-type channels open Potassium movement out decreases Calcium moves in 3 Repolarization Potassium delayed rectifier channels open Calcium L-type channels close Potassium moves out Calcium movement in decreases 4 Resting potential Potassium channels (both types) open Sodium and calcium channels are still closed Potassium moves out Little sodium or calcium moves in 2. There is a true resting potential during phase 0, -90mV. There is no real pacemaker activity under physiological conditions. These cells have fast conduction velocities, especially atrial myocytes and Purkinje fibers. Excitation Coupling (Slide 6, Fig 14.13) 1. A current spreads through gap junctions to contractile cells. 2. The action potentials travel along plasma membranes and T tubules. 3. Ca2+ channels open in the plasma membrane and sarcoplasmic reticulum. 4. Ca2+ induces Ca2+ release from the sarcoplasmic reticulum. 5. Ca2+ binds to troponin, exposing myosin-binding sites, and muscle fibers contract. 6. The crossbridge cycle begins. 7. Ca2+ is actively transported bacy into the sarcoplasmic reticulum and extracellular fluid. 8. Tropomyosin blocks myosin-binding sites, and muscle fibers relax. Summary And Conclusions (Slide 7) 1. There are slow myocytes in SA and AV nodes. 2. There are fast myocytes in atrial and ventricular myocardium. 3. Slow myocytes have 3 phases while fast myocytes have 5 phases. 4. Slow myocytes are “pacemakers.” 5. EC coupling and calcium-induced calcium release Use Of Electrocardiographic Leads To Record Electrical Activity Of Entire Heart (Slide 8, Fig 14.14) 1. An equilateral triangle is formed around the heart, and the vertices are extended until they land on the hands and legs. This is called Einthoven’s triangle. 2. One positive and one negative electrode is placed at each location to monitor the heart’s activity. What Are Cardiac Arrhythmias? (Slide 9, Fig 14.16) 1. The first picture is a normal heart rhythm. The PR wave has to occur at specific intervals. The ST segment should be flat. During a heart attack, it is either depressed or elevated. 2. The second picture is one of a heart attack during which the ST segment is depressed. This occurs when the resting heart rate is greater than 100 beats/minute and is called sinus tachycardia. 3. The sinus bradycardia, the third picture, is one of an athlete where the ST segment is extended. That is a very healthy heart. This occurs when the resting heart rate is less than 50 beats/minute. 4. The SA node controls the heart rate, called the sinus rhythm. Summary And Conclusions (Slide 10) 1. More women die each year from heart disease than from any other cause. So do more men. 2. Heart disease is the leading cause of death among former NFL, NHL and NBA players. 3. It is also a leading cause of death among former governors of New York, mayors of Chicago and winners of Miss America pageants. 4. Learn how to care for your hearts.