BIOL 244 Chapter 18: Heart, Part 2Lecture 10 (September 23) Route of blood flow through the heart Blood enters heart through inferior & superior vena cava into the right atrium. From right atrium blood goes through the tricuspid valve into the right ventricle then throughthe pulmonary semilunar valve to pulmonary trunk, then into the right & left pulmonary arteries to the lungs. The lungs oxygenate the blood & it returns through theright & left pulmonary veins into the left atrium then through the bicuspid vale into theleft ventricle then through the aortic semilunar valve into the ascending aorta to the aortic arch for distribution into the body. Physiology of the heart muscle Action Potential Unstimulated contractile cells of hear maintain stable resting membrane potential Cardiac pacemaker (autorhythmic) cells making up intrinsic conduction system have unstable resting potential that continuously depolarizes, drifting slowly toward threshold Pacemaker potentials/prepotentials- spontaneously changing membrane potential that initiate AP that spread through heart & trigger rhythmic contractions- Slow depolarization due to both opening of Na+ channels & closing of K+ channels- Membrane potential is never a flat line Depolarization AP begins when pacemaker potential reaches threshold Due to Ca2+ influx through Ca2+ channels Repolarization Due to Ca2+ channels inactivating and K+ channels opening Allows K+ efflux, which brings membrane potential back to its negative voltage Complete repolarization- K+ channels close, K+ efflux declines, & slow depolarization to threshold begins  Role of ions in cardiac muscle contraction Na+ (sodium) Depolarization opens a few fast voltage-gated Na+ channels in sacrollema allowing extracellular Na+ to enter Initiates positive feedback cycle to cause rising phase of action potential- Reversal of membrane potential from -90 mV to +30 mV) K+ (potassium) Few K+ channels open during plateau of AP to prolong plateau and prevent rapid repolarization Repolarization:- Due to Ca2+ channels inactivating & K+ channels opening- Allows K+ efflux, which brings membrane potential back to its resting voltage Ca++ 10-20% of calcium needed enters cardiac cells from extracellular space Stimulates SR to release other 80% of Ca2+ needed Ca2+ barred from entering nonstimulated cardiac fibers Na+ dependent membrane depolarization occurs  voltage channels opens that allow Ca2+ to enter from extracellular space (slow Ca2+ channels) Calcium surge across sarcolemma prolongs depolarization potential: plateau in AP As long as Ca2+ enters the cell it will continue to contract Cardiac Conduction System/Intrinsic System of heart SA Node Located in right atrial wall, inferior to entrance of superior vena cava Generates impulses about 75 times per minute Sets pace for heart as a whole b/c no other region of conduction system or myocardium has faster depolarization rate Heart’s pacemaker & sinus rhythm determines heart rate AV Node Located inferior portion of interatrial septum right above the tricuspid valve Impulse delayed 0.1 seconds, allowing atria to respond & complete contraction before ventricles contract- Reflects smaller diameter of fibers & once through here the signaling impulse passes rapidly through the rest of the cell AV Bundle “Bundle of His” Located superior part of interventricular septum Only electrical connection between atria & ventricles Fibrous cardiac skeleton is nonconducting & insulates the rest of the junction Right and left bundle branches course along the interventricular septum toward heart apex Purkinje Fibers Subendocardial conduction network Completes pathway through interventricular septum, penetrates into heart apex & turn superiorly into ventricular walls Excite septal cells, but bulk of ventricular depolarization depends on large fibers of conducting network and cell-to-cell transmission of impulses via gap junctions between ventricular muscle cells More elaborate on left side of heart Ventricular contraction almost immediately follows ventricular depolarization