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Heart Physiology Electrical events Se4ng the Basic Rhythm: • The heart's (cardiac muscle) ability to depolarize and contract is intrinsic. • Intrinsic cardiac conduc-on system – made up of autorhythmic cells conEnuously depolarize (a.k.a. driG towards threshold). • Pacemaker poten3als – spontaneous change in membrane potenEals. – The loss of K+ and the entry of Na+ depolarizes the membrane to the point of threshold (-‐40 mV). – Once threshold is reached, Ca2+ influx in the autorhythmic cells produces the acEon potenEal. LocaEon of Autorhythmic Cells: • Sinoatrial (SA) node – pacemaker "sinus rhythm". • Atrioventricular (AV) node • AV Bundle • Bundle branches • Purkinje fibers Sinus Rhythm: • Set by the SA node, the sinus rhythm is set at 75 impulses per minute. • The total Eme of heart depolarizaEon from SA node to the last of the ventricular muscle is 2/10 second. • Gap junc3ons connect the autorhythmic cells throughout the system. Extrinsic InnervaEon of the Heart • Located in the medulla oblongata: – Cardioacceleratory center (sympatheEc neurons) • Norepinephrine – Cardioinhibitory center (parasympatheEc neurons) • actetylcholine Electrocardiograph “ECG” P wave = atrial depolarizaEon QRS complex = ventricle depolarizaEon and contracEon T wave = ventricle repolarizaEon P-‐R interval = depolarizaEon through the enEre conducEon system Q-‐T interval = ventricle depolarizaEon followed by ventricle repolarizaEon Defects in the ICS: • Arrythmia: irregular heart rhythm. • FibrillaEons: rapid and irregular contracEons. Normal ECG Extrasystole Heart Block FibrillaEon FibrillaEon