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Heart activity • Modulation of autorhythmic cells • Electrical activity co-ordinates heart contraction • Pacemakers • Electrocardiogram shows heart electrical activity close Modulation of autorhythmic cells • NE (sympathetic) and E (adrenal hormone) • Autorhythmic cells have beta1 receptors • Cyclic AMP levels increase • Properties of If and Ca++ channels altered • More rapid Na+ and Ca++ entry • Rapid action potential • Rapid contractions Modulation of autorhythmic cells • Parasympathetic, acetyl choline • Muscarinic receptors • K+ channels open mb hyperpolarizes cell less excitable • Ca++ channel less likely to open slower depolarization cell is less excitable Electrical activity in the heart • Begins with action potential in autorhythmic cells. • Gap junctions spread depolarization through intercalated discs The heart’s main pacemaker is in the sinoatrial node (SA node) where a group of autorhythmic cells are located at the junction of the vena cava and the right atrium The conductive system of the heart consists of noncontractile autorhythmic cells Action potentials spread across atria. Action potentials encounter fibrous Tissue at the junction of the atria and Ventricles. Action potential reach the ventricles Through the AV node, then spread Rapidly down the bundle of His. Prukinje fibers rapidly transmit Impulses up the ventricle All contractile cells at apex Contract simultaneously Why does the AV node delay action potentials? • This allows the atria to complete contraction and allows the ventricles to fill before the ventricles contract Autorhythmic cells • All cells of the conducting fibers are autorhythmic • The cells of the SA node set the heart rate because their rhythm is fastest. • They set the pace. • If the SA node is damaged, the other pacemakers still function, the pace lowers. The electrocardiogram shows the electrical activity of the heart. • • • • ECG Electrodes placed on the skin On arms and leg ECG shows summated electrical activity during heart activity • Indirectly shows contraction and relaxation of atria and ventricles Einthoven’s triangle. Location of electrode placement For ECG The electrocardiogram. Know the points. ECG • P wave, atria depolarize • QRS complex, wave of ventricular depolarization • T wave, ventricles repolarize The heart cycle • Period from the beginning of one heart beat to the beginning of the next heart beat • Two phases: – Diastole, period of cardiac muscle relaxation – Systole, period of cardiac muscle contraction • The atria and ventricles do not contract and relax at the same time. 5 phases of the heart cycle • Heart at rest (atrial and ventricular diastole) • Completion of ventricular filling (atrial systole) • Early ventricular contraction (first heart sound) • The heart pumps (ventricular ejection) • Ventricular relaxation (second heart sound)