Guyton 2011 pages Berne 2008 pages

Excitatory and Conductive System of the Heart Natural Excitation of The Heart To cause rhythmical contraction of the heart muscle Conducting impulses rapidly through the heart. Is susceptible to damage by heart disease, especially by ischemia of the heart tissues

SA Node = normal pacemaker Natural Excitation of The Heart Have two cell type RMP is much less negative Lack the i Kl, then the ratio of g K to g Na during phase 4 is much less Pacemaker Cells (Small, round cells having few organelles and myofibril) Conductor Cells (Slender, elongated cells)

SA Node = normal pacemaker Natural Excitation of The Heart Change the slope of Phase 4 Change the RMP Change threshold Potential Change the Heart Rate Normal rhythm = 70 – 80 beats/min

SA Node = normal pacemaker Depolarization and repolarization is slower in slow responses Na, Ca Ca K Natural Excitation of The Heart

Internodal Pathways Natural Excitation of The Heart

AV Node Natural Excitation of The Heart Regions AN region (transitional zone) N region (mid portion) NH region Cause of the Slow Conduction Mainly by diminished numbers of gap junctions One-way conduction Normal rhythm = 40 – 60 beats/min

Purkinje fibers Natural Excitation of The Heart Cause of the Rapid Conduction mainly by increased numbers of gap junctions Normal rhythm = 15– 40beats/min

Summary of the Spread of the Cardiac Impulse Through the Heart (second) Natural Excitation of The Heart

Conduction Rate (m/s)Tissue 0.05SA node 1Internodal Pathways 0.05AV node 1Bundle of His 2Left and Right Bundles 4Purkinje System 1Ventricular Muscle Impulse Velocity in Different Part of the Heart

Natural Excitation of The Heart Points Overrdrive Suppression Stokes - Adams Syndrome Ectopic Pacemaker

Natural Excitation of The Heart Effect of ANS of cardiac rhythmicity

Natural Excitation of The Heart Parasympathetic division Sympathetic division Mainly innervates nodes Acetylcholine Increased permeability to K Increases the overall activity of the heart Norepinephrine Increased permeability to Na and Ca Innervates all areas Decreases Heart Rate

Reentry Phenomenon Natural Excitation of The Heart

Reentry Mechanisms Natural Excitation of The Heart Pathway around the circle is too long Like cardiac dilation Velocity of conduction becomes decreased Blockage of the Purkinje system, Ischemia of the muscle, High blood potassium levels The refractory period of the muscle shortened in response to various drugs, such as epinephrine or after repetitive electrical stimulation

Global Reentry Natural Excitation of The Heart

The End

Early afterdepolarizations More likely to occur when the prevailing heart rate is slow; a rapid heart rate suppresses EADs Induced by the addition of cesium in an isolated Purkinje fiber preparation. More likely to occur in cardiac cells with prolonged action potentials than in cells with short action potentials. Early afterdepolarizations More likely to occur when the prevailing heart rate is slow; a rapid heart rate suppresses EADs Induced by the addition of cesium in an isolated Purkinje fiber preparation. More likely to occur in cardiac cells with prolonged action potentials than in cells with short action potentials. Triggered activities When action potentials are sufficiently prolonged, some Ca++ channels that were activated at the beginning of the plateau have sufficient time to recover from inactivation

Delayed afterdepolarizations The amplitudes of the DADs are increased by interventions that raise intracellular Ca ++ concentrations. Increasing the extracellular [Ca ++ ] Administering toxic amounts of digitalis glycosides. Delayed afterdepolarizations The amplitudes of the DADs are increased by interventions that raise intracellular Ca ++ concentrations. Increasing the extracellular [Ca ++ ] Administering toxic amounts of digitalis glycosides. Triggered activities