Natural Excitation of the Heart

Natural Excitation of the Heart Specialized Excitatory and Conductive System of the Heart Function: 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

Natural Excitation of the Heart SA Node = normal pacemaker Have two cell type: Small, round cells having few organelles and myofibril (Pacemaker) Slender, elongated cell (Conductor) RMP is much less negative Lack the iKl, then the ratio of gK to gNa during phase 4 is much less. Vm deviates much more from the EK and reach to threshold. Normal rhythm = 70 – 80 beats/min

Natural Excitation of the Heart AV Node Normal rhythm = 40 – 60 beats/min The AV node is composed of three functional regions: AN region (transitional zone) N region (mid portion) NH region The principal delay is in the AN and N regions of the AV node. The conduction velocity is actually slower in the N region than in the AN region. However, the path length is substantially greater in the AN than in the N region. Cause of the Slow Conduction Mainly by diminished numbers of gap junctions

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

Natural Excitation of the Heart Summary of the Spread of the Cardiac Impulse Through the Heart (second) The last portions of the ventricles to be excited are the posterior basal epicardial regions and a small zone in the basal portion of the interventricular septum.

Natural Excitation of the Heart

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

Natural Excitation of the Heart Purkinje fibers and Reentry Mechanisms Wolff-Parkinson-White syndrome.

Natural Excitation of the Heart Triggered activities 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. 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

Natural Excitation of the Heart Triggered activities 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.