DR. HANA OMER Electrical Conduction System

The Electrical Conduction System of the Heart Cardiac cells have four properties: Excitability: allows response to electrical impulse Conductivity: ability to pass on electrical impulses Automaticity: can generate own electrical impulses Contractility: ability to contract

The Electrical Conduction System of the Heart Specialized conduction tissue propagates electrical impulses to the muscular tissue. Pacemaker: area where electrical activity arises

The Electrical Conduction System of the Heart Sinoatrial node Located in right atrium Receives blood from the RCA Fastest pacemaker Impulses are spread through internodal pathways, causing depolarization. Adapted from 12-Lead ECG: The Art of Interpretation, courtesy of Tomas B. Garcia, MD.

The Electrical Conduction System of the Heart Sinoatrial node (cont’d): Electrical impulses move from SA node to the atrioventricular (AV) node Impulse conduction is delayed so the atria will empty into the ventricles.

The Electrical Conduction System of the Heart Sinoatrial node (cont’d): Normally pass: Into bundle of His Into right and left bundle branches Into Purkinje fibers A dromotropic agent is one which affects the conduction speed in the AV node, and subsequently the rate of electrical impulses in the heart.

Resting stateBoth sodium & potassium channels are closed & membrane is in resting state Depolarization phase Activation gates of sodium channels open, but potassium channels remain closed & so sodium ions rush into the cells leading to more +ve charge inside the cells

Repolarization phaseInactivation gates close the sodium channels, but potassium channels open allowing potassium ions to leave the cells ---- leading to decrease in +ve charge inside the cell Back to resting state

The Electrical Conduction System of the Heart Depolarization: muscle fibers are stimulated to contract Occurs through changes in concentration of electrolytes across cell membranes Myocardial cells bathed in electrolyte solution Chemical pumps maintain ion concentrations.

The Electrical Conduction System of the Heart Depolarization (cont’d) Cell receives stimulus from conduction Permeability of the cell wall changes to allow sodium ions in Calcium ions also enter. Depolarization spreads, causing a mechanical contraction.

The Electrical Conduction System of the Heart Repolarization begins with the closing of sodium and calcium channels and opening of potassium channels.

The Electrical Conduction System of the Heart Myocardial cells must be fully polarized to respond normally to electrical stimulus. Refractory period: cell is depolarized or in the process of repolarizing Absolute refractory period: completely depolarized Relative refractory period: partially repolarized

The Electrical Conduction System of the Heart Secondary pacemakers Any conduction system component can act as a secondary pacemaker if the SA node is damaged. The farther removed from the SA node, the slower the intrinsic rate of firing.

The Electrical Conduction System of the Heart Measuring the heart’s electrical conduction activity Show as a series of waves and complexes on ECG Adapted from 12-Lead ECG: The Art of Interpretation, courtesy of Tomas B. Garcia, MD.

The Electrical Conduction System of the Heart