Cells outside the sinus node pacemaker, for example, ventricular muscle cells, possess pacemaker activity that is so weak that the normal electrical discharge from the sinus node suppresses them. Myocardial cells normally lack the ability for either spontaneous formation or rapid conduction of the electrical impulse. For these functions most cardiac cells are dependent on cardiac pacemaker cells and the conduction system that consists of nodes, bundle branches, and a terminal branching network of specialized conducting tissue — the Purkinje fibers that ramify the myocardium. The sinus node undergoes sponta¬neous depolarization and has no resting phase; myocardial tissue must be depolarized and have a resting potential (see the chapter Electrocardiography). Occasionally, pacemaker cells outside the SA node may interrupt the normal heart¬beat, causing a premature beat, which is also called an extra beat (see Fig. 1).
The electrical conducting system of the heart is vital to life. Damage to the electrical system can occur when the coronary arteries are blocked and fail to supply sufficient blood to the electrical system as may happen after several heart attacks. The electrical system can also be affected by certain degenerative diseases that cause calcification and hardening (sclerosis) of the bundles.

Страницы: 1 | 2