njit-etd2003-081 - New Jersey Institute of Technology

17
The heart, located in the chest, is a muscular organ, which is enclosing in a
fibrous sac called the pericardium [5]. The walls of the heart are primarily composed of
cardiac-muscle cells called the myocardium. Cardiac-muscle cells combine properties of
both skeletal muscle and smooth muscle. However, even more important, approximately
one percent of the cardiac-muscle fibers has specialized features that are essential for
normal heart excitation [6]. They constitute a network known as the conducting system
of the heart and are connected to other cardiac-muscle fibers by gap junctions. The gap
junctions allow action potentials to spread from one cardiac-muscle cell to another.
Figure 2.3 The intrinsic conduction system of the heart and succession of the action
potential through selected areas of the heart during one heart beat. B. The sequence of
potentials generated across the heart is shown from top to bottom beginning with the
pacemaker potential generated by the SA node and ending with an action potential.
(From E. N. Marieb, Human Anatomy and Physiology, 3 rd ed. New York: The
Benjamin/Cummings Publishing Company, Inc., 1995.)
Thus, the initial excitation of one myocardial cell results in excitation of all cells,
and as a result, the pumping action of the heart. The conducting system of the heart is