Computational Intelligence in Cardiology and Heart Disease Diagnosis - Computational Intelligence in Biomedical Engineering - page 150

heart beats about 90 times/min, which stabilizes to about 70 beats/min by
18 years and remains in this region throughout adulthood.
4.2.4 The Heart Conduction System
The heart possesses a specialized nerve conduction system that allows it to
beat autonomously even if devoid of control from the main nervous system. If
the heart is removed from the body, it will continue beating for several hours
if supplied with the appropriate nutrients.
There are four main components to the heart's conduction system: the
sinoatrial (SA) node, internodal fiber bundles, AV node, and the AV bundle.
The pacemaker of the heart responsible for controlling the heart rate is the
SA node composed of a small group of specialized cardiac muscle situated in
the superior (upper) part of the right atrium. It lies along the anterolateral
margin of this chamber between the opening of the superior vena cava and
the right auricle and has the special property of being able to produce self-
excitation, which initiates a heartbeat. The fibers of the SA node fuse with
the surrounding atrial muscle fibers allowing action potentials generated in
the nodal tissue to spread to both atria causing atrial contraction. The speed
of contraction is approximately 0.3 m/s. This action potential is carried at a
higher speed by several internodal fiber bundles, which lie among the atrial
muscle fibers. These fibers lead to the AV node, which is located in the right
atrium near the lower part of the interatrial septurn.
The AV node (also known as the bundle of His) induces a short delay
(approximately 0.1 s) in transmission of the action potential to the ventricles.
This delay is important because it allows the atria to complete their contrac-
tion and pump all the blood into the ventricles before contraction occurs. The
delay occurs not only within the fibers of the AV node itself but also at special
junctional fibers that connect the node with ordinary atrial fibers.
The AV bundle then descends a short distance into the interventricular sep-
turn, which divides into the right- and left-bundle branches. Each of these
branches descends along its respective side of the interventricular septum
immediately beneath the endocardium and spreads out into progressively
smaller branches. These specialized muscle fibers are called Purkinje fibers
(see Figure 4.1), which penetrate approximately one-third of the distance into
the myocardium. The primary purpose of the Purkinje fibers is to conduct
the action potential at approximately six times the velocity of an ordinary
cardiac muscle (1.5-4.0 m/s) allowing the Purkinje fibers to channel a very
rapid and simultaneous distribution of the action potential throughout the
muscular walls of both ventricles. The fibers themselves terminate at the car-
diac muscle within the ventricles, providing a pathway for action potentials to
enter the ventricular muscle. When the impulse enters the ventricle muscle, it
causes ventricular contraction, which proceeds upward from the apex of the
heart toward its base.
A heartbeat starts in the right atrium when the SA node dispatches an
impulse throughout the atria causing both atriums to contract. The impulse