Page 147 - 84 human physiolofy part-1

The Cardiovascular System

infarction occluding it will cause ischemia in the SA node unless there is a sufficiently good
anastomosis from the left coronary artery. If not, death of the affected cells will stop the SA node from
triggering the heartbeat

AV Node
The atrioventricular node (abbreviated AV node) is the tissue between the atria and the ventricles of the
heart, which conducts the normal electrical impulse from the atria to the ventricles. The AV node
receives two inputs from the atria: posteriorly via the crista terminalis, and anteriorly via the interatrial
septum. [1] An important property that is unique to the AV node is decremental conduction. This is the
property of the AV node that prevents rapid conduction to the ventricle in cases of rapid atrial rhythms,
such as atrial fibrillation or atrial flutter. The atrioventricular node delays impulses for 0.1 second
before spreading to the ventricle walls. The reason it is so important to delay the cardiac impulse is to
ensure that the atria are empty completely before the ventricles contract (Campbell et al, 2002). The
blood supply of the AV node is from a branch of the right coronary artery in 85% to 90% of
individuals, and from a branch of the left circumflex artery in 10% to 15% of individuals. In certain
types of supraventricular tachycardia, a person could have two AV Nodes; this will cause a loop in
electrical current and uncontrollably-rapid heart beat. When this electricity catches up with itself, it will
dissipate and return to normal heart-beat speed.

AV Bundle
The bundle of HIS is a collection of heart muscle cells specialized for electrical conduction that
transmits the electrical impulses from the AV node (located between the atria and the ventricles) to the
point of the apex of the fascicular branches. The fascicular branches then lead to the Purkinje fibers
which innervate the ventricles, causing the cardiac muscle of the ventricles to contract at a paced
interval. These specialized muscle fibers in the heart were named after the Swiss cardiologist Wilhelm
His, Jr., who discovered them in 1893. Cardiac muscle is very specialized, as it is the only type of
muscle that has an internal rhythm; i.e., it is myogenic which means that it can naturally contract and
relax without receiving electrical impulses from nerves. When a cell of cardiac muscle is placed next to
another, they will beat in unison. The fibers of the Bundle of HIS allow electrical conduction to occur
more easily and quickly than typical cardiac muscle. They are an important part of the electrical
conduction system of the heart as they transmit the impulse from the AV node (the ventricular
pacemaker) to the rest of the heart. The bundle of HIS branches into the three bundle branches: the
right left anterior and left posterior bundle branches that run along the intraventricular septum. The
bundles give rise to thin filaments known as Purkinje fibers. These fibers distribute the impulse to the
ventricular muscle. Together, the bundle branches and purkinje network comprise the ventricular
conduction system. It takes about 0.03-0.04s for the impulse to travel from the bundle of HIS to the
ventricular muscle. It is extremely important for these nodes to exist as they ensure the correct control
and co-ordination of the heart and cardiac cycle and make sure all the contractions remain within the
correct sequence and in sync.

Purkinje Fibers
Purkinje fibers (or Purkyne tissue) are located in the inner ventricular walls of the heart, just beneath
the endocardium. These fibers are specialized myocardial fibers that conduct an electrical stimulus or
impulse that enables the heart to contract in a coordinated fashion. Purkinje fibers work with the
sinoatrial node (SA node) and the atrioventricular node (AV node) to control the heart rate. During the
ventricular contraction portion of the cardiac cycle, the Purkinje fibers carry the contraction impulse
from the left and right bundle branches to the myocardium of the ventricles. This causes the muscle
tissue of the ventricles to contract and force blood out of the heart — either to the pulmonary
circulation (from the right ventricle) or to the systemic circulation (from the left ventricle). They were

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