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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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