Heart Nodes and Electrical Conduction

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What Are Heart Nodes?

Cardiac Electrical Conduction System
Cardiac Electrical Conduction System Showing Nodes and Bundle Fibers. OpenStax, Anatomy & Physiology/Wikimedia Commons/CC BY Attribution 3.0

A node is a specialized type of tissue that behaves as both muscle and nervous tissue. When nodal tissue contracts (like muscle tissue), it generates nerve impulses (like nervous tissue) that travel throughout the heart wall. The heart has two nodes that are instrumental in cardiac conduction, which is the electrical system that powers the cardiac cycle. These two nodes are the sinoatrial (SA) node and the atrioventricular (AV) node.

Sinoatrial (SA) Node

The sinoatrial node, also referred to as the pacemaker of the heart, coordinates heart contractions. Located in the upper wall of the right atrium, it generates nerve impulses that travel throughout the heart wall causing both atria to contract. The SA node is regulated by autonomic nerves of the peripheral nervous system. Parasympathetic and sympathetic autonomic nerves send signals to the SA node to either accelerate (sympathetic) or slow down (parasympathetic) heart rate depending on need. For example, heart rate is increased during exercise to keep up with the increased oxygen demand. A faster heart rate means that blood and oxygen are delivered to muscles at a more rapid rate. When a person stops exercising, heart rate is returned to a level appropriate for normal activity.

Atrioventricular (AV) Node

The atrioventricular node lies on the right side of the partition that divides the atria, near the bottom of the right atrium. When the impulses generated by the SA node reach the AV node, they are delayed for about a tenth of a second. This delay allows atria to contract, thereby emptying blood into the ventricles before ventricular contraction. The AV node then sends the impulses down the atrioventricular bundle to the ventricles. The regulation of electrical signals by the AV node ensures that electrical impulses do not move too rapidly, which can result in atrial fibrillation. In atrial fibrillation, atria beat irregularly and very rapidly at rates of between 300 to 600 times per minute. Normal heart rate is between 60 to 80 beats per minute. Atrial fibrillation can result in adverse conditions, such as blood clots or heart failure.

Atrioventricular Bundle

Impulses from the AV node are passed along to atrioventricular bundle fibers. The atrioventricular bundle, also called the bundle of His, is a bundle of cardiac muscle fibers located within the septum of the heart. This fiber bundle extends from the AV node and travels down the septum, which divides the left and right ventricles. The atrioventricular bundle splits into two bundles near the top of the ventricles and each bundle branch continues down the center of the heart to carry impulses to the left and right ventricles.

Purkinje Fibers

Purkinje fibers are specialized fiber branches found just beneath the endocardium (inner heart layer) of the ventricle walls. These fibers extend from atrioventricular bundle branches to the left and right ventricles. Purkinje fibers rapidly relay cardiac impulses to the myocardium (middle heart layer) of the ventricles causing both ventricles to contract. Myocardium is thickest in heart ventricles allowing ventricles to generate enough power to pump blood to the rest of the body. The right ventricle forces blood along the pulmonary circuit to the lungs. The left ventricle forces blood along the systemic circuit to the rest of the body.