Four Steps of Cardiac Conduction

Heart Electrical Conduction
The heart beat starts in the heart's natural pacemaker, the sinoatrial (SA) node (green). This small area of specialized muscle cells emits an electrical impulse about 70 times a minute. The impulse spreads through the muscles of the atria (upper heart chambers), making them contract, and is carried via special fibers to the atrioventricular (AV) node (orange). The AV node delays the impulse to allow the ventricles (lower heart chambers) to fill, before effecting ventricular contraction via branches of the bundle of HIS (red). JOHN BAVOSI/SCIENCE PHOTO LIBRARY/Getty Images

Cardiac Conduction System

Have you ever wondered what causes your heart to beat? Your heart beats as a result of the generation and conduction of electrical impulses. Cardiac conduction is the rate at which the heart conducts electrical impulses. These impulses cause the heart to contract and then relax. The constant cycle of heart muscle contraction followed by relaxation causes blood to be pumped throughout the body.

Cardiac conduction can be influenced by various factors including exercise, temperature, and endocrine system hormones.

Step 1: Pacemaker Impulse Generation

The first step of cardiac conduction is impulse generation. The sinoatrial (SA) node (also referred to as the pacemaker of the heart) contracts, generating nerve impulses that travel throughout the heart wall. This causes both atria to contract. The SA node is located in the upper wall of the right atrium. It is composed of nodal tissue that has characteristics of both muscle and nervous tissue.

Step 2: AV Node Impulse Conduction

The atrioventricular (AV) node lies on the right side of the partition that divides the atria, near the bottom of the right atrium. When the impulses from the SA node reach the AV node, they are delayed for about a tenth of a second. This delay allows atria to contract and empty their contents into the ventricles prior to ventricle contraction.

Step 3: AV Bundle Impulse Conduction

The impulses are then sent down the atrioventricular bundle. This bundle of fibers branches off into two bundles and the impulses are carried down the center of the heart to the left and right ventricles.

Step 4: Purkinje Fibers Impulse Conduction

At the base of the heart the atrioventricular bundles start to divide further into Purkinje fibers.

When the impulses reach these fibers they trigger the muscle fibers in the ventricles to contract. The right ventricle sends blood to the lungs via the pulmonary artery. The left ventricle pumps blood to the aorta.

Cardiac Conduction and the Cardiac Cycle

Cardiac conduction is the driving force behind the cardiac cycle. This cycle is the sequence of events that occur when the heart beats. During the diastole phase of the cardiac cycle, the atria and ventricles are relaxed and blood flows into the atria and ventricles. In the systole phase, the ventricles contract sending blood to the rest of the body.

Cardiac Conduction System Disorders

Disorders of the heart's conduction system can cause problems with the heart's ability to function effectively. These problems are typically the result of a blockage that diminishes the rate of speed at which impulses are conducted. Should this blockage occur in one of the two atrioventricular bundle branches that lead to the ventricles, one ventricle may contract more slowly than the other. Individuals with bundle branch block typically don't experience any symptoms, but this issue can be detected with an electrocardiogram (ECG). A more serious condition, known as heart block, involves the impairment or blockage of electrical signal transmissions between the heart's atria and ventricles.

Heart block electrical disorders range from first to third degree and are accompanied by symptoms ranging from light headedness and dizziness to palpitations and irregular heart beats.