The cardiac conduction system acts as the heart’s internal electrical wiring. This network of specialized cells generates and transmits electrical impulses throughout the heart muscle. Its primary function is to ensure a coordinated and efficient pump, directing blood flow through the body.
The Heart’s Natural Pacemaker
The heart’s electrical signal begins at the sinoatrial (SA) node, often called the heart’s natural pacemaker. This cluster of specialized cardiac muscle cells resides in the upper back wall of the right atrium. The SA node spontaneously generates electrical impulses, setting the pace for the entire heart. It typically fires between 60 to 100 times per minute at rest. The autonomic nervous system can influence this rate, increasing it during activity and decreasing it during rest.
The Electrical Signal’s Journey
Once generated by the SA node, the electrical impulse rapidly spreads across both atria. This activation causes atrial muscle cells to contract, pushing blood into the lower chambers. The signal then arrives at the atrioventricular (AV) node, located in the lower back section of the interatrial septum. A brief delay occurs here, lasting approximately 0.09 to 0.15 seconds. This pause allows the atria to fully empty their blood into the ventricles before ventricular contraction begins.
After the delay, the impulse travels from the AV node into the Bundle of His, a collection of specialized cells. This bundle descends through the interventricular septum, the wall separating the lower heart chambers. The Bundle of His then divides into the left and right bundle branches. These branches carry the electrical signal down to the apex, or bottom, of the heart. From there, the signal rapidly spreads throughout the ventricular walls via Purkinje fibers. These fibers ensure the impulse reaches ventricular muscle cells quickly and almost simultaneously.
Coordinating the Heartbeat
The initial impulse from the SA node triggers atrial contraction, pushing blood into the relaxed ventricles. The brief delay at the AV node allows the ventricles to fill completely before their own contraction begins. This sequential filling and subsequent powerful contraction of the ventricles propels blood out of the heart. The right ventricle pumps deoxygenated blood to the lungs, while the left ventricle pumps oxygenated blood to the rest of the body. This coordinated electrical-mechanical coupling results in a single heartbeat.
Disruptions in the Electrical System
Problems within the cardiac conduction system can lead to an abnormal heart rhythm, referred to as an arrhythmia. These disruptions occur when electrical impulses do not generate properly or do not travel along their normal pathways. Some issues manifest as a slow heart rate, known as bradycardia, stemming from a problem with the SA node’s ability to generate impulses or a blockage in signal transmission.
Conversely, an abnormally fast heart rate, termed tachycardia, can occur if other parts of the heart start generating electrical signals that override the SA node’s normal rhythm. Conditions like atrial fibrillation or atrial flutter involve very rapid electrical activity in the upper chambers, which can lead to a fast ventricular response if not managed by the AV node. Heart block represents a disruption or complete blockage of the electrical signal’s passage, often occurring at the AV node, preventing impulses from reaching the ventricles effectively. This can cause the ventricles to beat at a slower, intrinsic rate.