What is the electrical conduction system?
The heart functions thanks to an internal electrical system that ensures perfect coordination of the heartbeats. The heart’s electrical conduction system is a network of specialized cells that produce and transmit electrical impulses throughout the heart. This electrical activity triggers regular contractions of the heart, allowing blood to circulate efficiently throughout the body.
These contractions are triggered by successive changes in the electrical state of the heart cells. This process, known as depolarization and repolarization, occurs in an orderly fashion to coordinate each heartbeat.
The components of the electrical conduction system
The cardiac electrical conduction system is made up of several key structures:
- Sinus node: located in the wall of the right atrium, it acts as the “conductor” of the heart rhythm. This node initiates the electrical impulses of each heartbeat.
- Atrioventricular node: located between the atria and ventricles, it receives the electrical impulse from the sinus node and slows it down slightly. This slight delay allows the atria to contract and fill the ventricles with blood before they in turn contract.
- Bundle of His: this bundle of fibers carries the impulse from the atria to the ventricles.
- Branches of the bundle of His: once the impulse has been transmitted, the bundle of His divides into two branches, right and left, which allow the impulse to reach each ventricle.
- Purkinje fibers (or Purkinje network): located in the walls of the ventricles, these fibers distribute electrical impulses in a rapid, coordinated manner, ensuring simultaneous contraction of the ventricles for powerful, efficient blood pumping.
How does the electrical conduction system work?
What is depolarization and repolarization?
Both the inside and outside of heart cell membranes are electrically charged, forming what is known as a membrane potential. At rest, the inside of cells is negatively charged compared with the outside.
When the heart receives an electrical signal, ion channels (microscopic gates) open in the cell membrane, allowing positive ions (such as sodium and calcium) to enter the cell. This changes the internal charge of the cell, which then becomes more positive.
This change in charge spreads from cell to cell, causing the heart cells, called cardiomyocytes, to contract, like a wave moving through the heart. This process is called depolarization.
Then the heart has to prepare itself for the next beat. This is where repolarization comes in. This time, other channels open to let out ions (such as potassium), restoring the cell’s initial negative charge.
Once the cells have returned to their resting state, they are ready to receive a new electrical signal.
Electrical conduction in cardiac cells
The heart’s electrical impulse is initiated by specialized cells called pacemaker cells, located mainly in the sinus node. These cells generate electrical impulses spontaneously and regularly, without the need for an external signal.
This electrical impulse then travels through the atria, causing them to contract and send blood to the ventricles.
The electrical impulse then reaches the atrioventricular node, where it is slowed slightly to allow the ventricles to fill with blood.
The impulse then travels to the ventricles via the branches of the bundle of His and the Purkinje fibers. This causes the ventricles to contract in a coordinated manner, sending blood to the aorta and pulmonary arteries.
How is the heart’s electrical activity measured?
The electrical activity of the heart is measured using an electrocardiogram (also known as an ECG or EKG). This test records the electrical signals produced by each heartbeat using electrodes (sensors) placed on the patient’s torso, arms and legs.
The signals are translated into waves, visible on a screen and printed on paper. Each wave corresponds to a stage in the electrical conduction of the heart, from contraction of the atria to relaxation of the ventricles. These waves are called: P, Q, R, S and T.
Malfunctions of the electrical conduction system
Irregularities in cardiac activity are known as arrhythmias or heart rhythm disorders. They occur when the propagation of electrical impulses does not function normally. Depending on where and how the malfunction occurs, different pathologies may appear:
- Bradycardia: the heart beats abnormally slowly, which can prevent proper blood distribution throughout the body.
- Tachycardia: the heart beats abnormally fast, which can be tiring for the heart and reduce the efficiency of blood circulation.
- Atrial fibrillation: the electrical rhythm is disorganized in the atria (the upper chambers of the heart), resulting in irregular and often rapid contraction of the atria. This can cause the formation of blood clots and increase the risk of stroke.
- Ventricular fibrillation: a disturbance in the ventricles (the lower chambers) causes rapid, uncoordinated contractions, preventing the heart from pumping efficiently. Ventricular fibrillation is a medical emergency requiring immediate treatment.
- Bundle branch block: this is a delay or block in electrical conduction in one of the branches of the bundle of His (which transmits signals to the ventricles). This blockage can slow the contraction of one side of the heart.
These disorders may be of no consequence or may lead to a variety of symptoms, such as palpitations, dizziness, fatigue or even malaise. Some require regular monitoring, while others may require specific treatment or even urgent medical intervention.
