Wiktor Wise
The heart's electrical system is critical to its function. The heart is a pump made up of muscle tissue, and its pumping action is regulated by an electrical conduction system that coordinates the contraction of the various chambers of the heart. This electrical conduction system is controlled by the sinus node, a small mass of specialized tissue located in the right upper chamber (atria) of the heart. The sinus node generates an electrical impulse that travels through the heart, causing it to contract and pump blood. This electrical impulse can be measured by a test called an electrocardiogram (EKG), which traces the movement of electrical signals across the heart and allows for the assessment of any irregularities in the heart's electrical system.
| Characteristics | Values |
|---|---|
| Electrical impulse origin | Sinus node (also called the sinoatrial node, SA node, or the heart's "natural pacemaker") |
| Sinus node location | Upper right chamber (atrium) of the heart |
| Sinus node function | Generates electrical impulses/stimuli that travel through the heart |
| Normal sinus rhythm | 60 to 100 beats per minute |
| Electrical impulse pathway | From the sinus node, the impulse spreads across the right and left atria, then to the atrioventricular (AV) node, and finally to the ventricles |
| AV node function | Slows down electrical impulses before they reach the ventricles |
| Conduction pathway | After the AV node, the electrical current continues down the bundle of His, dividing into right and left bundle branches to stimulate the right and left ventricles |
| Abnormal conditions | Certain heart tissue can act as a secondary pacemaker, causing a faster heart rate; extra electrical pathways can cause abnormally fast heartbeats; abnormalities can lead to arrhythmias |
| Heart block | A condition where electrical signals cannot reach the ventricles, resulting in a slow or absent pulse |
| Treatment for heart block | Pacemaker implantation |
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What You'll Learn
The sinus node
The rate of action potentials produced by the sinus node, and therefore the heart rate, is influenced by various factors, including environmental and physiologic factors. The autonomic nervous system, for example, regulates the firing of the sinus node and influences the heart rate. Parasympathetic input slows down the rate of action potential production, decreasing the heart rate, while sympathetic input increases it. This regulated control of the sinus node allows the heart to adapt to various stressors placed on the body, such as increased oxygen demand during exercise.
Sinus node dysfunction (SND) can occur when the electrical pacemaker function of the sinus node is disrupted. This can be caused by ischemia, necrosis of pacemaker cells, or changes in the electrophysiology of the sinus node. SND, also known as sick sinus syndrome, can lead to symptoms such as dizziness and syncope due to reduced cerebral perfusion. Implantable electronic pacemakers are currently the only effective treatment for SND.
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Electrical impulses
The heart's electrical system is critical to its function. The heart requires a source of energy and oxygen to function, and its pumping action is regulated by an electrical conduction system that coordinates the contraction of its chambers. This electrical conduction system is controlled by electrical impulses that originate in the sinus node, a small mass of specialised tissue located in the right upper chamber (atria) of the heart. The sinus node is often referred to as the heart's "natural pacemaker".
The sinus node generates an electrical impulse regularly, 60 to 100 times per minute under normal conditions. This electrical impulse then spreads across the right atrium and the left atrium (the top two chambers of the heart), causing both atria to contract. This action pushes blood into the right and left ventricles, the bottom two chambers of the heart. As the electrical impulse passes through the atria, it generates a "P" wave on an electrocardiogram (EKG), which is used to assess irregularities in the heart's electrical system.
After stimulating the atria, the electrical impulse travels to the atrioventricular (AV) node, located in the middle of the heart between the atria and ventricles. The AV node acts as a gatekeeper, slowing down the electrical impulse for a very short period, allowing the atria to contract a fraction of a second before the ventricles. This delay is reflected in the EKG as the PR interval, a pause in electrical activity before the impulse reaches the ventricles.
From the AV node, the electrical impulse continues down the conduction pathway into the ventricles through a pathway called the bundle of His, which divides into right and left bundle branches to provide electrical stimulation to both ventricles. The ventricles then contract, sending blood throughout the body. This entire process of electrical impulse propagation and cardiac contraction is what ensures a regular, efficient, and coordinated heartbeat.
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Atrial depolarization
The electrical system of the heart is critical to its functioning. The heart generates its own electrical signal, which is produced by a tiny structure known as the sinus node (also called the sinoatrial node, or SA node). This is located in the upper portion of the right atrium. The sinus node generates an electrical stimulus regularly, 60 to 100 times per minute under normal conditions.
The electrical impulse originates in the sinus node and spreads across the right atrium and the left atrium (the top two chambers of the heart), causing both atria to contract. This is referred to as atrial depolarization. This action pushes blood into the right and left ventricles, the bottom two chambers of the heart. As the electrical impulse passes through the atria, it generates the so-called "P" wave on the EKG.
The P wave represents the electrical depolarization of the atria of the heart. In a healthy person, this originates at the sinoatrial node and disperses into both left and right atria. The depolarization of the right atrium is responsible for the early part of the P wave, while the depolarization of the left atrium is responsible for the middle and terminal portions of the P wave. The PR interval represents the time between atrial depolarization and ventricular depolarization.
Abnormalities in the timing of the PR segment can indicate pathology. For example, a PR interval of under 120 milliseconds (ms) may indicate that electrical impulses are travelling between the atria and ventricles too quickly, as in Wolff-Parkinson-White (WPW) syndrome. Conversely, a prolonged PR interval can indicate a first-degree atrioventricular (AV) block, caused by a delay in the electrical signal as it travels through the AV node.
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The AV node
Disruptions in the AV node's function can have significant consequences for the heart's electrical system and overall cardiac function. For example, third-degree heart block occurs when electrical signals fail to reach the ventricles, resulting in a slowed or absent pulse, often requiring a pacemaker. Bundle branch block is another conduction disorder where a blockage in one of the bundle branches causes one ventricle to contract more slowly than the other. While this condition may not always require treatment, it underscores the importance of the AV node's role in maintaining proper heart rhythm and timing.
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Conduction pathways
The electrical impulse that controls the heartbeat originates in the sinus node, a small mass of specialised tissue located in the right upper chamber (atria) of the heart. The sinus node is often referred to as the heart's "natural pacemaker". The sinus node generates an electrical stimulus regularly, 60 to 100 times per minute under normal conditions.
The electrical impulse then spreads across the right and left atria, causing both atria to contract. This is referred to as atrial depolarisation, an action that pushes blood into the right and left ventricles. As the electrical impulse passes through the atria, it generates the so-called "P" wave on an EKG.
When the wave of electricity reaches the AV disc, it is stopped, except in the AV node. The AV node is located in the middle of the heart, between the atria and ventricles. The electrical impulse travels through the AV node at a slow, controlled rate toward the ventricles, so there is a pause in the electrical activity on the EKG, referred to as the PR interval. The AV node acts as a gatekeeper, ensuring that the atria have time to contract and push blood into the ventricles before the ventricles contract.
After passing through the AV node, the electrical current then continues down the conduction pathway, through a pathway called the bundle of His, and into the ventricles. The bundle of His divides into right and left pathways (bundle branches) to give electrical stimulation to the right and left ventricles. The ventricles then contract, sending blood throughout the body.
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Frequently asked questions
The heart's electrical system is critical to how it functions. It controls the electrical impulses that cause the heart to beat and their conduction, which organises the beating of the heart.
The electrical impulse originates in a small mass of specialised tissue called the sinus node, which is located in the upper right chamber of the heart. The sinus node generates an electrical stimulus regularly, between 60 to 100 times per minute under normal conditions. The electrical impulse then spreads across the right and left atria (the two upper chambers of the heart), causing them to contract and push blood into the ventricles. The electrical signal then reaches the atrioventricular (AV) node, where it is slowed down before continuing into the ventricles (the two lower chambers of the heart).
The sinus node, also known as the sinoatrial node (SA node), is often referred to as the heart's "natural pacemaker". It controls the heart rate and ensures regular, efficient, and coordinated heartbeats.
Disruptions in the heart's electrical system can cause the heart to beat too quickly, too slowly, or irregularly. These variations in heart rhythm are called arrhythmia. Symptoms of arrhythmia can include heart palpitations, shortness of breath, dizziness, or fainting. Treatment for arrhythmia may include medication or procedures such as the use of an automated external defibrillator (AED) to restore a normal heartbeat.
