Molly Ayala
The heart is a pump made of muscle tissue. It requires an electrical impulse to start a heartbeat, which is generated in a part of the heart called the sinus node. The sinus node is a small mass of special tissue located in the upper right chamber of the heart. The electrical impulse travels from the sinus node across the cells of the heart's right and left atria, causing them to contract and push blood into the ventricles. The ventricles then contract, sending blood throughout the body. The electrical system of the heart is critical to its function, as it controls the electrical impulses that cause the heart to beat.
| Characteristics | Values |
|---|---|
| Purpose of the electrical system | To coordinate the pumping of the four chambers of the heart and control the heart rate |
| Heart's "natural pacemaker" | Sino-atrial (SA) node, a small area of special electrical tissue high on the right side of the heart |
| SA node function | Generates an electrical stimulus regularly, 60 to 100 times per minute under normal conditions |
| SA node location | Upper portion of the right atrium |
| Heart rate | Depends on a person's age, faster during exercise or excitement, and slower at rest or sleep |
| Bradycardia | Slow heart rate caused by SA node dysfunction or heart block |
| Tachycardia | Abnormally fast heart rate caused by an extra electrical pathway |
| Heart block | Electrical signals cannot get from the upper portion of the heart to the lower chambers |
| Bundle branches | Part of the electrical system that divides into right and left bundle branches to stimulate the right and left ventricles |
| Bundle branch block | Conduction disorder where one ventricle contracts slower than the other |
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The sinus node
The electrical impulse originates in the sinus node and travels across the cells of the heart's right and left atria. The atria are stimulated first and contract for a brief period before the ventricles, allowing blood to empty into the ventricles. The impulse then continues to the atrioventricular node (AV node), where it is slowed down momentarily before passing through the bundle of His into the ventricles.
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Electrical impulses and heart rate
The heart's electrical system is critical to its function, controlling the electrical impulses that cause the heart to beat. The heart's pumping action is controlled by an electrical conduction system that coordinates the contraction of the heart chambers. The sinus node, also called the sinoatrial node (SA node), is the natural pacemaker of the heart and controls the heart rate. It is a small mass of specialised tissue located in the right upper chamber (atria) of the heart. The sinus node generates an electrical stimulus regularly, 60 to 100 times per minute under normal conditions. This electrical pulse travels down through the conduction pathways, causing the heart's lower chambers (ventricles) to contract and pump out blood. The right and left atria are stimulated first and contract to push blood from the atria into the ventricles. The ventricles then contract to pump blood out into the blood vessels of the body. Each contraction of the ventricles represents one heartbeat.
The electrical impulse starts on the right side of the upper chamber in the sinus node and travels a set path through the upper chambers, causing them to contract and squeeze blood into the lower chambers. The electrical signal then reaches the atrioventricular (AV) node, located between the atria and ventricles. The AV node slows down the electrical impulses for a very short period, allowing the atria to contract a fraction of a second before the ventricles. This delay in the AV node creates a pause in the electrical activity, which is referred to as the PR interval. After passing through the AV node, the electrical current continues down the conduction pathway, through the bundle of His, and into the ventricles.
The bundle of His divides into right and left bundle branches to give electrical stimulation to the right and left ventricles. The bundle branches are a part of the heart's electrical system, which controls the heartbeat. The heart normally beats faster during exercise or excitement and more slowly when at rest or sleeping. The heart rate is determined by the number of electrical impulses, with each impulse generating one heartbeat.
The electrical impulses in the heart can be too slow or too fast. Slow electrical impulses, called bradycardia, can be caused by heart block or sinus node dysfunction. Heart block occurs when the electrical signal cannot get through to the lower chambers, resulting in a slower heartbeat. Sinus node dysfunction occurs when the sinus node, the heart's normal pacemaker, does not work regularly, causing a slow heartbeat. Fast electrical impulses, called tachycardia, can be caused by an extra electrical pathway between the atria and ventricles, allowing the electrical impulse to create a continuous loop.
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Bradycardia
The heart is a pump made of muscle tissue. Its pumping action is controlled by an electrical conduction system that coordinates the contraction of the heart chambers. An electrical stimulus is generated in a special part of the heart muscle called the sinus node or sinoatrial node (SA node). The SA node is a small mass of specialised tissue in the right upper chamber (atria) of the heart. In an adult, the SA node sends out a regular electrical pulse 60 to 100 times per minute. This electrical pulse travels through the heart, causing the heart to contract and pump out blood.
Borderline or occasional bradycardia may not require treatment. However, severe or prolonged bradycardia can be treated in several ways. For instance, if medication side effects are causing the slow heart rate, then the medication regimen can be adjusted or discontinued. A pacemaker can often regulate the heart’s rhythm, speeding up the heart rate as needed.
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Tachycardia
The heart is a pump made of muscle tissue. Its pumping action is controlled by an electrical conduction system that coordinates the contraction of the heart chambers. The heart's electrical activity is generated by a small mass of specialized tissue called the sinus node (also called the sinoatrial node, or SA node). This node is located in the right upper chamber (atria) of the heart. The sinus node generates an electrical stimulus regularly, 60 to 100 times per minute under normal conditions.
There are several types of tachycardia:
- Sinus tachycardia: An increase in heart rate due to faster electrical signals from the sinus node. While it is a common response to exercise, sinus tachycardia at rest may indicate an underlying issue.
- Ventricular tachycardia (VT): A fast heart rate originating in the lower chambers (ventricles) of the heart. VT is often associated with disorders that interfere with the heart's electrical conduction system, such as lack of coronary artery blood flow or abnormalities in heart tissue.
- Supraventricular tachycardia (SVT): A type of tachycardia where the heart rate exceeds 100 beats per minute and originates in the upper chambers (atria) of the heart. SVT is the most common heart arrhythmia in infants and children.
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The AV node
The heart is a pump made up of muscle tissue. Its pumping action is controlled by an electrical conduction system that coordinates the contraction of the heart chambers. The sinus node (also called the sinoatrial node or SA node) is a small mass of specialised tissue located in the right upper chamber (atria) of the heart. The sinus node generates an electrical stimulus regularly, 60 to 100 times per minute under normal conditions.
The electrical stimulus then travels from the sinus node to the atrioventricular node (also called the AV node). The AV node is a small structure in the heart, located in the Koch triangle, near the coronary sinus on the interatrial septum. The AV node receives two inputs from the right atrium: posteriorly, via the crista terminalis, and anteriorly, via the interatrial septum. The cells of the AV node are specialised cardiac muscle cells (cardiomyocytes), also known as nodal cardiac muscle cells, which are smaller than typical cardiomyocytes and lack intercalated discs.
An important feature of the AV node is its ability to slightly delay electrical signals, thus coordinating the contraction firstly of the atria and secondly of the ventricles. This delay in the cardiac pulse is extremely important: it ensures that the atria have ejected their blood into the ventricles before the ventricles contract. This also protects the ventricles from excessively fast rate responses to atrial arrhythmias. The AV node's normal intrinsic firing rate without stimulation (such as that from the SA node) is 40–60 times/minute.
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Frequently asked questions
The heart's electrical activity is what causes it to beat. The electrical system of the heart controls the heartbeat and coordinates the pumping of the four chambers of the heart.
The sinus node, also called the sinoatrial node (SA node), is a small mass of specialized tissue located in the right upper chamber (atria) of the heart. It is the heart's natural pacemaker and generates an electrical stimulus regularly, 60 to 100 times per minute under normal conditions.
When there is a problem with the electrical activity of the heart, it can cause an irregular heartbeat. This can be due to a blockage in the electrical signal, which may result in a slower or faster heartbeat. In some cases, a permanent pacemaker may be required to correct the issue.
