Molly Ayala
The electrical system of the heart is critical to its function. 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 a small mass of specialized tissue located in the upper right chamber (atrium) of the heart. It generates an electrical stimulus, or impulse, regularly, 60 to 100 times per minute under normal conditions, which causes the heart to beat.
| Characteristics | Values |
|---|---|
| Heart's natural pacemaker | Sinoatrial node (SA node) or sinus node |
| Location of SA node | Upper right chamber (atrium) of the heart |
| SA node function | Generates electrical stimulus 60-100 times per minute under normal conditions |
| Heartbeat | Electrical impulse causes the heart to contract 60-100 times per minute at rest, depending on the person's age |
| Heartbeat during exercise or excitement | Faster than at rest |
| Heartbeat during rest or sleep | Slower than during exercise or excitement |
| Atrioventricular node (AV node) | Located between the atria and ventricles |
| AV node function | Slows down impulses for a very short period |
| Bundle of His | A collection of cells that is part of the electrical system of the heart |
| Bundle branches | Right and left pathways that give electrical stimulation to the right and left ventricles |
| Electrical system function | Controls the heartbeat and coordinates the pumping of the four chambers of the heart |
| Electrical system dysfunction | Can cause abnormally fast or slow heartbeats |
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What You'll Learn
The sinus node
The rate of action potentials produced by the sinus node can vary depending on various environmental and physiological factors. The autonomic nervous system, through sympathetic and parasympathetic input, regulates the firing of the sinus node and thus influences the heart rate. For example, during exercise, sympathetic input increases the rate of action potential production, allowing the heart to respond to the body's increased oxygen demand.
Sinus node dysfunction (SND) can occur when there is a blockage of the arterial blood supply to the SA node, often due to myocardial infarction or progressive coronary artery disease. This can lead to conditions known as sick sinus syndrome, characterised by irregular heartbeats that are typically too slow or exhibiting pauses in function. Implantable electronic pacemakers are currently the only effective treatment for SND.
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Electrical conduction pathways
The heart's pumping action is controlled by an electrical conduction system that coordinates the contraction of the heart chambers. The electrical conduction system of the heart consists of a collection of nodes and specialised conduction cells that initiate and coordinate the contraction of the heart muscle. This group of muscle cells is called the cardiac conduction system. The main parts of the system are the SA node, AV node, bundle of His, bundle branches, and Purkinje fibres.
The SA node (sinoatrial node) is a collection of specialised cells (pacemaker cells) that can spontaneously generate electrical impulses. The SA node is located in the upper wall of the right atrium, at the junction where the superior vena cava enters. The rate at which the SA node generates impulses is influenced by the autonomic nervous system. The sympathetic nervous system increases the firing rate of the SA node, which increases heart rate, while the parasympathetic nervous system decreases the firing rate, decreasing heart rate.
The SA node starts the sequence by causing the atrial muscles to contract. The signal then travels to the AV node (atrioventricular node), where it is delayed for a very short period. This delay allows the atria to contract a fraction of a second before the ventricles, so their blood empties into the ventricles before the ventricles contract.
After passing through the AV node, the electrical current then continues down the conduction pathway, through the bundle of His, and into the ventricles. The bundle of His is a continuation of the specialised tissue of the AV node and serves to transmit the electrical impulse from the AV node to the Purkinje fibres of the ventricles. The bundle of His divides into right and left bundle branches to stimulate the right and left ventricles. The Purkinje fibres are a network of specialised cells that are able to rapidly transmit cardiac action potentials from the bundle of His to the myocardium of the ventricles.
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The AV node
The atrioventricular (AV) node, also known as the Node of Tawara, is a collection of specialized cardiac muscle cells, bundled together to form a node within the wall of the interatrial septum. It is located in the Koch triangle, near the coronary sinus on the interatrial septum. In a right-dominant heart, the AV node is supplied by the right coronary artery, while in left-dominant individuals, it originates from the left circumflex artery.
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Heart rate
The heart's electrical system controls the heart rate. The heart's pumping action is controlled by an electrical conduction system that coordinates the contraction of the heart chambers. The heart rate is determined by the number of electrical impulses generated by the sinus node, also called the sinoatrial node or SA node. This is a small mass of specialized tissue located in the right upper chamber (atria) of the heart. The SA node is the heart's "natural pacemaker" and controls the heart rate by generating an electrical stimulus regularly, 60 to 100 times per minute under normal conditions. This electrical pulse travels down through the conduction pathways and causes 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.
The electrical impulse originates in the SA node and spreads across the right and left atria, causing both atria to contract. This is referred to as atrial depolarization, which pushes blood into the ventricles. As the electrical impulse passes through the atria, it generates a "P" wave on an EKG or electrocardiogram. An EKG is a test that records the electrical activity of the heart and allows doctors to assess irregularities in the heart's electrical system.
After stimulating the atria, the electrical impulse travels to the atrioventricular node (AV node), located between the atria and the ventricles. The AV node slows down the electrical impulses for a very short period, allowing the atria to contract before the ventricles. This delay in electrical activity on the EKG is referred to as the PR interval. After passing through the AV node, the electrical impulse continues down the conduction pathway, through the bundle of His, and into the ventricles.
The heart rate can vary depending on different factors such as age, exercise, and emotional state. For example, the heart normally beats faster during exercise or excitement and slower when at rest or sleeping. Additionally, certain conditions such as bradycardia and tachycardia can affect heart rate. Bradycardia is when the electrical impulses are too slow, causing a decrease in heart rate. Tachycardia is when the electrical impulses are abnormally fast, resulting in an increased heart rate.
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Electrocardiograms (ECG/EKG)
During an ECG, electrodes (small, sticky plastic patches) are placed on the chest, arms, and legs. These electrodes are connected to an ECG machine by lead wires, which measure and record the electrical activity of the heart. The process is non-invasive and does not cause any discomfort, although some people may experience skin irritation if the patches are left on for too long. It is important for the patient to remain still and not talk during the test to ensure accurate results.
The ECG machine interprets the electrical activity and provides a printed output, known as an electrical tracing of the heart. This tracing represents the electrical impulses travelling through the heart, which cause the heart's chambers to contract and pump blood. The tracing can be used to identify any abnormalities in the heart's rhythm, which may require further evaluation or treatment.
The procedure for an ECG is generally straightforward and does not require any special care before or after the test. However, patients may be asked to remove clothing from the waist up and any objects that could interfere with the test, such as jewellery. The patient will then lie flat on a table or bed, and the electrodes will be attached to the skin. Once the test is complete, the electrodes are removed, and the patient can resume normal activities unless otherwise instructed by their healthcare provider.
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Frequently asked questions
The electrical system of the heart coordinates the pumping of the four chambers of the heart and controls the heart rate.
The electrical system of the heart involves the sinus node, which is the heart's "'natural pacemaker', generating an electrical impulse that stimulates the heart's upper chambers (atria) to contract and send blood to the lower chambers (ventricles). The electrical impulse then travels to the atrioventricular node (AV node), where it is slowed down before continuing down the conduction pathway, through the bundle of His, and into the ventricles.
The sinus node, also called the sinoatrial node (SA node), is a small mass of specialised tissue located in the right upper chamber (atrium) 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 the electrical system of the heart malfunctions, it can result in conditions such as bradycardia or tachycardia. Bradycardia is when the electrical impulses are too slow, causing a decrease in heart rate. Tachycardia is when the electrical impulses are abnormally fast, which can be caused by an extra electrical pathway between the atria and ventricles.
The electrical activity of the heart can be measured using an electrocardiogram (ECG or EKG), which involves recording the electrical activity using small stickers attached to the chest.
