Wiktor Wise
The electrical activity within our heart is a fascinating process that is critical to its functioning. The heart's electrical system, known as the cardiac conduction system, is a network of nodes, cells, and signals that work together to control our heartbeat. This intricate system ensures that electrical impulses travel through the heart, causing it to contract and pump blood throughout our body. The process begins with an electrical stimulus generated by a small mass of specialized tissue called the sinus node, located in the upper right chamber of the heart. From there, the electrical impulse travels through the heart's chambers, causing them to contract in a coordinated manner. Any disruptions in this electrical pathway can lead to serious complications, including cardiac arrest. Understanding and maintaining the health of our heart's electrical system is crucial for overall cardiovascular well-being.
| Characteristics | Values |
|---|---|
| Electrical impulse origin | Sinus node (also called the sinoatrial node or SA node) |
| Sinus node location | Upper portion of the right atrium (upper chamber of the heart) |
| Sinus node function | Generates an electrical stimulus 60-100 times per minute under normal conditions |
| Electrical impulse pathway | From the sinus node to the atrioventricular node (AV node) |
| AV node function | Slows down impulses for a very short period before continuing to the ventricles |
| Ventricles function | Contract and pump blood out into the blood vessels of the body |
| Atria function | Contract to push blood from the atria into the ventricles |
| Heart rate control | Autonomic nervous system regulates SA node speed based on activity and rest |
| Heart rate increase | Sympathetic nervous system (fight or flight response) |
| Heart rate decrease | Parasympathetic nervous system (rest and digest response) |
| Heart rate measurement | Electrocardiogram (EKG or ECG) traces electrical signals and rhythm of the heart |
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What You'll Learn
The sinus node
The electrical impulse originates in the sinus 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 right and left ventricles. As the electrical impulse passes through the atria, it generates a "P" wave on an EKG or electrocardiogram.
Sinus node dysfunction, also known as sick sinus syndrome, is a group of irregular heartbeat conditions caused by faulty electrical signals in the heart. When the sinus node is defective, the heart's rhythms become abnormal, typically too slow or exhibiting pauses in its function. Implantable electronic pacemakers are currently the only effective treatment for this condition.
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Electrical impulses
The heart is a pump made up of muscle tissue. The heart's pumping action is controlled by an electrical conduction system that coordinates the contraction of the heart chambers. The cardiac conduction system is the network of nodes, cells, and signals that controls the heartbeat.
The electrical impulse originates in 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 sinus node generates an electrical stimulus regularly, 60 to 100 times per minute under normal conditions. The right and left atria are stimulated first and contract to push blood from the atria into the ventricles. This is referred to as atrial depolarization.
The electrical impulse then travels from the sinus node across the cells of the heart's right and left atria. As the electrical impulse passes through the atria, it generates a P" wave on the EKG. When the wave of electricity reaches the AV disc, it is stopped, except in the AV node. The AV node is located between the atria and ventricles. The 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. This pause 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 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 to pump blood out into the blood vessels of the body.
The electrical impulses in the heart can be too slow, causing a decrease in heart rate, or bradycardia. This can be caused by heart block, which occurs when the electrical signal that starts in the upper portion of the heart cannot get through to the lower chambers. Bradycardia can also be caused by sinus node dysfunction, where the sinus node does not work with regularity. On the other hand, the impulses can be abnormally fast, which is called tachycardia. In children with abnormally fast heartbeats, there may be an extra electrical path in addition to the normal path, causing the electrical impulse to make a continuous loop.
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Electrocardiogram (ECG/EKG)
The electrical system of the heart is critical to its function. It controls the electrical impulses that cause the heart to beat and their conduction, which organises the beating of the heart. The cardiac conduction system is the network of nodes, cells and signals that controls the heartbeat. The sinus node (also called the sinoatrial node or SA node) generates an electrical stimulus regularly, 60 to 100 times per minute under normal conditions. The sinus node is a small mass of specialised tissue located in the right upper chamber (atria) of the heart.
An electrocardiogram (ECG or EKG) is a quick, easy and simple test used to evaluate the heart's electrical activity. It is a recording of the heart's electrical activity and is an integral part of the initial evaluation of a patient suspected of having a cardiac-related problem. The word ECG is derived from the German word "elektro-kardiographie". Electrodes (small, plastic patches that stick to the skin) are placed at certain locations on the chest, arms, and legs. When the electrodes are connected to an ECG machine by lead wires, the electrical activity of the heart is measured, interpreted, and printed out. The procedure is non-invasive and carries minimal risk.
The movement of electrical signals across the heart is what is traced on an EKG. As the electrical impulse passes through the atria, it generates the "P" wave on the EKG. When the wave of electricity reaches the AV disc, it is stopped, except in the AV node. The 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 ECG procedure may be done on an outpatient basis or as part of a hospital stay. The patient lies flat on a table or bed for the test and it is important that they lie still and do not talk during the ECG so as not to change the results. The ECG will take only a short time for the tracing to be completed. Once the tracing is completed, the technician will disconnect the leads and remove the electrodes.
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Cardiac conduction system
The cardiac conduction system is the network of nodes, cells, and signals that controls the heartbeat. It is the heart's electrical system, generating and conducting electrical impulses that cause the heart to beat. The cardiac conduction system is made up of impulse-generating nodes and the impulse-propagating His-Purkinje system.
The SA node (sinoatrial node) is a collection of specialised cells (pacemaker cells) that can spontaneously generate electrical impulses. These pacemaker cells are located in the upper wall of the right atrium, where the superior vena cava enters. The SA node generates an electrical stimulus regularly, 60 to 100 times per minute under normal conditions. The wave of excitation created by the SA node spreads across the atria, causing them to contract. This is known as atrial depolarisation, pushing blood into the ventricles.
The electrical impulse then reaches the AV node (atrioventricular node), where it is slowed down for a very short period. The AV node is located near the central area of the heart, and its function is to delay the SA node's electrical signal by a fraction of a second. This delay ensures that the atria are empty before the contraction stops. After the AV node, the electrical impulse travels down the conduction pathway via the bundle of His, a collection of specialised conduction cells, into the ventricles. The bundle of His and the Purkinje fibres spread the wave impulses along the ventricles, causing them to contract.
The cardiac conduction system is critical to the functioning of the heart. The electrical signals generated by this system control the beating of the heart, regulating blood flow through the heart and body.
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Heart rate and autonomic nervous system
The human heart is a muscular organ responsible for pumping blood throughout the body. The heart's electrical system, also known as the cardiac conduction system, is a network of nodes, cells, and signals that controls the heartbeat. This system generates electrical impulses that organise the heart's beating, with each heartbeat comprising a contraction of the ventricles.
The autonomic nervous system, which consists of the sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS), plays a crucial role in regulating heart rate. The SNS, often associated with the "'fight-or-flight' response, increases heart rate by releasing hormones like epinephrine and norepinephrine. On the other hand, the PNS, associated with the "rest and digest" response, slows down the heart rate by releasing acetylcholine.
The SNS and PNS work together to maintain heart rate balance. During mild exercise, for example, a decrease in parasympathetic activity leads to a slight increase in heart rate. However, as exercise intensity increases, the SNS becomes more prominent, resulting in a more substantial acceleration of the heart rate. Similarly, when transitioning from a resting state to an upright posture, the SNS demonstrates a more significant role in increasing heart rate compared to the PNS.
Regular participation in cardiovascular exercise can decrease resting heart rate over time by increasing heart size, contractile strength, and the duration of blood filling the heart. This reduction in resting heart rate is attributed to increased activity of the parasympathetic nervous system and potentially decreased activity of the sympathetic nervous system.
Heart rate is typically measured when the body is at rest, and a normal resting heart rate ranges from 60 to 100 beats per minute. Resting rates above 100 bpm may indicate that the heart is working excessively hard to circulate blood, warranting medical attention. Conversely, resting rates below 60 bpm are common among endurance-trained athletes, whose bodies are more efficient at utilising oxygen from the blood.
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Frequently asked questions
The heart's electrical system controls the electrical impulses that cause the heart to beat. The electrical impulse originates in the sinus node, which is a small mass of specialized tissue located in the upper right chamber (atria) of the heart.
The sinus node generates an electrical stimulus regularly, 60 to 100 times per minute under normal conditions. The electrical impulse then travels through the conduction pathways, causing the heart's ventricles to contract and pump out blood.
An electrocardiogram (ECG or EKG) is used to assess the rhythm of the heart by recording its electrical activity.
Certain conditions are associated with a slow heartbeat, known as bradycardia, or a fast heartbeat, known as tachycardia. In some cases, an extra electrical path or an abnormal "focus" can cause abnormally fast heartbeats.
