Beatrice Lang
The heart is a muscle that pumps blood around the body. To do this, it needs an electrical impulse to stimulate a heartbeat. This impulse is generated by a group of cells called the sinus node (also known as the sinoatrial node or SA node), which acts as a pacemaker. The sinus node is located in the right upper chamber (atrium) of the heart. The electrical signal then moves down through the conduction pathways to the ventricles, causing the heart to contract and pump blood. The rate at which the sinus node sends out electrical signals determines the heart rate.
| Characteristics | Values |
|---|---|
| Name | Sinus Node, SA Node, Sinoatrial Node |
| Location | Right upper chamber (atrium) of the heart |
| Composition | A small mass of specialized tissue |
| Function | Generates electrical stimulus/impulse |
| Normal Rate | 60-100 times per minute at rest |
| Rate Influencers | Body's nervous system, hormones, endocrine system, oxygen needs, exercise |
| Slow Rate Condition | Bradycardia |
| Fast Rate Condition | Tachycardia |
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What You'll Learn
The sinus node
The rate at which the sinus node sends out electrical impulses determines the heart rate. Under normal conditions, the sinus node generates an electrical stimulus 60 to 100 times per minute in adults at rest. However, the heart rate can adjust higher or lower depending on the body's needs. For example, during exercise, signals from the body cause the heart rate to increase, delivering more blood and oxygen to the body.
Sinus node dysfunction (SND), also known as sick sinus syndrome, can occur when the sinus node does not function properly. This can lead to irregular heart rhythms, such as bradycardia (slow heart rate) or tachycardia (fast heart rate). SND can be caused by changes in the electrophysiology of the SAN, known as electrical remodelling, which has been associated with heart failure, ageing, diabetes, and atrial fibrillation. Implantable electronic pacemakers are currently the only effective treatment for SND.
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The atrioventricular node
The AV node is an important component of the cardiac conduction system, which coordinates the contraction of the heart's chambers. It receives electrical impulses from the sinoatrial (SA) node, which is the heart's pacemaker, and transmits them to the ventricles. The SA node generates an electrical stimulus 60 to 100 times per minute under normal conditions, although this can vary depending on a person's age and level of physical activity.
An important feature of the AV node is its ability to slightly delay electrical signals by approximately 40 milliseconds or 0.09 seconds. This delay ensures that the atria have ejected their blood into the ventricles before the ventricles contract and pump blood out to the body. This protects the ventricles from excessively fast rate responses to atrial arrhythmias.
The AV node also has the ability to independently produce impulses, a process known as self-excitation. This allows it to act as a secondary pacemaker for the heart when the SA node or connected conducting fibres are dysfunctional. The normal AVN firing rate is 20 to 60 beats per minute without stimulation, compared to 60 to 100 beats per minute for the SA node.
The AV node receives its blood supply from the atrioventricular nodal artery, which is a branch of the right coronary artery in most hearts.
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The conduction pathway
The heart is a pump made up of muscle tissue. It requires a source of energy and oxygen to function. The heart's pumping action is regulated by an electrical conduction system that coordinates the contraction of the various chambers of the heart. The sinus node, a small mass of specialised tissue located in the right upper chamber (atria) of the heart, generates an electrical stimulus. This stimulus is generated regularly, 60 to 100 times per minute under normal conditions.
The electrical impulse travels from the sinus node to the atrioventricular node (AV node). At the AV node, the impulses are briefly slowed down before continuing down the conduction pathway via the bundle of His into the ventricles. The bundle of His divides into right and left pathways, called bundle branches, to stimulate the right and left ventricles.
The atria contract for a short period before the ventricles, allowing blood to empty into the ventricles. The ventricles then contract and pump blood out of the heart. The entire heartbeat process starts again, beginning with an impulse in the sinus node.
Abnormalities in the conduction pathway can lead to conditions such as bradycardia (slow heart rate) or tachycardia (fast heart rate). In some cases, an extra electrical pathway may be present, contributing to abnormal heart rhythms.
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The bundle of His
The left bundle branch further divides into the left anterior fascicle and the left posterior fascicle, giving rise to thin filaments known as Purkinje fibres. These Purkinje fibres are responsible for distributing the electrical impulse to the ventricular muscle, causing the ventricles to contract in a paced and coordinated manner.
Disorders affecting the bundle of His can lead to heart blocks, resulting in conduction delays or even a complete dissociation between the atria and ventricles. These blocks are categorised based on the location of cellular damage, with blocks in the bundle branches referred to as "bundle branch blocks" and blocks in the fascicles called "fascicular blocks".
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The ventricles
The heart's pumping action is regulated by an electrical conduction system that coordinates the contraction of the various chambers of the heart. An electrical stimulus is generated by the sinus node (also called the sinoatrial node, or SA node). This is a small mass of specialised tissue located in the right upper chamber (the atria) of the heart. The sinus node generates an electrical stimulus regularly, 60 to 100 times per minute under normal conditions. The atria are then activated, causing them to contract and squeeze blood into the ventricles. The electrical signal then reaches the atrioventricular node (AV node), which is located in the middle of the heart, between the atria and the ventricles. The AV node slows down the electrical impulses for a very short period before they continue down the conduction pathway into the ventricles. The bundle of His, which is a part of the conduction pathway, divides into right and left pathways, called bundle branches, to stimulate the right and left ventricles. The ventricles then contract, sending blood throughout the body. Each contraction of the ventricles represents one heartbeat.
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Frequently asked questions
The heart sends electrical signals to itself. The electrical impulse starts on the right side of the upper chamber in an area called the sinus node (also called the sinoatrial node, or SA node).
The sinus node is a small mass of specialized tissue located in the right upper chamber (atria) of the heart. These cells act as a pacemaker for the heart.
The electrical impulse travels from the sinus node to the atrioventricular node (also called the AV node). The signal is delayed in the AV node and then spreads through the lower chambers.
The electrical signals cause the heart to contract and pump blood. The contraction of the atria and ventricles makes a heartbeat.
