Dorothy FarrarThe sinus node
The sinus node (SA node or sino-atrial node) is the primary electrical pacemaker of the heart , which is primarily responsible for the generation of excitation and the heart rate.
Definition
The sinus node is located in the right atrium of the heart and is responsible for the heart’s action. What is felt as palpitations is the job of the sinus node. It regulates this heart movement and is therefore crucial for the functionality and reactions of the entire body.
Anatomy
The sinus node is located on the so-called epicardium (outer heart wall layer) and has the shape of a spindle. However, the size and location are individually different and often differ significantly from each other.
An atrial branch of the coronary artery supplies the node with blood . It is made up of specialized heart muscle cells that can spontaneously depolarize, resulting in electrical excitation. Three fiber bundles go from the sinus node to the AV node: anterior internodal bundle (Bachmann-James bundle) middle internodal bundle (Wenckebach bundle) posterior internodal bundle (Thorel bundle)
The heart does not require stimulation through the nerves , but pumps independently, since there are so-called pacemakers or pulse generators. These discharge spontaneously, with the most important center being the sinus node. This is where the superior vena cava joins the right atrium. It is not a knot that can be felt, but an accumulation of cells in the form of a spindle.
The blood supply comes from the right coronary artery. The normal rate of the sinus node is 60 to 80 beats per minute. However, it depends on the level of training, age and other individual factors. During physical exertion it is between 120 and 200 beats, but at night it drops to 50 beats per minute.
From the sinus node, the excitation then spreads to the atria to the AV node (atrioventricular node), which is located between the ventricles and the atria. Here the excitation is somewhat delayed and then the transmission takes place via His bundle , Tawara limbs or Purkinkje fibers. If the excitation reaches the working muscles of the ventricles, they contract and the blood is ejected from the heart.
Function
The sinus node is the heart’s clock. It creates the so-called sinus rhythm. The movement of the heartbeat is initiated on the basis of electrical impulses, with the impulses reaching the AV node via the atria.
This is a kind of filter: If, for example, two heartbeats reach the AV node due to an error, the second is filtered out so that the heart function is not impaired.
Electrical activity is carried from the AV node to the apex of the heart. If there is a disturbance in spontaneous excitation, then there is a change in the cardiac rhythm or a cardiac arrhythmia.
Diseases
- Bradycardia
- cardiac arrest
There are various disorders associated with the sinus node, which are summarized under the term “sick sinus syndrome”. These mainly include frequency changes: if the frequency is too fast, this disturbance is called tachycardia, if the frequency is too slow, it’s called bradycardia. If the sinus node stops working, the heart stops working and acute cardiac arrest occurs.
After a brief pause, a secondary pacemaker usually kicks in, which can take over the job of the sinus node. This works at a lower frequency, which is sufficient for a healthy person. Depending on the level of frequency, different symptoms appear. When heart rate drops, sufferers experience fainting and dizziness . If the heart rhythm is permanently slowed down, the symptoms of cardiac insufficiency appear. These include shortness of breath , reduced physical performance and water retention in the legs or lungs.
Overactivity of the sinus node leads to breathing difficulties , tachycardia and feelings of tightness in the chest. The pain in the chest also very often radiates to the neck or left arm . In addition, phases with a reduced number of beats and phases with increased pacing can alternate, which is referred to as atrial flutter or atrial fibrillation.
Sinus node syndrome often occurs in patients who suffer from high blood pressure or coronary artery disease in particular, since the heart muscle is not supplied with enough oxygen . If there is no increase in heart rate during exercise, this is referred to as chronotropic incompetence.
In addition, an AV block can also occur if the transmission of impulses from the sinus node to the ventricle is disturbed. Three different forms can be distinguished: First-degree AV block: The transmission of the impulses is delayed. Treatment is usually not required. Second-degree atrioventricular block: Signal transmission occasionally fails. If patients suffer from heart disease, treatment is necessary. Third-degree atrioventricular block: There is a complete disruption of the conduction pathways and sufferers experience the symptoms of bradycardia.
Hello and welcome to my Health Guide & Encyclopedia! My name is Dorothy Farrar, and I'm the founder and main author of this platform.
My passion for health and wellness started at a young age when I became interested in the connection between the food we eat and the way we feel. This fascination led me to study nutrition and dietetics in college, where I learned about the importance of a balanced diet and the impact of various nutrients on the body.