The sinoatrial node, better known as the sinuatrial or SA node is the key structure responsible for the generation of a regular heartbeat and is therefore often referred to as the pacemaker of the heart. It is the first component of the cardiac conduction system and is composed of specialized cardiac muscle cells, which are bundled together into a node within the right atrium. These cells have the ability of self-excitation, so that the heart is able to contract independently of any extrinsic innervation.
The following article will discuss the anatomy and function of the SA node.
|Node of specialized cardiac muscle cells (pacemaker cells) located just beneath the epicardium in the wall of the right atrium
|Generates impulses for the contractions of the heart; known as pacemaker of the heart
|Sinuatrial nodal branch of right coronary artery
|Sympathetic and parasympathetic branches from cardiac plexus
The sinuatrial node is found in the posterolateral wall of the sinus of venae cavae in the right atrium (hence ‘sinu-’ + atrial) close to the junction of the superior vena cava with the right atrium, deep to the crista terminalis. It is made up of specialized cardiomyocytes, also known as nodal cardiac muscle cells or ‘pacemaker’ cells which are grouped together into an elongated ellipsoid bundle with a length of 8 to 25 mm. Nodal cardiac muscle cells are smaller than typical cardiomyocytes and lack intercalated discs. They are capable of self-excitation, meaning they are able to independently produce spontaneous electrical impulses. They communicate directly with and excite adjacent/perinodal cardiomyocytes through gap junctions and thus induce the contractions of the heart.
The sinuatrial node is one of several structures of the cardiac conduction system. This intrinsic conduction system generates impulses for the contractions of the heart and ensures the coordinated flow of blood through the chambers of the heart. The components of this system include:
- Sinuatrial (SA) node
- Atrioventricular (AV) node
- Atrioventricular (AV) bundle (of His)
- Right and left bundle branches
- Purkinje cells
As the cells of the SA node send impulses at the fastest rate, the SA node acts as the pacemaker of the heart and the remaining components of the cardiac conduction system are synchronized to its rhythm. The impulses from the SA node spread across the atria, causing them to contract, before moving on to the AV node and further components of the conduction system mentioned above. Specialized conducting fibers between these different structures allow for quick transfer of impulses (four times faster than across normal myocardium). Each of these structures has a different localization and is responsible for the excitation of cardiac muscle cells in a particular area of the heart; this allows for coordinated blood flow through the chambers of the heart.
Would you like to find out more about the anatomy and function of the remaining structures of the cardiac conduction system? Read all about them in the following article:
The sinuatrial node receives its arterial blood supply from the sinuatrial nodal branch of the right coronary artery in the majority (60%) of individuals but can also arise from the left coronary artery/circumflex artery in around 40% of individuals. In the former example, the supply originates near the origin of the right coronary artery and ascends posteriorly to the superior vena cava to supply the SA node as well as the pulmonary trunk.
Although the heart contains its own intrinsic cardiac conduction system, it relies on external autonomic innervation received via the cardiac plexus to modulate its function. The parasympathetic portions of the cardiac plexus receive contributions from the vagus nerve only and are responsible for reducing the heart rate and force of contraction. The sympathetic part of the cardiac plexus is received via the cardiac nerves which originate from the cervical and upper thoracic sympathetic ganglia. Sympathetic input to the heart is responsible for increasing heart rate and force of contraction, such as during the ‘fight or flight’ response, which causes our heart to beat faster.
Test your knowledge on the nerves of the heart with this quiz.
Would you like to master the neurovascular components of the heart? Work through the following study units, which provide you with interactive materials, such as videos and quizzes:
Sick sinus syndrome
Sick sinus syndrome is a collective term used in reference to disorders of the sinuatrial node. The umbrella term addresses disorders that result in abnormally fast heart rates (tachycardia) or abnormally slow heart rates (bradycardia). It also includes some disorders that can cause the heart rate to switch between tachycardic and bradycardic states (bradycardia-tachycardia syndrome). Sick sinus syndrome can also include pauses in sinuatrial node activity longer than 2 or 3 seconds. It is a relatively rare disorder that becomes more prevalent with increasing age.
In addition to aging, sick sinus syndrome may also be caused by medication used to slow down the heart (beta-blockers, calcium channel blockers and digitalis), or to lower blood pressure levels and abnormal electrolyte levels (hyperkalemia). Other causes of sick sinus syndrome include (but are not limited to) previous heart attack, hypothyroidism and amyloidosis (abnormal deposition of amyloid tissue throughout the body).
Some patients living with sick sinus syndrome may experience palpitations, presyncopal or syncopal episodes, fatigue, weakness, acute onset of confusion and chest pain. The abnormal heart rate may also cause sleep disturbances as well. The formal diagnosis can be made using a continuous electrocardiogram (ECG) known as a Holter monitor. ECG tracing is collected over 24 to 48 hours and evaluated for any abnormality. Ultimately, these patients may require placement of an artificial pacemaker to help regulate the heart rate.
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