Sinoatrial node
The sinoatrial node is the impulse-generating (pacemaker) tissue located in the right atrium
Right atrium
The right atrium is one of four chambers in the hearts of mammals and archosaurs...

 of the heart
The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...

, and thus the generator of normal sinus rhythm
Sinus rhythm
In medicine, sinus rhythm is the normal beating of the heart, as measured by an electrocardiogram . It has certain generic features that serve as hallmarks for comparison with normal ECGs.- ECG structure :...

. It is a group of cells positioned on the wall of the right atrium, near the entrance of the superior vena cava
Superior vena cava
The superior vena cava is truly superior, a large diameter, yet short, vein that carries deoxygenated blood from the upper half of the body to the heart's right atrium...

. These cells are modified cardiac myocytes. Though they possess some contractile filaments, they do not contract.

It was first described in 1907 by Arthur Keith
Arthur Keith
Sir Arthur Keith was a Scottish anatomist and anthropologist, who became a fellow of the Royal College of Surgeons of England and Hunterian Professor and conservator of the Hunterian Museum of the Royal College of Surgeons in London...

 and Martin Flack.

Role as a pacemaker

Although all of the heart's cells have the ability to generate the electrical impulses (or action potential
Action potential
In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...

s) that trigger cardiac contraction, the sinoatrial node normally initiates it, simply because it generates impulses slightly faster than the other areas with pacemaker potential
Pacemaker potential
In the pacemaking cells of the heart , the pacemaker potential is the slow, positive increase in voltage across the cell's membrane that occurs between the end of one action potential and the beginning of the next action potential...

. Cardiac myocyte
A myocyte is the type of cell found in muscles. They arise from myoblasts.Each myocyte contains myofibrils, which are long, long chains of sarcomeres, the contractile units of the cell....

s, like all muscle cells, have refractory periods following contraction during which additional contractions cannot be triggered; their pacemaker potential is overridden by the sinoatrial or atrioventricular nodes.

In the absence of extrinsic neural and hormonal control, cells in the sinoatrial node (SA node), situated in the upper right corner of the heart, will naturally discharge (create action potentials) at about 60-100 beats/minute. Because the sinoatrial node is responsible for the rest of the heart's electrical activity, it is sometimes called the primary pacemaker.

If the SA node does not function, or the impulse generated in the SA node is blocked before it travels down the electrical conduction system, a group of cells further down the heart will become the heart's pacemaker. These cells form the atrioventricular node
Atrioventricular node
The atrioventricular node is a part of the electrical control system of the heart that coordinates heart rate. It electrically connects atrial and ventricular chambers...

 (AV node), which is an area between the atria and ventricles
Ventricle (heart)
In the heart, a ventricle is one of two large chambers that collect and expel blood received from an atrium towards the peripheral beds within the body and lungs. The Atria primes the Pump...

, within the atrial septum. If the AV node also fails, Purkinje fibers
Purkinje fibers
For the nervous cells, see Purkinje cellPurkinje fibers are located in the inner ventricular walls of the heart, just beneath the endocardium...

 (or known by some as the bundle of His) are capable of acting as the pacemaker. The reason Purkinje cells do not normally control the heart rate is that they generate action potentials at a lower frequency than the AV or SA nodes.


The sinoatrial node is submyocardial at the lateral aspect of the junction of the superior vena cava
Superior vena cava
The superior vena cava is truly superior, a large diameter, yet short, vein that carries deoxygenated blood from the upper half of the body to the heart's right atrium...

 and right atrium
Right atrium
The right atrium is one of four chambers in the hearts of mammals and archosaurs...

. Its deep aspect abuts cardiac myocytes belonging to the right atrium. Its superficial aspect is covered by adipose tissue. The SA node fibres vaguely resemble cardiac myocytes; however, they are measurably thinner, more tortuous and stain less intensely (on H&E) than cardiac myocytes.


The SA node is richly innervated by parasympathetic nervous system
Parasympathetic nervous system
The parasympathetic nervous system is one of the two main divisions of the autonomic nervous system . The ANS is responsible for regulation of internal organs and glands, which occurs unconsciously...

 fibers (CN X: Vagus Nerve
Vagus nerve
The vagus nerve , also called pneumogastric nerve or cranial nerve X, is the tenth of twelve paired cranial nerves...

) and by sympathetic nervous system
Sympathetic nervous system
The sympathetic nervous system is one of the three parts of the autonomic nervous system, along with the enteric and parasympathetic systems. Its general action is to mobilize the body's nervous system fight-or-flight response...

 fibers (T1-4, Spinal Nerves
Spinal nerve
The term spinal nerve generally refers to a mixed spinal nerve, which carries motor, sensory, and autonomic signals between the spinal cord and the body...

). This unique anatomical arrangement makes the SA node susceptible to distinctly paired and opposed autonomic
Autonomic nervous system
The autonomic nervous system is the part of the peripheral nervous system that acts as a control system functioning largely below the level of consciousness, and controls visceral functions. The ANS affects heart rate, digestion, respiration rate, salivation, perspiration, diameter of the pupils,...

  • Stimulation of the vagus nerves (the parasympathetic fibers) causes a decrease in the SA node rate (thereby decreasing the heart rate). Parasympathetic fibers cannot change the force of contraction, however,because they only innervate the SA node and AV node (which control heart rate only)***(NOTE: This statement is not entirely accurate. The following study: demonstrates how the parasympathetic nervous system, through the action of vagus nerve, exerts a negative inotropic effect upon the heart)
  • Stimulation via sympathetic fibers causes an increase in the SA node rate (thereby increasing the heart rate and force of contraction). Sympathetic fibers can increase the force of contraction because in addition to innervating the SA and AV nodes, they innervate the atria and ventricles themselves.

Blood supply

In the majority of individuals, the SA node receives blood from the right coronary artery
Right coronary artery
The right coronary artery originates above the right cusp of the aortic valve. It travels down the right atrioventricular groove, towards the crux of the heart.At the origin of the RCA is the conus artery....

, meaning that a myocardial infarction
Myocardial infarction
Myocardial infarction or acute myocardial infarction , commonly known as a heart attack, results from the interruption of blood supply to a part of the heart, causing heart cells to die...

 occluding it will cause ischaemia in the SA node unless there is a sufficiently good anastomosis
An anastomosis is the reconnection of two streams that previously branched out, such as blood vessels or leaf veins. The term is used in medicine, biology, mycology and geology....

 from the left coronary artery. If not, death of the affected cells will stop the SA node from triggering the heartbeat, and pacemaker function will be manifest more distal in the cardiac system (e.g. AV node).

Sinus node dysfunction

Sinus node dysfunction describes an irregular heartbeat caused by faulty electrical signals of the heart. When the heart's sinoatrial node is defective, the heart’s rhythms become abnormal – either too fast, too slow, or a combination.

See also

  • Cardiology
    Cardiology is a medical specialty dealing with disorders of the heart . The field includes diagnosis and treatment of congenital heart defects, coronary artery disease, heart failure, valvular heart disease and electrophysiology...

    • Cardiac pacemaker
      Cardiac pacemaker
      right|thumb|350px|Image showing the cardiac pacemaker which is the SA nodeThe contraction of heart muscle in all animals with hearts is initiated by chemical impulses. The rate at which these impulses fire controls the heart rate...

    • Electrical conduction system of the heart
      Electrical conduction system of the heart
      The normal intrinsic electrical conduction of the heart allows electrical propagation to be transmitted from the Sinoatrial Node through both atria and forward to the Atrioventricular Node. Normal/baseline physiology allows further propagation from the AV node to the ventricle or Purkinje Fibers...

  • Heart block
    Heart block
    A heart block can be a blockage at any level of the electrical conduction system of the heart .* Blocks that occur within the sinoatrial node are described as SA nodal blocks....

  • Sinus bradycardia
    Sinus bradycardia
    Sinus bradycardia is a heart rhythm that originates from the sinus node and has a rate of under 60 beats per minute.-Signs and symptoms:The decreased heart rate can cause a decreased cardiac output resulting in symptoms such as lightheadedness, dizziness, hypotension, vertigo, and syncope...

  • Sinus tachycardia
    Sinus tachycardia
    Sinus tachycardia is a heart rhythm with elevated rate of impulses originating from the sinoatrial node, defined as a rate greater than 100 beats/min in an average adult. The normal heart rate in the average adult ranges from 60–100 beats/min...

External links

- "The conduction system of the heart."
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