Sinoatrial node

Sinoatrial node

Infobox Anatomy
Name = Sinoatrial or (el) node
Latin = nodus sinuatrialis
GraySubject =
GrayPage =

Caption = Interior of right side of heart. (SA node not labeled, but region visible at left, near crista terminalis.

Caption2 = Schematic representation of the atrioventricular bundle of His.
System =
Artery = sinuatrial nodal artery | MeshName = Sinoatrial+Node
MeshNumber = A07.541.409.819
DorlandsPre = n_09
DorlandsSuf = 12577255
The Sinoatrial node (abbreviated SA node or SAN, also called the sinus node) is the impulse generating (pacemaker) tissue located in the right atrium of the heart, and thus the generator of sinus rhythm. It is a group of cells positioned on the wall of the right atrium, near the entrance of the superior vena cava. These cells are modified cardiac myocytes. Though they possess some contractile filaments, they do not contract.

Role as a pacemaker

Although all of the heart's cells possess the ability to generate the electrical impulses (or action potentials) that trigger cardiac contraction, the sinoatrial node is what normally initiates it, simply because it generates impulses slightly faster than the other areas with pacemaker potential. Because cardiac myocytes, like all muscle cells, have refractory periods following contraction during which additional contractions cannot be triggered, their pacemaker potential is overridden by the sinoatrial node. In the absence of extrinsic neural and hormonal control, cells in the SA node will naturally discharge (create action potentials) at about 100 times/minute. [ [ AnatomySAnode ] ] 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 (AV node), which is an area between the atria and ventricles, within the atrial septum.


The SA node is richly innervated by parasympathetic nervous system fibers (CN X: Vagus Nerve) and by sympathetic nervous system fibers (T1-4, Spinal Nerves). This makes the SA node susceptible to autonomic influences.
* 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).
* 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 patients, the SA node receives blood from the right coronary artery, meaning that a myocardial infarction occluding it will cause ischaemia in the SA node unless there is a sufficiently good anastomosis from the left coronary artery. If not, death of the affected cells will stop the SA node from triggering the heartbeat.

ee also

**Cardiac pacemaker
**Electrical conduction system of the heart


External links

* - "The conduction system of the heart."
* [ at]
* (NormanAnatomyFig|thoraxheartinternalner)

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Look at other dictionaries:

  • sinoatrial node — [sī΄nōô rik′yə lərsī΄nō ā′trē əl] n. PACEMAKER (sense 2a): sometimes called sinoauricular node [sī΄nōô rik′yə lər] …   English World dictionary

  • Sinoatrial node — The sinoatrial node (the SA node) is one of the major elements in the cardiac conduction system, the system that controls the heart rate. This stunningly designed system generates electrical impulses and conducts them throughout the muscle of the …   Medical dictionary

  • sinoatrial node — SA node the pacemaker of the heart: a microscopic area of specialized cardiac muscle located in the upper wall of the right atrium near the entry of the vena cava. Fibres of the SA node are self excitatory, contracting rhythmically at around 70… …   The new mediacal dictionary

  • sinoatrial node — noun Date: 1913 sinus node …   New Collegiate Dictionary

  • sinoatrial node — /suy noh ay tree euhl, suy /, Anat. a small mass of tissue in the right atrium functioning as pacemaker of the heart by giving rise to the electric impulses that initiate heart contractions. Also called sinus node. [1920 25; SINO + ATRIAL] * * * …   Universalium

  • sinoatrial node — si′no•a′tri•al node′ [[t]ˈsaɪ noʊˈeɪ tri əl, ˌsaɪ [/t]] n. anat. a small mass of tissue in the right atrium functioning as pacemaker of the heart by giving rise to the electric impulses that initiate heart contractions • Etymology: 1920–25; sin… …   From formal English to slang

  • sinoatrial node — noun a specialized bit of heart tissue that controls the heartbeat • Syn: ↑pacemaker, ↑cardiac pacemaker, ↑SA node • Hypernyms: ↑cardiac muscle, ↑heart muscle …   Useful english dictionary

  • sinoatrial node — [ˌsʌɪnəʊ eɪtrɪəl] noun Anatomy a small body of specialized muscle tissue in the wall of the heart whose contractions regulate the heartbeat. Origin early 20th cent.: from sinus + atrial (see atrium) …   English new terms dictionary

  • Sinoatrial arrest — or sinus arrest is a medical condition wherin the sinoatrial node of the heart transiently ceases to generate the electrical impulses that normally stimulate the myocardial tissues to contract and thus the heart to beat. Since the heart contains… …   Wikipedia

  • sinoatrial block — sinoatrial exit block a disturbance in which the atrial response is delayed or omitted because of partial or complete interference with the propagation of impulses from the sinoatrial node to the atria …   Medical dictionary

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