- Sinoatrial node
Name = Sinoatrial or (el) node
Latin = nodus sinuatrialis
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.
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 atriumof 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. [ [http://sprojects.mmi.mcgill.ca/cardiophysio/AnatomySAnode.htm 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 systemfibers (CN X: Vagus Nerve) and by sympathetic nervous systemfibers (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.
In the majority of patients, the SA node receives blood from the
right coronary artery, meaning that a myocardial infarctionoccluding it will cause ischaemiain the SA node unless there is a sufficiently good anastomosisfrom the left coronary artery. If not, death of the affected cells will stop the SA node from triggering the heartbeat.
Electrical conduction system of the heart
* - "The conduction system of the heart."
* [http://user.gru.net/clawrence/vccl/chpt2/adlt46.gifDiagram at gru.net]
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