The Sinoatrial node
(abbreviated SA node
, 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. 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
- 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 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.
- - "The conduction system of the heart."
- Diagram at gru.net