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Sinoatrial node
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The heart beats without anyone asking it to, and the command comes from a very small place. The sinoatrial node is a crescent-shaped cluster of myocytes, which are heart muscle cells. It sits where the crista terminalis, a ridge on the wall of the right atrium, meets the mouth of the superior vena cava, the large vein that brings blood back from the upper half of the body.
Those cells generate the electrical impulse on their own, without any signal reaching them from outside. Then their neighbors, the perinodal or transitional cells, carry it to the right atrium and from there to the rest of the heart's conduction system.
There is a detail here that surprises people. Even though it commands the whole heart, the node does not show up on the electrocardiogram: its mass is too small to leave a signal of its own. What you see is the P wave, generated by the atrium as it activates, and from that you infer that the node did its job.
A few square millimeters that set the pace for the whole heart.
In brief
The sinoatrial node is a crescent-shaped cluster of myocytes where the crista terminalis meets the opening of the superior vena cava.
A few square millimeters that set the pace for the whole heart.
Those myocytes generate the impulse on their own, and the perinodal, or transitional, cells carry it to the right atrium and the conduction system.
Its mass is too small to leave a signal of its own on the electrocardiogram: its activity is inferred from the atrial P waves.
Sources (1)
- National Center for Biotechnology Information, StatPearls PublishingPhysiology, Sinoatrial Node, StatPearlsPublished
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