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Cardiac pacemaker cell
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The heart beats because a group of cells tells it to. They sit together in the sinoatrial node, a cluster of heart muscle cells, or myocytes, with pacemaker activity. Under normal conditions, they generate the electrical impulses that set the rhythm and rate of the whole heart.
What makes them different is that they never rest. The cells of the atria and the ventricles have a resting phase between one beat and the next; pacemaker cells do not. As soon as an action potential ends, which is the cell's electrical discharge, they start to depolarize again automatically, meaning they charge themselves back up toward the next discharge. They wait for no signal: the impulse arises inside them, again and again.
At rest they do this at an intrinsic rate of 60 to 100 times per minute. That rate is not fixed: input from the parasympathetic nervous system, the one that governs rest, slows impulse production, and input from the sympathetic system, the one that governs the alarm response, speeds it up.
In brief
The sinoatrial node is a cluster of myocytes with pacemaker activity that normally generates the electrical impulses setting the heart's rhythm and rate.
Unlike atrial and ventricular cells, they have no resting phase: once an action potential ends, they begin to depolarize on their own.
They wait for no signal: the impulse arises inside them, again and again.
At rest they do this at an intrinsic rate of 60 to 100 beats per minute; parasympathetic input slows impulse production and sympathetic input speeds it up.
Sources (1)
- National Center for Biotechnology Information, StatPearls PublishingPhysiology, Sinoatrial Node, StatPearlsPublished
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