| 21. | In addition to the above blocks, the SA node can be suppressed by any other arrhythmia that reaches it.
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| 22. | Tbx18 turns on genes that drive SA node cell development, simultaneously turning off genes that create atrial muscle cells.
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| 23. | If the SA node fails to initialize, the AV junction can take over as the main pacemaker of the heart.
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| 24. | The problem in sick sinus syndrome is that the SA node is not functioning properly and is causing an irregular heartbeat.
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| 25. | Stimulation of the vagus nerves cause a " decrease " in the SA node rate, decreasing the heart rate.
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| 26. | These converted SA node cells are able to respond to the nervous system, allowing the heart to be regulated as normal.
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| 27. | Indeed, since the sinus venosus is incorporated into the right atrium in amniotes, it is likely homologous with the SA node.
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| 28. | The other foci attempt to fire at their intrinsic rate, but they are activated by the SA node before they can fire.
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| 29. | The total time elapsed from the initiation of the impulse in the SA node until depolarization of the ventricles is approximately 225 ms.
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| 30. | Although they make up a relatively small portion of the heart SA node cells play a crucial role in the heart s function.
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