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