Pacemaker Cells

[G1SG2]

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I know pacemaker cells are in the SA node, but are they also in the AV node, Bundle of His, and Purkinje Fibers???

Also, TPR says that the SA node has sodium leak channels, but no voltage gated sodium channels...I thought everyone had voltage gated sodium channels? Or does it only have the "fast acting sodium/sodium leak channels"?

 

[matth87]

I know pacemaker cells are in the SA node, but are they also in the AV node, Bundle of His, and Purkinje Fibers???

Also, TPR says that the SA node has sodium leak channels, but no voltage gated sodium channels...I thought everyone had voltage gated sodium channels? Or does it only have the "fast acting sodium/sodium leak channels"?

Pacemaker cells are in the SA node, but all cells in the heart are permeable to Na+ to some degree. The SA node sets the pace because it is the most permeable.

A good example is that if the SA node was removed the heart would still be autorhythmic. If the SA node is removed since the other cells are not "as" permeable to Na+ the pace of the heart will be slowed down. I think default is around 80beats/min so it would be slowed to say 60beats/min.

The SA node has Na+ Leak Channels and Slow Na+ VG Channels. The Na+ Leak Channels cause the resting membrane potential to be unstable.

Once threshold is reached VG Ca+ Channels and Slow VG Na+ Channels will open. These are slow Na+ Channel instead of the fast Na+ Channels that cause depolarization in all other cell types.

Ca+/Na+ will influx which is phase 0 and then the Ca+/Na+ channels will close (no plateau) and K+ will open to reestablish the "unstable" resting membrane potential that is around -60mV, a little lower then normal cells. This is phase 3. The unstable RMB is phase 4.

Any questions?

Where exactly do the TPR books say that the SA node doesn't have Na+ Channels. I know they have a graph in the book with the SA node Action Potential and in phase 3 it only shows Ca+ channels but there are suppose to be both Na+ and Ca+ channels. Just the Ca+ are very prominent.

 

[Charles_Carmichael]

The upstroke of the SA node action potential is due to calcium channels, not Na+ channels like in other cells. The slope of this upstroke is not as steep as it would be if Na+ was involved.

Na+ ions are involved in the SA node action potential only during it's unstable resting membrane potential (phase 4); this instability is due to the funny channels that are permeable to both Na+ and other cations (hence the name, "funny"). You could say that these are voltage gated Na+ channels, since repolarization activates them and they allow Na+ to pass through. This ensures that as soon as repolarization occurs, the membrane potential rises towards depolarization again; this is the reason behind the automaticity of pacemaker cells.

To answer your initial question, yes, the AV node, Bundle of His, and Purkinje fibers are latent pacemakers. They fire at a slower rate than the SA node and the SA node overrides them. This is due to the fact that the pacemaker with the fastest rate of phase 4 depolarization (ie. the unstable membrane potential) controls the heart rate. That's why the SA node is dominant. These latent pacemakers can become the pacemaker of the heart if the SA node is damaged/suppressed. Hope this helps.

 

[G1SG2]

Got it, thanks everyone!

 

[doctor_hunt]

i want to ask...how can purkinje cells became a paceymaker when purkinje cells or fibers were Fast Action poptensials cells