nodal Action potential

[aspiringmd1015]

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in the nodal cells(slow response fibers), the normal voltage gated sodium channels dont play a role in the action potential like they do in other cells of the body, and my book says its due to the lower resting membrane potential of nodal cells which is at -70mv, and bc of this the voltage gated sodium channels are in inactivated states bc these cells are partially depolarized being at a lower resting potential. how does a lower resting potential cause the normal sodium channels to become inactivated, and thus not play a role in depolariztion(fast upstroke of phase 0 for example in ventricular muscle action potential)?

 

[cbrons]

Probably because the nature of the pacemaker Na+ channels - the "funny" channels or w/e - are different and don't function in the same way as the fast Na+ channels in myocytes.
Just my surmisal.

 

[aspiringmd1015]

i guess not imp, also for regulation of the heart rate by the ANS, it says an increase in cAMP will increase slope of phase 4 making it steeper, makes sense bc you phosphorylate the ca, k, and sodium channels which allow sodium and calcium influx leading to quicker depolarization, but it says itll shorter then ap duration bc of increased Ik(delayed potassium channel) even though this K channel should be phosphrylated and thus inactive. ANyone?

 

[worldbeater]

Your question isn't clear to me, can you make your question a little more precise?

Are these nodal cells referring to the SA node or the AV node? Those are two separate graphs and have two separate shapes.

All sodium channels are voltage gated and have a slow phase and fast phase. The slow phase is called "m" and the fast phase is called "h". Remember it by saying "move home". The m portion of the gate is activated, sodium trickles in. Then when it hits a certain depolarization number, the the h gate is activated, cause a rush of sodium to flow into the cell.

For your second question, yes, the thorocolumbar area (and the heart specifically) of body is controlled by the sympathetic system. This would make it's second messenger cAMP. The heart rate is controlled by the SA node (while the AV node controls the ventricular rate). So the SA node only has 3 phases 0, 3, 4.

The o phase is an upswing that causes depolarization due to calcium, since we are talking about the atrium here and that is the main depolarization ion. Phase 3 is repolarization due to the efflux of K+ channels, that why the graph drops down. Phase 4 is automaticity, Na+ in/Ca+ out exchange, which causes a slight upswing.

Phase 4 has nothing to do with K channels, so I think you may be a bit confused. Clear up your Q and I'll try to explain further from what I know.