Action potential affects only skeletal muscles?

[brood910]

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I am totally confused by TPRH workbook bio #58.
It did not say anything other than voltage-gated sodium channels are inhibited by a certain drug.

So, since the channels are blocked, depolarization cannot occur, and A.P. will not happen.
But, how the hell do you know whether this will cause only respiratory failure, but not cardiac arrest?

Their answer says preventing action potentials leads to skeletal muscle paralysis, but cardiac muscle is not affected... I thought action potentials occur in all kinds of muscles..?

 

[sakabato93]

I am totally confused by TPRH workbook bio #58.
It did not say anything other than voltage-gated sodium channels are inhibited by a certain drug.

So, since the channels are blocked, depolarization cannot occur, and A.P. will not happen.
But, how the hell do you know whether this will cause only respiratory failure, but not cardiac arrest?

Their answer says preventing action potentials leads to skeletal muscle paralysis, but cardiac muscle is not affected... I thought action potentials occur in all kinds of muscles..?

Depolarization in cardiac muscles occurs due to the influx of calcium ions, not sodium ions.

 

[brood910]

Depolarization in cardiac muscles occurs due to the influx of calcium ions, not sodium ions.

I did not know this at all. Thanks a lot. Am I supposed to know this tho? Neither EK nor TPR mentioned about this. Are there any other differences between APs in cardiac + skeletal muscles?

 

[1TB4RKSB4CK]

I did not know this at all. Thanks a lot. Am I supposed to know this tho? Neither EK nor TPR mentioned about this. Are there any other differences between APs in cardiac + skeletal muscles?

I think one of the EK 30 minute exams had a passage with a graph and all specifically on Cardiac AP and you can compare it to what you know about the other. S

 

[brood910]

I think one of the EK 30 minute exams had a passage with a graph and all specifically on Cardiac AP and you can compare it to what you know about the other. S

Thanks!
But do we really need to know this information?

 

[brood910]

Depolarization in cardiac muscles occurs due to the influx of calcium ions, not sodium ions.

I just found out that this is wrong, btw.
Cardiac muscles get depolarized by both Ca and Na influx.
There are Na voltage gated channel and Na/Ca channel.
So, can we assume that cardiac muscles wont get affected by blocking Na voltage gated channel much as they have another channel? But, I am kinda confused as this channel is really really slow, so it wont depolarize fast enough to let heart work properly without Na voltage gated channel..

 

[1TB4RKSB4CK]

http://faculty.unlv.edu/jyoung/SmoothMuscle2006.pdf

I browsed through this pdf for a bit for smooth muscle

 

[sakabato93]

I just found out that this is wrong, btw.
Cardiac muscles get depolarized by both Ca and Na influx.
There are Na voltage gated channel and Na/Ca channel.
So, can we assume that cardiac muscles wont get affected by blocking Na voltage gated channel much as they have another channel? But, I am kinda confused as this channel is really really slow, so it wont depolarize fast enough to let heart work properly without Na voltage gated channel..

Let me clarify.

Pacemaker potential is generated by the influx of Calcium ions only. All subsequent contractions (i.e. atria and ventricles) require sodium and calcium ions, so you're right.

As for other things you should know about cardiac muscles, you should understand the idea of a functional syncitium. The cardiac muscle cells all have special gap junctions between cells which allows for depolarization to spread from cell to cell forming a "functional syncitium" which allows for coordinated cell contractions.

You should be familiar with the method of conduction of nerve impulses in the hear as well. You don't necessarily need to memorize it, but remember that the signal for heartbeating starts at the pacemaker (SA node) and travels to the AV node, where it pauses for a moment, then spreads out. Of course this is a crude description, but if you want to know more, google a bit.

Good luck!

 

[mcloaf]

Pacemaker potential is generated by the influx of Calcium ions only.

Spontaneous depolarization of cardiac pacemaker cells during phase 4 is due to the inward current of sodium ions initially, called the funny current. The actual phase 0 upstroke of the nodal action potential is from calcium channels, but the spontaneity is due to sodium.

 

[workethic712]

ALL action potentials require sodium influx. In SA node, it still begins with slow Na+ channels opening first bringing the potential to threshold before Ca2+ channels open for the majority of the depolarization (pacemaker potential). However, in cardiac muscle cells, the depolarization is primarily due to Na+ channels--upon repolarization, Ca2+ channels open to maintain the depolarized state (plateau phase).

I think the phase 0, phase 4, etc. info is a little more in depth specifically than you need to know for the MCAT but definitely have to understand the big picture and difference between the pacemaker vs muscle cells and how their APs differ.

 

[brood910]

So... why is it that the drug only caused respiratory failure, but not cardiac arrest?

 

[KGflyboy]

The sodium channels in cardiac muscle are structurally and functionally different than the ones in skeletal muscle. At this point, I would not stress over the specifics. I doubt it comes up on the MCAT. There are plenty of drugs that bind selectively to specific receptors, which is why drugs can target skeletal muscles without affecting the heart or GI tract.