PHYS2008 Question Thread

[crescendo]

Also, just going back through a few key PRS questions...I came across this one:

Removing ECF calcium would prevent which of the following cells from firing an action potential when stimulated directly?
1. Neurons
2. Skeletal muscle cells
3. Cardiac autorhythmic cells
4. Cardiac contractile cells
5. Smooth muscle cells

Apparently 3 and 5 is the answer.

I understand why 1 and 2, and 4 would be fine if they were stimulated directly.

And I understand why 3 would not be able to fire an action potential.

However, why wouldn't 5 be able to fire it were stimulated directly, even if the ECF Ca2+ was removed?
Smooth muscle cells have a sarcoplasmic reticulum, which releases calcium...so this doesn't make sense?

I am obviously very confused somewhere!

Smooth muscle also uses the calcium-dependent calcium coupling... See page 429 of 6th edition Silverthorn "an increase in cytosolic Ca initiates contraction..." hehe

[crescendo]

I found something else that we need to know for the exam according to Charles: "*For exam, how do skeletal muscles and cardiac muscles differ in terms of AP and EC coupling?".

I understand how they differ in terms of action potential generation/spread etc...
But, I don't quite understand what he means in terms of EC coupling? Or how they differ? Because it is not in the notes clearly..

Can anyone help me?

In line with what I posted just now, skeletal muscle needs acetylcholine to initiate contraction. Cardiac muscles need ECF Ca+...

[Turtle]

In line with what I posted just now, skeletal muscle needs acetylcholine to initiate contraction. Cardiac muscles need ECF Ca+...

Thanks Crescendo!

[Turtle]

For anyone who did PHYS20009, lets pray for a question on Restrictive vs. Obstructive lung diseases haha!
I will cry with joy if that appears on the exam, because we all know is so well know after spending so much time on it!

[Starlight]

For anyone who did PHYS20009, lets pray for a question on Restrictive vs. Obstructive lung diseases haha!
I will cry with joy if that appears on the exam, because we all know is so well know after spending so much time on it!

Agreed! The respiration stuff came quite naturally for me because I had already learnt nearly half of it before!

[Turtle]

Ok, I am still having serious issues with one area on muscles.
For smooth muscles:
If we remove ECF Ca2+, but still stimulate the muscle, shouldn't it be able to fire?
Because it has a sarcoplasmic reticulum that releases calcium itself.

I don't understand how a cardiac muscle can fire when stimulated directly, while a smooth muscle cannot.
Cardiac muscles also have Ca2+ dependant Ca2+ release from the SR, just like smooth muscles?

[pink0829]

Ok, I am still having serious issues with one area on muscles.
For smooth muscles:
If we remove ECF Ca2+, but still stimulate the muscle, shouldn't it be able to fire?
Because it has a sarcoplasmic reticulum that releases calcium itself.

I don't understand how a cardiac muscle can fire when stimulated directly, while a smooth muscle cannot.
Cardiac muscles also have Ca2+ dependant Ca2+ release from the SR, just like smooth muscles?

Yea, I'm a bit confused too. I think it has something to do with the rising phase of the action potential for cardiac contractile cells being Na+ dependent.

[Aurelian]

Ok, I am still having serious issues with one area on muscles.
For smooth muscles:
If we remove ECF Ca2+, but still stimulate the muscle, shouldn't it be able to fire?
Because it has a sarcoplasmic reticulum that releases calcium itself.

I don't understand how a cardiac muscle can fire when stimulated directly, while a smooth muscle cannot.
Cardiac muscles also have Ca2+ dependant Ca2+ release from the SR, just like smooth muscles?

In smooth muscles, the Ca²⁺ in the sacroplasmic reticulum is only released as a result of calcium-induced calcium release due to an influx of Ca²⁺ from the ECF. This stands in contrast to skeletal muscle cells in which the DHP receptors in the t-tubules are mechanically linked to the calcium release channels in the sarcoplasmic reticulum (i.e. no calcium-induced calcium release). As such, ECF Ca²⁺ is required for smooth muscle contraction.

In addition, smooth muscle action potentials use Ca²⁺ instead of Na⁺ for depolarization, so if you remove ECF Ca²⁺ action potentials won't be able to fire in the first place, even if the cells are directly stimulated.

Now, cardiac contractile muscle cells also have calcium-induced calcium release. As such, their contraction requires ECF Ca²⁺, but the propagation of action potentials in these cells does not require ECF Ca²⁺.

Does that help...? I think part of the problem might be that you're forgetting to separate whether or not a given cell will be able to contract from whether a given cell will be able to fire an action potential.

[Starlight]

Easiest way to remember -> Depolarization of Smooth muscle cells & cardiac autorhythmic cells depends on calcium

Depolarization of calcium= SMAC (SM= Smooth muscle AC= autorhythmic cardiac) the mechanisms will only confuse you.

[vea]

Did you guys learn anything about smooth muscle action potentials in phys? I read somewhere that the rising phase is actually due to Ca2+ influx but we didn't learn it in our course and i think i recall seeing it in one of your past exams.

[Starlight]

Did you guys learn anything about smooth muscle action potentials in phys? I read somewhere that the rising phase is actually due to Ca2+ influx but we didn't learn it in our course and i think i recall seeing it in one of your past exams.

No we didn't have a diagram with a rising phase or anything but it's definitely good to know. We only learnt about excitation-contraction coupling. 

[Aurelian]

No we didn't have a diagram with a rising phase or anything but it's definitely good to know. We only learnt about excitation-contraction coupling. 

I'm pretty sure that Charles did actually mention it in the lecture though (even though it's not on the slides).

[Starlight]

I'm pretty sure that Charles did actually mention it in the lecture though (even though it's not on the slides).

I'm pretty sure we would only need to know about the depolarization of the AP cycle for smooth muscle in that case

[Aurelian]

I'm pretty sure we would only need to know about the depolarization of the AP cycle for smooth muscle in that case

Everything else is the same as a "standard" action potential anyway =)

[Starlight]

Everything else is the same as a "standard" action potential anyway =)

Cool, thanks!