cardiac muscle

[chiddler]

A longer refractory period in cardiac cells helps prevent disorganized transmission of impulses throughout the heart and makes summation and tetanus impossible.

I have a few questions regarding this:

1. Why is tetanus a bad thing?

2. Is the refractory period = the plateau phase? (phase 2)

3. I don't understand how a longer refractory period prevents tetanus.

thanks!

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[ljc]

1. Because tetanus causes a muscle to contract for an extended period. In other words, your heart stops beating.

2. Refractory period is phase 4. It is the period between one impulse completing and another beginning.

3. A longer refractory period ensures that the heart cannot contract twice in a row too quickly. This allows the muscle to fully relax before it contracts again, giving the maximum amount of "pump" possible. If the heart were to contract again too soon, it may only be able to "pump" a fraction of that amount because it did not relax all the way.

 

[chiddler]

Ok I understand my first and last question, thank you.

I think you're mistaken about refractory period. It is defined as the moment where a new action potential can not be initiated. It can clearly be initiated at phase 4 if i'm not mistaken because the muscle cell returns to RMP.

 

[MrNeuro]

Ok I understand my first and last question, thank you.

I think you're mistaken about refractory period. It is defined as the moment where a new action potential can not be initiated. It can clearly be initiated at phase 4 if i'm not mistaken because the muscle cell returns to RMP.

theres absolute refractory period and relative refractory which are you talking about?

absolute refractory period would be the depolarization region where voltage gated sodium channels are inactivated

relative refractory period would be the region where those voltage gated sodium channels are now in an active closed state and potassium channels are open causing a repolarization.

based on the information above you could deduce that the first part of the depolarization/plateu and part of the down slope is the absolute refractory period
and
the relative refractory period is during the down slope so starts @ half of phase 3 and ends at start of phase 4

 

[ljc]

You're correct in the definition. I might be wrong on what number that corresponds to on the graph, but I was just guessing as I'm not really familiar with smooth-muscle action potentials, and if they follow the same rules. Usually a refractory period is during the hyperpolarization of the membrane, which initially prevents a repeat action potential altogether, and then after that requires an extra strong action potential to depolarize. As long as you understand that refractory period = cannot be depolarized, you should be good.

 

[ThirdEye]

You want the atria to have enough time to refill and prevent the ventricles from contracting at the same time as the atria; preventing flow back into the atria

 

[chiddler]

theres absolute refractory period and relative refractory which are you talking about?

either.

You're correct in the definition. I might be wrong on what number that corresponds to on the graph, but I was just guessing as I'm not really familiar with smooth-muscle action potentials, and if they follow the same rules. Usually a refractory period is during the hyperpolarization of the membrane, which initially prevents a repeat action potential altogether, and then after that requires an extra strong action potential to depolarize. As long as you understand that refractory period = cannot be depolarized, you should be good.

this is cardiac muscle action potential! only cardiac muscle has the distinctive plateau phase.

i think you're right about knowing the definition, but better safe than sorry i suppose. i'll still take the answer if anybody knows it

 

[MrNeuro]

either.



this is cardiac muscle action potential!

i think you're right about knowing the definition, but better safe than sorry i suppose. i'll still take the answer if anybody knows it

look at my edited answer... hope that helps if not just tell me and maybe i can be of some help

 

[chiddler]

absolute refractory period would be the depolarization region where voltage gated sodium channels are inactivated.

This makes sense.

relative refractory period would be the region where those voltage gated sodium channels are now in an active closed state and potassium channels are open causing a repolarization.

Relative refractory is by definition potentially depolarizable with a strong enough action potential. But in this case, maybe there is no relative refractory period. Maybe there's only absolute! What I mean is cardiac muscle action potentials do not have that "dip".

Or maybe this is just out of scope of mcat lol

 

[MrNeuro]

absolute refractory period would be the depolarization region where voltage gated sodium channels are inactivated.

This makes sense.

relative refractory period would be the region where those voltage gated sodium channels are now in an active closed state and potassium channels are open causing a repolarization.

Relative refractory is by definition potentially depolarizable with a strong enough action potential. But in this case, maybe there is no relative refractory period. Maybe there's only absolute! What I mean is cardiac muscle action potentials do not have that "dip".

Or maybe this is just out of scope of mcat lol

there doesn't need to be a dip all the dip signifies is that its lagging over time think of it like this

in relative refractory period there is K+ or (+) charge exiting the cardiac muscle cell let us now assume that during the down slope there are 10 + charges leaving every second in order to reach the threshold potential from baseline we'd need 5+ charges entering the neuron. now during a down slope with 10 + charges leaving every second and with a basic stimulation of Na channels open and allow 5+ charges HOWEVER due to the 10+ charges leaving our net charge inside the neuron is still - 5 + charges (from the 10 K+ leaving) so again there doesn't need to be a hyperpolarization all the hyperpolarization signifies is 1) a greater disparity from threshold and 2) K+ channels have a lag causing a repolarization.

does that help?

 

[chiddler]

Thanks for your help. I'm having difficulty comprehending what you wrote, it's kind of confusing.

 

[MedPR]

A longer refractory period in cardiac cells helps prevent disorganized transmission of impulses throughout the heart and makes summation and tetanus impossible.

I have a few questions regarding this:

1. Why is tetanus a bad thing?

2. Is the refractory period = the plateau phase? (phase 2)

3. I don't understand how a longer refractory period prevents tetanus.

thanks!

1. Think about a cramp in your foot. Besides the pain, what else happens? You lose control and basically function of those muscles. You wouldn't want this to happen to your heart, for obvious reasons.

2. Yes, the refractory period is the plateau phase. The whole reason this phase is so much longer than in skeletal muscle is to prevent overlapping of action potentials, which could lead to tetani.

3. Kind of answered this is #2. Remember absolute refractory period in skeletal muscle? The period of time where absolutely no amount of depolarization could cause another contraction? And the relative refractory period, where a large enough (larger than normal) depol could cause a contraction? In cardiac muscle, there is an effective refractory period (the plateau) where K+ channels are open (so K+ goes out of the cell) and certain Ca2+ channels are also open (Ca2+ goes into the cell). This is how the cell prevents rapid repolarization. While all this is going on, the Na+ channels are closed. I don't know exactly what keeps the Na+ channels closed, but I assume it has something to do with the membrane potential being higher than a certain mVoltage.