Which of the following is longer?
(a) The refractory pe...

Question

Which of the following is longer?

(a) The refractory period of a cardiac contractile cell.

(b) The refractory period of skeletal muscle fiber.

Accepted Answer

Welcome back. Our next problem says the brief period of time during an action potential in the ventricular contractile cell when it is completely unresponsive to any additional stimulus is called a absolute refractory period. B, relative refractory period C, rapid depolarization or D repolarization. Well, let's think about the different uh periods in terms of membrane potential that happen after an action potential has been triggered. You have this initial action potential which causes rapid depolarization of the membrane and then you have this plateau and then back down as the cell rep polarizes and then back to normal. So this would be our rapid depolarization. You've had your accident potential come along and calcium channels have opened those voltage gated channels and calcium ions positively charged calcium ions have come rushing in. And now the inside of the membrane has a positive charge, it's flipped. So you've had this rapid depolarization, then these calcium channels remain open for a short time and then close at which point, some of the potassium channels have opened and you now have positively charged potassium. We'll see K plus channels open at the end of the plateau here and potassium goes out of the cell and you have a process of re polarization where you restore that usual negative membrane potential. So in this period of time, which period of time would represent the time when the cell cannot respond to any other stimulus. And that would be from this rapid depolarization point to the end of the plateau. Here at this point, the calcium cha channels are either already open. So another potential coming along won't trigger any changes or they have closed and are inactive, they are unable to open again. So they can't respond to any sort of signal. A period of time when the cell can't respond normally to a normal stimulus is the refractory period. But this point specifically where it won't respond to anything is the absolute refractory signal, no matter how strong the stimulus the cell is physically unable to respond to it. Or the reasons we mentioned when we look at choice b, the relative refractory period that is going to apply to this down slope here, that repolarization process. So that will be a relative refractory period because at this point, the calcium channels are closed. So all right, calcium channels closed, but they can open. So if exposed to a strong enough electrical signal, those calcium channels can open again. And because our membrane has started to rep polarize an influx of calcium ions will change things. It will make that membrane potential positive again, depolarize the membrane again. So it can respond to a strong signal in this period because the threshold is higher. So that threshold to reach an action potential is higher because the membrane hasn't completely re polarized. But it's not the same thing as the absolute period where it just can't respond at all. So that was perhaps our most tempting choice since it is a refractory period. But to be thorough, we'll look at our other two answer choices. Choice c rapid depolarization as we saw, that's that initial action potential point where those calcium ions rush in and flip the polarization of the membrane. So that's part of this absolute refractory period. But it's not the entire thing. As we saw in our little diagram, the absolute period happens throughout that whole plateau as well. So absolute period in that area there. So that's why that's not correct. It's not complete. And then choice D repolarization again, that's that period on the down slope where the membrane is being rep polarized as the potassium ions that are positively charged are going out of the cell. So K plus ions exiting cell and restoring the resting membrane potential. So not our correct answer because at that point, again, that is the relative refractory period where there is some capability of responding to a strong signal. But again, we were looking for the brief period of time during an action potential in the ventricular contractile cell when it is completely responsive completely. Excuse me, unresponsive to any additional stimulus and that's choice a the absolute refractory period. See you in the next video.

Which of the following is longer?
(a) The refractory period of a cardiac contractile cell.
(b) The refractory period of skeletal muscle fiber.

Key Concepts

Refractory Period

Cardiac Contractile Cells

Skeletal Muscle Fibers

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