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Old 07-05-2011
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EKG Absolute Refractory Period versus Effective Refractory Period

Iīm reviewing cardiac physiology (Constanzo BRS). I get the basic concept of Absolute Refractory Period (due to inactivation of Sodium channels) and its difference form Relative Refractory Period (opening of voltage gated potassium channels).
Just in case I got a pretty good understanding after I read this article on Wikipedia


But I donīt get the difference between Absolute Refractory Period (ARP) and Effective Refractory Period (ERP)....what do they mean by conducted action potential can not be elicited?

I will appreciate any input...:sorry:
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Old 07-05-2011
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Absolute refractory period means that "absolutely" no stimulus is large enough to generate another action potential- no matter how much inward current is supplied because most Na+ channels are closed.

Effective refractory period means that a "conducted" action potential cannot be generated, meaning that there is not enough inward current to "conduct" the action potential to the next adjacent site, hence the term conducted. At the end of this period Na+ channels start to recover (become available to carry inward current).

Hope this help.

Last edited by tootsie; 07-05-2011 at 05:41 PM.
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Old 07-05-2011
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Remember that in order to generate an action potential, the sodium channels need to open to depolarize the cell to the threshold for action potential firing. Once the cell is depolarized, these Na+ channels are inactive, so no matter how much you stimulate the cell, you can't elicit another action potential. This is absolute refractory period.

Once the cell is repolarized, the cells go from inactive to closed, and an action potential can be generated.

During repolarization via K+ channels opening, the cell can hyperpolarize if too much K+ leaving the cell. During this time, since the Na+ are closed, you can elicit an action potential if you give a big enough stimulus to open the Na+ channel. However, since hyperpolarized means more negative potential relative to repolarized, a bigger stimulus is needed to elicit a action potential; this is the relative refractory period.

Mathematical example of relative refractory period:
Say the resting membrane potential (i.e. the point where it's repolarized to) is -80mV. Imagine the threshold for action potential firing is +60mV. That means you need 140mV to get to threshold to fire an action potential.
Now imagine the cell is hyperpolarized to -100mV. That means you would need 160 mV to elicit an action potential. So 140 mV won't do anything (based on all-or-none phenomenon) and the cell is technically refractory, but it's relative because you can get an action potential if the stimulus is big enough.

Bottom line:
Absolute refractory- absolutely no action potential can be generated
Relative refractory- a larger than normal stimulus is required to generate an action potential.
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Cardiovascular-, Physiology-

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