Zat
Forum Senior
Topics: 2
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I've noticed a previous post questioning the ERP. This is the response:
Absolute Refractory Period----> Cell cannot be excited.(period)
Effective Refractory Period----> a CONDUCTED action potential cannot be elicited.
Therefore, it is a halfway excitablity period between the ARP and the RRP, and therefore longer than ARP.
You can also put it this way; in ERP a local response or excitation can be elicited but it cannot be conducted/propagated. (i.e. if the inward current is strong enough)
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| Dragonfly
Forum Elite
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i think you are confusing things, during ARP you can not have any AP at all, b/c cells are fully already depolorized. this period is followed by RRP (to my best knowledge there is no GAP between these two) during which you can excite memb and generate AP only with a higher threshold b/c some Na channels are closed by then. and once AP is generated it is CONDUCTED, remember "all or non low".
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| Zat
Forum Senior
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Posts: 26
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Thank you Dragonfly for the explanation. i have two points:
1- I was explaining the "Effective" Refractory Period (ERP) not ARP. All Physiology sources mention and discuss the ARP & RRP but not many explain the ERP. But it was mentioned in the Kaplan Lecture Notes and was kinda confusing.
The ERP is midway between both the ARP & RRP. In other words; during ARP the inactivation gates for Na channels are all closed and therefore action potential is IMPOSSIBLE. In ERP some begin to open, but only a local non-conducted potential is elicited (electrotonic potential). In RRP more Na channels are open, and a propagated potential is possible if the stimulus is strong enough.
2- "All or none law". This law is mostly exhibited in the alpha motor neurons, motor-neuron plate, ect. It's not exhibited in all excitable cells of the body. Whether it's exhibited in the Cardiac muscle cells, i'm not sure. But one thing i do know, is that, the law is not present in smaller neurons in the brain, and these form what's called an "electrotonic potential" or what's called "subthreshold potential change".
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| mildus
Forum Guru
Topics: 19
Posts: 614
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ERP = ARP + 1/2 RRP
it is true that in ARP an action potential can't be elicited, and that in RRP it can be elicited only if stimulus is strong enough; but, in practice, action potential can't be elicited during ERP because there are not enough Na+ channels recovered from refractority (after ERP, there are more than 50% of Na+ channels open, so it is possible for an action potential to happen).
Note that if you talk about a single channel, it only has two states (1. refractory, 2. not refractory), and when you talk about membrane containing many channels then you have 3 possible states (1. ARP, 2. RRP, 3. exitable) because not all the channels are in the same state in the same time (so, it is always critical mass of channels that is important)
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| Zat
Forum Senior
Topics: 2
Posts: 26
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Thanx for the valuable detailed info mildus.
True an action potential cannot be elicited during ERP, what differs it from ARP is that only a local non-propagated response can be elicited.
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