NextStep (NS) Question 17, Psych, FL3. Strength of AP Propagation

[Alpha-ketogluterate]

In the question above we are asked to identify what is it that myelin does NOT do. I believe it does all of the above. The proposed answer is C. The explanation reads that myelin does not impact the strength of the AP. I agree with this statement. However, the answer choice is not talking about the strength of the AP. It is talking about the strength of the action potential Propagation. Myelin acts as an insulator and along with the nodes of ranvier ensures that the AP is essentially recreated over and over again as it moves down the axon, this ensures that the AP's original strength is maintained. Without myelination not only does the speed of the AP suffer but also its strength. We witness this in MS patients where the damage to myelin also impacts the actual strength of the AP delivered at the bottom of the axon as compared to the AP generated at the soma. This khan article sums it up well (I cannot post links so google "Action potential velocity khan" it is the first link, go down to demyelination. I have posted the relevant line here.)

So again while I agree that myelin does not impact the all or none nature of the AP, it certainly does impact the strength of the impulse propagation. Myelin DOES protect the neuron, it DOES increase the velocity of the propagation (by lowering resistance and capacitance within the neuron), it DOES provide insulation and it DOES protect against the loss of strength through propagation down the axon. So I believe all of the above are correct. Thoughts?

"... When the myelin coating of nerves degenerates, the signals are either diminished or completely destroyed..."

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[sapientnarwhal]

Yah I had an issue with that question as well and submitted feedback to the testwriters. I recall in my cellular neurobio class an experiment where the middle of a squid axon was depolarized, resulting in bidirectional AP propagation. The strength of each AP attenuated with distance due to current leak.

 

[NextStepTutor_1]

Hi all --

So, it seems like a lot is going on here. First, the principle that APs are all-or-nothing events is AFAIK solid, and is different from the magnitude of the associated depolarization, even if ambiguous language is sometimes used. For example, look at this textbook excerpt, where the author refers to both "magnitude of the depolarization" and "magnitude of the action potential" to refer to the same thing (i.e. the potential difference expressed in mV). Likewise, the principle that demyelinating diseases primarily involve decreased frequency of AP transmission seems fairly solid (see this article, as well as the textbook excerpt above). As such, I have to think that the KA quote uses "diminished" to mean "diminished frequency" because the context of that paragraph is talking about these syndromes on the tissue level.

That said, @Alpha-ketogluterate, your point is taken that talking about "the strength of action potential propagation" could be taken as referring to the magnitude of depolarization, rather than an (incorrect) assertion that action potentials are gradient rather than all-or-nothing events, so I will bring that to our FL team to clarify. Best of luck, and hope that this response has shed some light on this issue.

 

[Alpha-ketogluterate]

It has, thank you. You do agree then that if we have an unmyelinated neuron axon, the actual STRENGTH of the AP can be decreased due to ion leakage? This is very similar to a wire where current is allowed to dissipate out, sure the original AP created cannot change in its set magnitude, but this does not mean that it follows that the AP along the entire length of the neuron will have the same strength if it is allowed to leak (which is one consequence of demyelination).

 

[Alpha-ketogluterate]

Hi all --

So, it seems like a lot is going on here. First, the principle that APs are all-or-nothing events is AFAIK solid, and is different from the magnitude of the associated depolarization, even if ambiguous language is sometimes used. For example, look at , where the author refers to both "magnitude of the depolarization" and "magnitude of the action potential" to refer to the same thing (i.e. the potential difference expressed in mV). Likewise, the principle that demyelinating diseases primarily involve decreased frequency of AP transmission seems fairly solid (see as well as the textbook excerpt above). As such, I have to think that the KA quote uses "diminished" to mean "diminished frequency" because the context of that paragraph is talking about these syndromes on the tissue level.

That said, @Alpha-ketogluterate, your point is taken that talking about "the strength of action potential propagation" could be taken as referring to the magnitude of depolarization, rather than an (incorrect) assertion that action potentials are gradient rather than all-or-nothing events, so I will bring that to our FL team to clarify. Best of luck, and hope that this response has shed some light on this issue.

Also, I would like to point out that while your point is true, it also validates what I was saying as well. IF the frequency is diminished, it is due to the fact that some of the AP is not reaching the end of the axon with enough strength to propagate the signal. The decrease in strength then can (but not always) also cause a decrease in frequency.

 

[NextStepTutor_1]

Here's another way of looking at it. Talking about the strength of APs is dangerous because you don't want to fall into the trap of thinking that a "strong" action potential will have a greater effect than a "weak" action potential. From the point of view of the next link in the chain, an action potential is a yes-or-no thing, and it's quite possible that a trap answer choice could be designed to target that misconception. At the same time, yes, it is true that depolarization drives APs and has a magnitude that we can measure and think about. I would just urge caution around using the word "strength" in this context because it can easily lead to misconceptions.

The point that this question is trying to make is that whether an AP is transmitted (which is what was meant by "propagation," I think) is a yes-or-no phenomenon for which it is not meaningful to talk about "strength". But I definitely do agree with you that this phrasing should be changed if it is leading people to think about depolarization as a gradient phenomenon instead.

 

[Alpha-ketogluterate]

Here's another way of looking at it. Talking about the strength of APs is dangerous because you don't want to fall into the trap of thinking that a "strong" action potential will have a greater effect than a "weak" action potential. From the point of view of the next link in the chain, an action potential is a yes-or-no thing, and it's quite possible that a trap answer choice could be designed to target that misconception. At the same time, yes, it is true that depolarization drives APs and has a magnitude that we can measure and think about. I would just urge caution around using the word "strength" in this context because it can easily lead to misconceptions.

The point that this question is trying to make is that whether an AP is transmitted (which is what was meant by "propagation," I think) is a yes-or-no phenomenon for which it is not meaningful to talk about "strength". But I definitely do agree with you that this phrasing should be changed if it is leading people to think about depolarization as a gradient phenomenon instead.

Thank you again. I understand where the question is coming from. I have a standalone question, it is simply inspired by this question. Imagine the following scenario:

We are told that there exists a neuron that does not have myelin, if we were to measure the AP generated at the Soma, and the AP at the bottom of the axon of the same neuron connected to the soma from which we made the initial measurement, would the 2 values be equal or would the value at the end of the axon be different (ie lesser) due to ion leakage?

This is not directly related to the exam question, the question simply made me think of this scenario, and the attenuation the other student above described in the squid axon is something I had read about as well. I just want to check with you if this assumption is correct, that indeed the AP is changed if the axon is not myelinated. I fully understand the point you made earlier, graded potential refers to the change in output in relation to varying stimulus (such as the bipolar cells of the retina), and that APs are all or none in neurons, I am not talking about a change in output due to a change in stimulus, I am thinking if the all or none AP is reduced due to leakage or not. And again thanks for the detailed response.

 

[NextStepTutor_1]

Hi @Alpha-ketogluterate -

Yes, I think the scenario you're describing is correct. The textbook on PubMed I've been consulting for this seems pretty clear on this point: "Also, the magnitude of the depolarization diminishes with distance from the site of initial depolarization, as some of the excess cations leak back across the membrane through resting cation channels." That said, I don't think this level of detail tends to be tested, whereas the knowledge that APs are all-or-nothing in terms of their effects is much more of a "core" piece of information, so to speak. Paradoxically, for the MCAT, having a really extensive knowledge of certain science areas can actually be a danger, because you have to look at the question through the lens of the level of knowledge they're likely to be testing. Just something to keep in the back of your head, in any case.

 

[Alpha-ketogluterate]

Hi @Alpha-ketogluterate -

Yes, I think the scenario you're describing is correct. The 've been consulting for this seems pretty clear on this point: "Also, the magnitude of the depolarization diminishes with distance from the site of initial depolarization, as some of the excess cations leak back across the membrane through resting cation channels." That said, I don't think this level of detail tends to be tested, whereas the knowledge that APs are all-or-nothing in terms of their effects is much more of a "core" piece of information, so to speak. Paradoxically, for the MCAT, having a really extensive knowledge of certain science areas can actually be a danger, because you have to look at the question through the lens of the level of knowledge they're likely to be testing. Just something to keep in the back of your head, in any case.

Thank you, this was all very helpful. Your FLs are excellent, I feel like along with EK they've made me better at reading research and interpreting data. Thanks again for the help. And yes your point is taken, I will definitely listen to that advice.

 

[NextStepTutor_1]

Very glad to have been of help, and that's wonderful to hear 🙂.