BR answer vs PR Answer to a similar question

[fuhgidabowdit]

BR Question: Section 1 Passage VIII # 47


BR: It has been determined that the frequency of action potentials increases dramatically in axons once they have left the optic nerve. The most likely explanation for this increase is:

A. A higher density of sodium channels are found in the axons leaving the optic disc.
B. A lower density of sodium channels are found in the axons leaving the optic disc.
C. the axons are myelinated by Schwann cells
D. the axons are myelinated by oligodendrocytes

I chose A, but the answer is D. How does myelination increase the frequency of action potentials. I thought that it just affected its speed through the axons.

In the PR the question is :

Which one of the following is true concerning myelinated and unmyelinated axons?

One of the answer choices is:

Myelinated axons can conduct many more action potentials per second than can unmyelinated axons.

The solution says that this answer is wrong. (even though the answer is similar to what BR was saying: that the oligodendrocytes myelinate the neurons in the CNS leading to a frequency that increases dramatically) So which is it?

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[MedPR]

BR Question: Section 1 Passage VIII # 47


BR: It has been determined that the frequency of action potentials increases dramatically in axons once they have left the optic nerve. The most likely explanation for this increase is:

A. A higher density of sodium channels are found in the axons leaving the optic disc.
B. A lower density of sodium channels are found in the axons leaving the optic disc.
C. the axons are myelinated by Schwann cells
D. the axons are myelinated by oligodendrocytes

I chose A, but the answer is D. How does myelination increase the frequency of action potentials. I thought that it just affected its speed through the axons.

In the PR the question is :

Which one of the following is true concerning myelinated and unmyelinated axons?

One of the answer choices is:

Myelinated axons can conduct many more action potentials per second than can unmyelinated axons.

The solution says that this answer is wrong. (even though the answer is similar to what BR was saying: that the oligodendrocytes myelinate the neurons in the CNS leading to a frequency that increases dramatically) So which is it?

For TBR, neither answer A nor B can be correct because the optic disc is the start of the optic nerve. The question is asking about action potentials once they have left the optic nerve.

What are the other choices for the PR question?

 

[muhali3]

I think D might be right. If there is myelination, then an action potential can pass through a neuron faster due to saltatory conduction. This allows a higher amount of action potentials per second in that neuron, since each AP takes less time to pass. Unmyelinated neurons probably even have a higher density of sodium channels than myelinated neurons, since the only sodium channels in myelinated neurons are at the nodes of Ranvier.

Hope that made sense.

 

[fuhgidabowdit]

^^ Yea that gave me a different perspective on it. Thanks

 

[MedPR]

^^ Yea that gave me a different perspective on it. Thanks

It doesn't explain why the TPR answer is wrong..

 

[aspiringdoc09]

What were the other answer choices given in PR? Think in terms of a fiber optic cable. A frayed cable (unmyelinated) doesn't work as well as one that is unfrayed (myelinated) because the signal isn't traveling efficiently. Another example would be cars on a pot-hole filled highway vs a smoothly paved highway. Cars wouldn't travel as quickly on a pot-holed highway. Again, if we knew all the choices, then we can give you a better explanation of why PR said that answer is wrong (could be errata). Sometimes it may be wording that throws it off. Choices A and B have nothing to due with frequency of AP. Only other choices are C and D but Schwann cells are found in the PNS and Oligodendrocytes in CNS. You would need to know where optic nerve lies to choose correctly. All 12 cranial nerves are considered CNS. Knowing PNS and CNS cell types is more detail.

BR Question: Section 1 Passage VIII # 47


BR: It has been determined that the frequency of action potentials increases dramatically in axons once they have left the optic nerve. The most likely explanation for this increase is:

A. A higher density of sodium channels are found in the axons leaving the optic disc.
B. A lower density of sodium channels are found in the axons leaving the optic disc.
C. the axons are myelinated by Schwann cells
D. the axons are myelinated by oligodendrocytes

I chose A, but the answer is D. How does myelination increase the frequency of action potentials. I thought that it just affected its speed through the axons.

In the PR the question is :

Which one of the following is true concerning myelinated and unmyelinated axons?

One of the answer choices is:

Myelinated axons can conduct many more action potentials per second than can unmyelinated axons.

The solution says that this answer is wrong. (even though the answer is similar to what BR was saying: that the oligodendrocytes myelinate the neurons in the CNS leading to a frequency that increases dramatically) So which is it?

 

[MedPR]

What were the other answer choices given in PR? Think in terms of a fiber optic cable. A frayed cable (unmyelinated) doesn't work as well as one that is unfrayed (myelinated) because the signal isn't traveling efficiently. Another example would be cars on a pot-hole filled highway vs a smoothly paved highway. Cars wouldn't travel as quickly on a pot-holed highway. Again, if we knew all the choices, then we can give you a better explanation of why PR said that answer is wrong (could be errata). Sometimes it may be wording that throws it off. Choices A and B have nothing to due with frequency of AP. Only other choices are C and D but Schwann cells are found in the PNS and Oligodendrocytes in CNS. You would need to know where optic nerve lies to choose correctly. All 12 cranial nerves are considered CNS. Knowing PNS and CNS cell types is more detail.

That's not true. Structurally all 12 cranial nerves are considered PNS. CNS = brain and cord, PNS = sensory neurons, 31 pairs of spinal nerves, and 12 pairs of cranial nerves.

The optic nerve is considered CNS (therefore oligodendrocytes instead of schwann cells) because, unlike PNS, it doesn't regenerate if damaged (e.g, if you cut the optic nerve, it won't ever grow back) and also because of its embryonic origin.

Edit: Also, why wouldn't Na+ channel density affect the frequency of action potentials? If the cell can depolarize faster (due to more Na+ channels) following the absolute refractory period, wouldn't that increase it's action potential frequency?

 

[aspiringdoc09]

That's not true. Structurally all 12 cranial nerves are considered PNS. CNS = brain and cord, PNS = sensory neurons, 31 pairs of spinal nerves, and 12 pairs of cranial nerves.

The optic nerve is considered CNS (therefore oligodendrocytes instead of schwann cells) because, unlike PNS, it doesn't regenerate if damaged (e.g, if you cut the optic nerve, it won't ever grow back) and also because of its embryonic origin.

Edit: Also, why wouldn't Na+ channel density affect the frequency of action potentials? If the cell can depolarize faster (due to more Na+ channels) following the absolute refractory period, wouldn't that increase it's action potential frequency?

I've seen sources that show the 12 CN as part of CNS and PNS. Majority say PNS, so I will agree with you on that. We can both agree that optic is part of CNS. I was trying to help simplify the OP's critical thinking for choosing the correct answer.

Yes, you need Na-K channels for APs to work. I can't say for sure that having more Na+ channels will increase the frequency. To me having more isn't always better/faster especially if you think about how enzyme kinetics work. Eventually you will have saturation where adding more doesn't do anything for the rate. As for the refractory/resting period, all Na+ channels are closed, so no sodium can move into the cell. And any other APs are prevented and required to move unidirectional. Can having more Na+ channels increase the frequency? Maybe a little but not significantly. This just makes me think about the All-or-none principle.

But the real question was dealing with why the frequency increases in axons. APs begin at axon hilocks of neuron not at axon. What keeps the AP going is the myelinated sheaths that insulate the axon because the energy isn't being loss. That's the real purpose of the question. You and the OP are getting bogged down in details.

Maybe a person with an in depth neuroscience background can help us figure out if Na+ channel density affects the frequency of the APs? If I am wrong, I can admit it. I just don't know for sure, MedPR.

 

[aspiringdoc09]

Also, the following website has interesting information about action potential frequencies and general info from a physiological perspective. Look at the sections detailing the refractory period, concentration of Na+ and K+, and the frequency coding, particularly.

http://www.csupomona.edu/~seskandari/physiology/lecture_6.html

 

[MedPR]

I've seen sources that show the 12 CN as part of CNS and PNS. Majority say PNS, so I will agree with you on that. We can both agree that optic is part of CNS. I was trying to help simplify the OP's critical thinking for choosing the correct answer.

Yes, you need Na-K channels for APs to work. I can't say for sure that having more Na+ channels will increase the frequency. To me having more isn't always better/faster especially if you think about how enzyme kinetics work. Eventually you will have saturation where adding more doesn't do anything for the rate. As for the refractory/resting period, all Na+ channels are closed, so no sodium can move into the cell. And any other APs are prevented and required to move unidirectional. Can having more Na+ channels increase the frequency? Maybe a little but not significantly. This just makes me think about the All-or-none principle.

But the real question was dealing with why the frequency increases in axons. APs begin at axon hilocks of neuron not at axon. What keeps the AP going is the myelinated sheaths that insulate the axon because the energy isn't being loss. That's the real purpose of the question. You and the OP are getting bogged down in details.

Maybe a person with an in depth neuroscience background can help us figure out if Na+ channel density affects the frequency of the APs? If I am wrong, I can admit it. I just don't know for sure, MedPR.

I'm not trying to say you're wrong, but if you are right, I would like to understand so I don't miss it if it pops up on MCAT.

I don't think saturation kinetics applies to the number of sodium channels. You can think of the sodium channels as the enzyme, and the neurotransmitter released from the presynaptic cell as the substrate. Saturation kinetics says that increasing [substrate] will increase the rate until all of the available binding sites (enzymes) are saturated. So adding more enzymes (sodium channels) doesn't follow the same concept.

Correct me if I'm wrong, but depolarization via sodium pumps is what causes graded potentials, which are as the name implies, not all-or-none like an action potential is. More sodium pumps therefore allow the cell to reach threshold more quickly, thereby increasing the frequency of action potentials.

Again, it would be nice to have the other TPR answers.

 

[aspiringdoc09]

I'm not trying to say you're wrong, but if you are right, I would like to understand so I don't miss it if it pops up on MCAT.

I don't think saturation kinetics applies to the number of sodium channels. You can think of the sodium channels as the enzyme, and the neurotransmitter released from the presynaptic cell as the substrate. Saturation kinetics says that increasing [substrate] will increase the rate until all of the available binding sites (enzymes) are saturated. So adding more enzymes (sodium channels) doesn't follow the same concept.

Correct me if I'm wrong, but depolarization via sodium pumps is what causes graded potentials, which are as the name implies, not all-or-none like an action potential is. More sodium pumps therefore allow the cell to reach threshold more quickly, thereby increasing the frequency of action potentials.

Again, it would be nice to have the other TPR answers.

I was generalizing again with the enzyme kinetics example. All-or-none refers more to action potentials. I would like to see the other answer choices. I will look through my TPRH Bio because I haven't gotten that far yet. Also, that link I posted stated the frequency is determined by the refractory period. Any neuro experts want to pipe in about APs?

 

[CoolWhipp]

I'm studying for this MCAT and it sucks.

The myelin sheath increases the speed of the signal, you know this.

If you didn't have the myelin sheath and instead had channels all the way across the length of the axon, the signal would be slower.

The benefit of the action potential is such that if its triggered, you get an all out response. The channel opens and a lot cations move across the membrane, but they work slowly.

This influx concentration of cations move through brownian movement, which means they move fast. But the further you get away from the source, the concentration gets exponentially weaker. So, the ideal situation is that you want the channels separated at the maximum distance that can still produce a concentration high enough to trip the next channel. This is what the myelin sheath does.

Time Lapse:

1) Trigger
................(+) (+) (+) (+) (+) (+) (+) (+) (+) (+) (+) (+)
(Body/dentrites) ===|1|==[myelinsheath]==|2|== (tail?)


2) Channel Response
.........................(+).... (+)... (+) (+) (+) (+) (+) (+) (+)
(Body/dentrites) ===|1|==[myelinsheath]==|2|== (tail?)
............................(+) (+) (+)

3) Saltatory Conduction (Brownian movement of cations inside axon)
......................(+).... (+)... (+) (+) (+) (+) (+) (+) (+)
(Body/dentrites) ===|1|==[myelinsheath]==|2|== (tail?)
......................---moves fast-> ...(+)...... (+)... (+)

-Notice the (+) charge moving to the right, closer to the next channel
-The next channel is far from the first channel, this is because the cations equilibriate fast

4) Next Channel gets tripped
.......................(+)....... (+).... (+) (+) ......(+)..... (+)
(Body/dentrites) ===|1|==[myelinsheath]==|1|== (tail?)
.................................-----> (+).... (+) ....(+)... (+).... (+). (+)...


The same thing would happen without the myelin sheath, but the channels are slower than saltoratoryaroary (sp) conduction. In summary, as the axon is signalling through the channel it is slow but powerful and as it signals through the myelin sheath it is fast but quickly diminishing . Your body tries to create a balance for the best of both worlds.

I'm sleepy so I'll come back to this thread to see if I made sense later.

 

[fuhgidabowdit]

Sorry, let me post the rest of the answers from the TPR.

Which one of the following is true concerning myelinated and unmyelinated axons?

A. The amount of energy consumed by the Na+/K+ ATPase is much less in myelinated axons then unmeylinated axons.
B. Myelinated axons can conduct many more Action potentials per second than can unmyelinated axons
C. The size of the action potential depolarization is much greater in myelinated axons than in unmyelinated axons
D. Voltage-gated potassium channels do not play a role in repolarization in unmyelinated axons

Solution: Since area of membrane that is conducting is much less in myelinated axons, Na+,K+ ATPase only works to maintain the resting potential in the nodes of Ranvier, wheras in unmyelinated axons the Na+/K+ ATPase hydrolyzes ATP to maintain the resting potential across the entire membrane. B is false: The length of the refractory period is based on the characteristics of the voltage=gated sodium and potassium channels, which do not change. C is false. The size of the depolarization in an action potential does not vary greatly. Action potentials are an all-or-nothing response. D> is false: Voltage=gated potassium channels are the same in both neurons

 

[muhali3]

TPR phrased it badly. The total amount of energy consumed by all Na+/K+ ATPases in an unmyelinated axon is greater. However, the answer kind of implies that the amount of energy consumed per each Na+/K+ ATPase is less in an unmyelinated axon than in a myelinated axon.

I'm still unsure about the frequency thing though.

 

[Whiteshoes]

sorry to bump an old thread but i just did this problem and picked A because i thought more Na+ channels = more opportunity for action potentials within a given time assuming it is still myelinated. So what is the consensus. myelinaiton increases the speed but does it increase frequency as well? And why does not having more Na+ channels increase frequency?