AP frequency

[chiddler]

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It has been determined that the frequency of AP's increases dramatically in axons once they have left the optic nerve. This is most likely because:

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.

Answer: D.

"Myelination have the ability to increase the frequency of action potential conduction."

I know that myelination increases conduction velocity via saltatory conduction. But this is suggesting that frequency of APs increase. I don't see how this is possible given that myelination causes APs to be further apart and therefore less frequent. The reason it is faster with myelination is because moving charge via cytosol to initiate depolarization is faster than action potential depolarization.

halp plox

 

[typicalindian]

It has been determined that the frequency of AP's increases dramatically in axons once they have left the optic nerve. This is most likely because:

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.

Answer: D.

"Myelination have the ability to increase the frequency of action potential conduction."

I know that myelination increases conduction velocity via saltatory conduction. But this is suggesting that frequency of APs increase. I don't see how this is possible given that myelination causes APs to be further apart and therefore less frequent. The reason it is faster with myelination is because moving charge via cytosol to initiate depolarization is faster than action potential depolarization.

halp plox

Finally a topic that I am good at 🙂

C should be out of the question immediately because schwann cells are in the PNS and the question refers to the CNS. A and B should also be out because no matter how many sodium channels you add or remove, they are all going to have the same refractory period length. myelination of the axon increases frequency because the actions potentials are conduced through the axons much more quickly, this means that there is less time spent getting the action potential through the axon, so the time that would normally be spent getting the action potential through the axon could be used for the sodium channels to go back to their un-inactivated state. If I remember correctly, the frequency of an action potential is increased with myelination and/or by strengthening the stimulus. strength of an action potential is static though.

Hope I helped. If not let me know and I'll try to explain it differently.

 

[SaintJude]

Lol, I picked C. The axons of the ganglion cells bundle to form the optic nerve, which then exist the back of the eye.

Once the axons leave the optic nerve, do they move directly to the CNS system? If so, is that why this question deals with CNS?

 

[syoung]

Finally a topic that I am good at 🙂

C should be out of the question immediately because schwann cells are in the PNS and the question refers to the CNS. A and B should also be out because no matter how many sodium channels you add or remove, they are all going to have the same refractory period length. myelination of the axon increases frequency because the actions potentials are conduced through the axons much more quickly, this means that there is less time spent getting the action potential through the axon, so the time that would normally be spent getting the action potential through the axon could be used for the sodium channels to go back to their un-inactivated state. If I remember correctly, the frequency of an action potential is increased with myelination and/or by strengthening the stimulus. strength of an action potential is static though.

Hope I helped. If not let me know and I'll try to explain it differently.

Good lord I gotta read this again.

Lol, I picked C. The axons of the ganglion cells bundle to form the optic nerve, which then exist the back of the eye.

Once the axons leave the optic nerve, do they move directly to the CNS system? If so, is that why this question deals with CNS?

This too.

 

[chiddler]

Finally a topic that I am good at 🙂

C should be out of the question immediately because schwann cells are in the PNS and the question refers to the CNS. A and B should also be out because no matter how many sodium channels you add or remove, they are all going to have the same refractory period length. myelination of the axon increases frequency because the actions potentials are conduced through the axons much more quickly, this means that there is less time spent getting the action potential through the axon, so the time that would normally be spent getting the action potential through the axon could be used for the sodium channels to go back to their un-inactivated state. If I remember correctly, the frequency of an action potential is increased with myelination and/or by strengthening the stimulus. strength of an action potential is static though.

Hope I helped. If not let me know and I'll try to explain it differently.

1. How do you know this question refers to the CNS? I thought this was PNS because it is going from sensory to CNS. The path should be PNS, no? More, CNS is just brain and spinal cord.

2. Sure, same refractory length. But if we increase number of voltage gated ion channels, we can reduce the time it takes for that initial depolarization (the first leap into positive voltage as sodium rushes in) which therefore causes a slightly faster action potential.

3. As I wrote earlier, I know that myelination increases conduction velocity via saltatory conduction. But this is suggesting that frequency of APs increase. I don't see how this is possible given that myelination causes APs to be further apart and therefore less frequent. Can you please explain how AP frequency increases, given this?

 

[typicalindian]

1. How do you know this question refers to the CNS? I thought this was PNS because it is going from sensory to CNS. The path should be PNS, no? More, CNS is just brain and spinal cord.

2. Sure, same refractory length. But if we increase number of voltage gated ion channels, we can reduce the time it takes for that initial depolarization (the first leap into positive voltage as sodium rushes in) which therefore causes a slightly faster action potential.

3. As I wrote earlier, I know that myelination increases conduction velocity via saltatory conduction. But this is suggesting that frequency of APs increase. I don't see how this is possible given that myelination causes APs to be further apart and therefore less frequent. Can you please explain how AP frequency increases, given this?

1. It says after it leaves the optic nerve, im in class right now but I'm pretty sure the pathway synapses to some structure in the brain ( I think the lateral geniculate nucleus, not sure) which is in the CNS.
2 and 3. When you say "the AP are farther apart so they should be less frequent" is not the right way to think of it. The amount by which myelination increases the speed is really large. The very tiny effect that adds sodium channels would have pales in comparison to this

Anyways. Pop quiz in class ttyl!

 

[chiddler]

1. It says after it leaves the optic nerve, im in class right now but I'm pretty sure the pathway synapses to some structure in the brain ( I think the lateral geniculate nucleus, not sure) which is in the CNS.
2 and 3. When you say "the AP are farther apart so they should be less frequent" is not the right way to think of it. The amount by which myelination increases the speed is really large. The very tiny effect that adds sodium channels would have pales in comparison to this

Anyways. Pop quiz in class ttyl!

thanks. good luck lol

1. Ok lets forget about this. Question wasn't specific enough and I reread the passage which doesn't give any useful info in this regard.

2.3. But we're not talking about speed of conduction. We're talking about AP frequency. These are two completely different things because if you have an unmyelinated neuron, it will have much more AP's going on but it is still slower. This is because the action potentials via Na+ and K+ channels and all that are slow. Action potential propagation via ionic current is fast. Myelination forces the neuron to utilize ionic current and therefore speeds up propagation.

What i'm trying to show is that AP frequency is not necessarily equal to speed of conduction.

 

[Class1P]

okay I had no idea that oligodendrocytes myelnated anything. I thought they were supportive cells. I just assumed Schwann cells were the only cell that myelinated

 

[chiddler]

okay I had no idea that oligodendrocytes myelnated anything. I thought they were supportive cells. I just assumed Schwann cells were the only cell that myelinated

PNS = schwanns. CNS = oligos.

Lets see...COPS! oh wow what a perfect mnemonic!

 

[typicalindian]

thanks. good luck lol

1. Ok lets forget about this. Question wasn't specific enough and I reread the passage which doesn't give any useful info in this regard.

2.3. But we're not talking about speed of conduction. We're talking about AP frequency. These are two completely different things because if you have an unmyelinated neuron, it will have much more AP's going on but it is still slower. This is because the action potentials via Na+ and K+ channels and all that are slow. Action potential propagation via ionic current is fast. Myelination forces the neuron to utilize ionic current and therefore speeds up propagation.

What i'm trying to show is that AP frequency is not necessarily equal to speed of conduction.

I can see where you're confused. I've just had the concept drilled into my head through my many undergraduate courses that I just see it as common sense now and I can't really explain it well. Maybe I'll let someone else handle the explaining. =/

okay I had no idea that oligodendrocytes myelnated anything. I thought they were supportive cells. I just assumed Schwann cells were the only cell that myelinated

the way I remember it is the brain has astroCYTES and therefore oligodendroCYTES must be in the CNS. Therfore schwann must be the other one (pns). It's not the most helpful way to remember, and probably not the smartest way, but that is how I originally remembered it for school and it just stuck lol... I'm weird 🙁

 

[chiddler]

I can see where you're confused. I've just had the concept drilled into my head through my many undergraduate courses that I just see it as common sense now and I can't really explain it well. Maybe I'll let someone else handle the explaining. =/




the way I remember it is the brain has astroCYTES and therefore oligodendroCYTES must be in the CNS. Therfore schwann must be the other one (pns). It's not the most helpful way to remember, and probably not the smartest way, but that is how I originally remembered it for school and it just stuck lol... I'm weird 🙁

that's fine thanks very much for your help!

 

[syoung]

Giving it a try now.
Would it be because, as the speed goes up from let's say 100ms to 50ms with myelation, then I could have 2 AP in the time it takes one AP for the non-myelinated neuron. Hence AP goes up?

Plus where does it say that more myelination = less AP's?

 

[SaintJude]

PNS = schwanns. CNS = oligos.

Lets see...COPS! oh wow what a perfect mnemonic!

Perfect. Always been struggling with this. No more. 👍

 

[chiddler]

Giving it a try now.
Would it be because, as the speed goes up from let's say 100ms to 50ms with myelation, then I could have 2 AP in the time it takes one AP for the non-myelinated neuron. Hence AP goes up?

Plus where does it say that more myelination = less AP's?

here is what i'm trying to explain. top is myelinated. bottom is unmyelinated. it doesn't say this explicitly anywhere, but i'm inferring based on my knowledge of neurobio.

 

[syoung]

You're drawing the number of times it takes an AP to travel down an unmyelinated neuron right?

But question asks for frequency of AP, which I infer to be how often or the number of APs can travel down the neuron in a given time frame (1/s). IE the frequency is 10 AP per second. Whereas in the unmyelinated, it takes longer for the AP to travel down due to the lack of insulation, thus you might get 1 AP / second and therefore a smaller frequency.

Anyone want to back that up?

 

[MrNeuro]

But question asks for frequency of AP, which I infer to be how often or the number of APs can travel down the neuron in a given time frame (1/s). IE the frequency is 10 AP per second. Whereas in the unmyelinated, it takes longer for the AP to travel down due to the lack of insulation, thus you might get 1 AP / second and therefore a smaller frequency.

Anyone want to back that up?

👍

 

[chiddler]

thanks for the responses. i want to sleep on this question. i went through a lot of neurobio today so i want to make sure i understand this.

 

[chiddler]

You're drawing the number of times it takes an AP to travel down an unmyelinated neuron right?

But question asks for frequency of AP, which I infer to be how often or the number of APs can travel down the neuron in a given time frame (1/s). IE the frequency is 10 AP per second. Whereas in the unmyelinated, it takes longer for the AP to travel down due to the lack of insulation, thus you might get 1 AP / second and therefore a smaller frequency.

Anyone want to back that up?

An action potential is the depolarization and repolarization along the neuron. In a myelinated neuron, the action potentials are separated by node of ranviers which are insulators, right? So in between each action potential there is an ionic current to carry over the AP since the membrane cannot host an action potential. Therefore there is AP-ionic current-AP-ionic current.

In an unmyelinated, the action potential is not separated by a current therefore an AP occurs along every nanometer of the neuron. Therefore, there is continuous AP's: AP-AP-AP-AP. The frequency increases.

I'm not really sure what else to say. You didn't really refute anything i've argued for 😕

 

[syoung]

An action potential is the depolarization and repolarization along the neuron. In a myelinated neuron, the action potentials are separated by node of ranviers which are insulators, right? So in between each action potential there is an ionic current to carry over the AP since the membrane cannot host an action potential. Therefore there is AP-ionic current-AP-ionic current.

In an unmyelinated, the action potential is not separated by a current therefore an AP occurs along every nanometer of the neuron. Therefore, there is continuous AP's: AP-AP-AP-AP. The frequency increases.

I'm not really sure what else to say. You didn't really refute anything i've argued for 😕

Yarggh. If I didn't refute what you said, did I agree with what you said? I'm getting more lost on this topic than before.

But although you say there is continuous aP's traveling down the neuron, isn't it still the "same" action potential that came from the axon to the dendrite? Frequency (in my head) is the number of times something happens. Well ONE AP is when it goes from axon to dendrite, no? And passed on it's "message" to the next postsynaptic body?

 

[chiddler]

Yarggh. If I didn't refute what you said, did I agree with what you said? I'm getting more lost on this topic than before.

But although you say there is continuous aP's traveling down the neuron, isn't it still the "same" action potential that came from the axon to the dendrite? Frequency (in my head) is the number of times something happens. Well ONE AP is when it goes from axon to dendrite, no? And passed on it's "message" to the next postsynaptic body?

no you didn't agree either lol

But I see where you might be mistaken with your thinking: One action potential is one depolarization repolarization cycle along the membrane. That means that it takes many action potentials to traverse the neuron's plasma membrane.