How does myelin sheath increase nerve impulse speed?

[zut212]

How does myelin sheath increase nerve impulse speed? The sheath is an insulator, and it should slow down the conduction speed, but also "focus" and the impulse. I would think that the sheath prevents the nerve from sending out impulses in all directions normal to the axon.

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[pmahesh107]

The way an action potential propagates is by opening voltage-gated sodium channels which depolarize the cell. Think about how long it would take to conduct an action potential if you had to open channels along the entire length of the axon and wait for sodium influx. In the case of the myelinated nerve fibers, the myelin sheath covers large portions of the axon, leaving uncovered spaces known as nodes of Ranvier. The sodium channels in a myleinated nerve fiber are only at the nodes of Ranvier. So one sodium channel opening depolarizes a much greater length of the axon until it reaches the next node of Ranvier, where the voltage-gated sodium channels open and this cycle continues. In essence the AP "hops" around, covering much greater distance in a shorter amount of time.

 

[zut212]

Your answer was very intelligent and concise. You'll make an excellent physician and - if you choose to - professor.

I suppose that one aspect of nerve conduction is how long it takes for the Na-channels to "reload" or repolarize. The way that I look at it is this: By having myelanated sheets to "focus" the electrical pulse, you prevent scatter of voltage, which has the effect of SLOWING DOWN THE PULSE. However, I would think that by "focusing" the direction of the pulse, you also prevent the impulse from scattering out in all directions, much like an atomic chain reaction which is fast and in every direction! So the myelinated sheets "organize" the impulse direction a lot better.

Thanks a lot.

 

[neurosnap]

"Between areas of myelin are non-myelinated areas called the nodes of Ranvier. Because fat (myelin) acts as an insulator, membrane coated with myelin will not conduct an impulse. So, in a myelinated neuron, action potentials only occur along the nodes and, therefore, impulses 'jump' over the areas of myelin - going from node to node in a process called saltatory conduction (with the word saltatory meaning 'jumping'):"

http://people.eku.edu/ritchisong/301notes2.htm

The myelin sheath acts as an electrical resistance, which means that it doesn't conduct the electricity, forcing the charge to "jump" making it faster.

 

[zut212]

"Between areas of myelin are non-myelinated areas called the nodes of Ranvier. Because fat (myelin) acts as an insulator, membrane coated with myelin will not conduct an impulse. So, in a myelinated neuron, action potentials only occur along the nodes and, therefore, impulses 'jump' over the areas of myelin - going from node to node in a process called saltatory conduction (with the word saltatory meaning 'jumping'):"

http://people.eku.edu/ritchisong/301notes2.htm

The myelin sheath acts as an electrical resistance, which means that it doesn't conduct the electricity, forcing the charge to "jump" making it faster.

Aha! But the theoretical speed of the electrical impulse is the speed of light, which is not realized due to the insulating material. This is the thing that confused the dickens out of me. Now, you're saying that this same hindrance - insulation - is what helps speed up the flow?

 

[neurosnap]

Aha! But the theoretical speed of the electrical impulse is the speed of light, which is not realized due to the insulating material. This is the thing that confused the dickens out of me. Now, you're saying that this same hindrance - insulation - is what helps speed up the flow?

I'm not an expert but I think that's why it's theoretical. When we are talking about electricity that passes electrons through an axon, I think it moves something closer to the speed of sound. So if it's given that the kind of electricity that is involved in action potentials don't move at the speed of light, its conductive velocity could be improved by this myelin sheath acting as an insulator, creating an electromagnetic gradient at each subsequent node of ranvier, causing the synaptic impulse to act not as a wave, but as an impulse that jumps.

I'm not sure if all this is true, I'm just free ballin'.

 

[pmahesh107]

Aha! But the theoretical speed of the electrical impulse is the speed of light, which is not realized due to the insulating material. This is the thing that confused the dickens out of me. Now, you're saying that this same hindrance - insulation - is what helps speed up the flow?

Who told you this? The speed of light can't be the same theoretical speed of a nerve impulse, they are conducted in entirely different manners. Speed of light would be comparable to speed of electrons creating current but a nerve impulse is not this at all.

 

[Charles_Carmichael]

Aha! But the theoretical speed of the electrical impulse is the speed of light, which is not realized due to the insulating material. This is the thing that confused the dickens out of me. Now, you're saying that this same hindrance - insulation - is what helps speed up the flow?

Action potentials absolutely do not travel anywhere near the speed of light. I believe in mammalian neurons, the fastest speeds an action potential can attain is around 120 m/s (notice how very far away it is from 3E8 m/s).

Myelination works by significantly increasing membrane resistance (since the myelin sheath is a lipid insulator). What this means is that a depolarizing current cannot flow across the membrane and travels through the interior, which has low resistance. The increase in membrane resistance increases the length constant, which means that the depolarizing current can travel farther down the neuron. The action potentials essentially jump from node to node (areas of low membrane resistance) and this is what results in the increased conduction velocity.

 

[Quinoline]

Could someone else try to explain this, I still do not understand.
If you have myelin, you have an AP at a node, then have to wait for the charges to diffuse down to the next node. Without myelin, you can have voltage gated channels everywhere, how is this not faster?

 

[sazerac]

I've revisited this topic every six months since 2010 and it still makes no sense to me.

With myelin, sodium enters at a node A, travels a distance D inside the cell, and triggers the gates at the next node B.

Without myelin, wouldn't sodium also enter at the same position as the first node A, diffuse along the inside of the cell at the same rate for a distance D, and trigger the voltage gated channels in the location of the second node B at the same time? Sure the sodium might also trigger some intervening voltage gated channels between locations A and B as well, but that is irrelevant to the discussion. The sodium is going to travel inside the cell at the same velocity whether there is a myelin sheath or not, right?

And if not, why does sodium diffuse slower along a cell without myelin?

 

[Quinoline]

Yes, I think you have stated a good question.

For the arguments that myelin allow the action potential to jump along and this is faster, why don't then unmyelinated axons have channels at intervals like the myelin nodes? Couldn't you say that this would cause the same jump? And how does interval spacing of channels change diffusion rate?

The only thing I've read that I feel like may make sense is that without myelin, ions can leak out more, and myelin prevents leakage. Still, why does bolstering the signal more often result in slower speed?

 

[Maguro San]

Saltatory conduction.

Myelin insulates the nerve axon and conducts electricity faster than unmyelinated sheaths. If you have unmyelinated axons, the action potential propagates by depolarization of voltage-gated channels - ie. one channel will open and cause depolarization of the membrane causing its neighbor channel to depolarize and so forth. The process of depolarizing channels is relatively slow compared to propagation of the electrical charge across myelin. Myelin allows the electrical signal to jump between nodes and depolarize the voltage-gated channels at the nodes of Ranvier instead. Faster.

 

[Quinoline]

Maguro San, I do not think you reply answers my question "why don't then unmyelinated axons have channels at intervals like the myelin nodes?" Could you please explain this?

 

[Maguro San]

Saltatory conduction

Maguro San, I do not think you reply answers my question "why don't then unmyelinated axons have channels at intervals like the myelin nodes?" Could you please explain this?

Unmyelinated axons have voltage-gated channels that only depolarize the cell membrane in close proximity to the channel. Hence, for the signal to propagate, the voltage-gated channels must in literally right beside each other. If you have myelin, you can bypass this requirement as the myelin allows you to conduct an electrical signal over a longer distance (between the nodes of Ranvier). If you were to space out the voltage-gated channels and NOT have myelin between them, the signal would not propagate as the voltage-channels would not depolarize enough of the membrane to reach the next voltage-gated channel.

 

[Quinoline]

So you saying that with unmyelinated systems, the channels can only create weaker potentials while in myelinated the channels are more powerful? Or the channels are the same, but myelin prevents leakage?

 

[Maguro San]

So you saying that with unmyelinated systems, the channels can only create weaker potentials while in myelinated the channels are more powerful? Or the channels are the same, but myelin prevents leakage?

I don't have any references to cite but I believe that the channels are the same type regardless if axon myelinated or not. The myelin (by insulation and other physical properties I can't explain) allow the signal to propagate.

 

[amy_k]

The channels are the same whether myelinated or not in terms of the structure and time constant for activation. But we couldnt achieve saltatory conduction without myelination, because the fact that the membrane is insulated between the node causes the signal to jump from one node to the next in order to complete the circuit. Yes, sodium ions can flow through the axon as well which triggers the opening of the next voltage-gated channels, but they flow much more slowly this way, so if we removed the myelin (this demyelination is actually what happens in multiple sclerosis), in order for the signal to propagate we are relying on sodium ions flowing from one channel to the next. This is not only extremely slow, but the flow loses momentum between the channels because of leakage, which myelin also prevents. This would be slower than having channels all down the axon. If myelinated nerves become demyelinated they cannot transmit nerve impulses properly, as the signal trickles rather than jumps along the axon, losing momentum as it goes, to transmit a signal that is required to be transmitted quickly, hence the myelination. The last point about myelin is that it not only increases signal speed because of saltatory conduction, and preventing ion leakage, it also increases the effective thickness of the membrane massively, as it consists of many layers of schwann cells wrapped around the axon. Increasing the thickness of the nerve (increased diameter) increases velocity of ion movement, as in the squid giant axon, which increases the speed of the impulse. I hope that answers the question.

 

[Quinoline]

Are you saying that saltatory conduction is not ions diffusing? If not, what carries the charge? If you are, how does myelin increase diffusion speed?

Also, I don't think myelin increases speed by increasing diameter because the charge can't flow through myelin. For example if I have a wire and wrap it in plastic, current will not move faster through it because it is an insulator.

 

[Quinoline]

I think your first 2 points of your first comment are the correct reasons + myelination prevents leakage.
To simplify unmyelinated axons: say 1 channel depolarizing takes 1 msec and this channel depolarizing sends a voltage above threshold for 1 um. So each channel is placed 1 um apart. Then to go 10 um, it will take 10 msec, with the assumption that channel depolarization takes much longer than ion diffusion.
To simplify myelinated axons: in myelinated axons, channels are in high concentration at the nodes. If each node has 5 channels, these will compoundly send a voltage above threshold for 5 um. So now to go 10 um, it only takes 2 msec. Unmyelinated axons could also concentrate their channels at farther spaces like this, but because they are unmyelinated, the ions would leak out before reaching 5 um.

 

[Quinoline]

I think the logic of your second comment is correct, except that myelin does not increase the diameter of the conductor. Myelin is an insulator. What you are saying is like wrapping a wire in rubber and saying that the diameter of the wire increases.

 

[Labrat07]

@amy_k
Can you explain last part for me??? When I think of anything flow, I'm thinking about f= Av. Are you saying velocity increase from the area increase = resistance decrease = current increase = velocity increase. So my logic is flaw and i should only thinking Av = flow rate with fluid only and nothing else??

 

[amy_k]

The statement i made about the diameter increase due to myelin increasing the speed of propagation is correct. It acts like a thick capacitor and as the distance between the plates of a capacitor increases the capacitance decreases. Because of this the actual action potential (discharge of the capacitor) at the nodes becomes much faster and much more efficient (im not talking about movement along/through the myelin, im referring to the speed of the action potential.

 

[amy_k]

Looking back i just said "ion movement" not action potential so it was a bit vague, sorry.