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Medical Question about saltatory conduction in neurons

  1. Sep 14, 2008 #1
    I’ve been trying to understand salutatory conduction in myelinated axons and want a better understanding of how depolarization at one node of Ravier causes an action potential at the next. Is it caused by the actual mechanical movement of sodium ions through the axon? There is an animated diagram at http://www.edumedia-sciences.com/a503_l2-saltatory-conduction.html that shows positive ions passing along the axon from one node to the other. I have also read at http://en.wikipedia.org/wiki/Saltatory_conduction in the “Speed” section that, “It can be compared with a line of marbles pushing on each other - when poking the marble in one end then each marble only moves slightly, but this small effect on the marble in the other end is almost instantaneous.”
    Is it sort of like the movement of electricity in alternating current, where it is a shoving back and forth?
    If the effect from one node to the other is supposed to be at the speed of light -- (The speed of the signal from one node to the other is the speed of the induced electromagnetic wave, that is, the speed of light in interaction with transparent materials like the cytoplasm. http://en.wikipedia.org/wiki/Saltatory_conduction) – how could it be the movement of atoms (ions)? Or is it the passing of electrons from one ion to the next?
    I have limited math experience, so try to explain conceptually if possible.
     
  2. jcsd
  3. Sep 14, 2008 #2
    I dont know. I'm as confused as you are. even more confusing, someone once posted something to the effect that after the action potential passes the ions arent ACTIVELY pumped out of the cell as is commonly thought because its possible to suppress those pumps with chemicals and the cell will continue to function normally for a considerable period of time. the height of the action potential decreasing only very slowly.
     
  4. Sep 15, 2008 #3
    I think what you're thinking of - if I, understand it! - is that there are so many ions around that they aren't "used up" during the action potential. That is, it's not that the movement of ions through the ion gates is stopped because the ions are used up, but rather because of the change in voltage across the membrane closes the gates. Then the gates have to "re-set" before the next action potential. I think of it kind of like a bagpipe - blowing to inflate the bag is analogous to the ion pumps moving sodium out and potassium in, and the gates are like the pipes through which the air is expelled. This is a very sloppy analogy, I know, but it is a way I conceptualize the relationship between the pumps and the gates, and that the two don't have to be synchronized. As long as the pumps keep enough ions on the right sides of the cell wall (the bag is kept inflated), there will be ions to pass through the gates (air expelled from the bag) during action potentials.
     
  5. Sep 15, 2008 #4
    I see what youre saying. but the usual understanding is that the polarity of the cell completely reverses implying that all the charge not only has been used up but that in some mysterious way ions of the opposite charge somehow flood in and that this then obviously requires the ion pumps to reestablish the original gradient.

    now I can see perfectly well that that cant be true but I have no idea what is really happening.
     
  6. Sep 15, 2008 #5

    Dale

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    Hi bunburryist,

    The charge carriers in an electrolyte are the ions themselves, they don't "pass electrons" from one to the next. Basically, the ions in an electrolyte are free to move in response to an externally imposed E-field. Of course, there is a lot of bumping around in the fluid, so the motion of an individual ion is still very much a diffusion "random walk" but with a slight bias in the direction of the E-field. This slight bias causes a net "drift velocity" for the ions, which is the basis of the current.

    This "drift velocity" is very small, on the order of mm/s or even less. But the E-field which causes the movement propagates at the speed of light (in the material, not c the speed of light in vacuum). This is analogous to turning on a garden hose, it might take a full minute for a drop of water to make it from the faucet to the end of the hose, but the pressure wave which starts the flow leaving the hose takes less than a second.
     
  7. Sep 15, 2008 #6

    atyy

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    It could be that the thing that caused the depolarizing channels to open also caused repolarizing channels to open with slower kinetics than the depolarizing channels.
     
  8. Sep 15, 2008 #7
    see post 2
     
  9. Sep 15, 2008 #8

    atyy

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    Probably a different use of the word "active" - ie. requiring ATP. There are two uses of the word "active". Biochemists mean "using ATP". Electricians mean "not modelled by elements such as resistors and capacitors whose properties don't change with voltage".

    There is a lot of sodium outside the cell, but little inside, so sodium likes to rush into the cell.
    There is a lot of potassium inside the cell, but little outside, so potassium likes to rush outside the cell.

    The initial voltage change opens both sodium and potassium channels. The sodium channels have fast kinetics, so they open first and sodium rushes into the cell. Only a small amount of sodium is required to cause a large change in voltage, so although the cell has become oppositely polarized, the sodium concentration is the same. Then the sodium channels close.

    The potassium channels have slow kinetics, so they open later, and cause potassium to rush out of the cell, reversing the voltage change caused by sodium rushing in. Again, a small number of potassium ions can cause a large voltage change, so the cell is back to its starting voltage, and essentially unchanged potassium concentration.
     
  10. Sep 15, 2008 #9

    Dale

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    You are exactly right about the action potential currents moving a very small number of ions relative to the bulk concentrations. It is basically just a few ions right next to the membrane.
     
  11. Sep 16, 2008 #10
    DaleSpam - So are you saying that it's analogous to those things with a number of metal balls hanging from strings, where you pull back and drop the ball on one end and the force is transmitted to the last ball - only in the case of the neuron it's not a mechanical force but rather the ions pushing against one another (since they are of like charge)? I guess I'm still trying to understand how the "signal" is transmitted from one node to the other. Is it that the electric fields of the incoming ions push on the electric fields of the ions they encounter, and then they push on the fields of the ions they encounter, etc.? Would it be a good analogy to think of the metal ball concept, only having ions hanging from strings (obviously this couldn't really be done), and that the field of the falling ion would push on the field of the next ion, and so on, until the last ion received the force of the falling ion? And just as in the case of the metal balls the middle balls don't really move at all, but only pass on the force, so in the case of the ions they "middle ones" don't move, but only pass of the "electric pressure," so to speak?
     
  12. Sep 16, 2008 #11

    Dale

    Staff: Mentor

    Yes, that is a good analogy. The balls don't move much, but the "wave" of force does.
    Your word "electric pressure" hints at the usual analogy of current as a flow of an incompressible fluid like water. All of the ions do move a little (the hanging balls also do move a little). But the point is that the "electric pressure" is transmitted at the speed of light in the medium whereas the ions themselves move at the drift velocity.
     
  13. Sep 16, 2008 #12
    75 m/s isnt even the speed of sound in water much less the speed of light in water.
     
  14. Sep 16, 2008 #13

    Dale

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    Yes, and typical drift velocities are even lower: on the order of mm/s.
     
  15. Sep 19, 2008 #14
    Do the ions find their way into the gates by random jostling? Is it possible for ions to accidentally go the wrong way? Is it simply that since there are more sodium ions outside than inside that the odds are better that one from the outside will find its way into the gate?

    Is there any electric repulsion resulting from an excess of positive ions that would help to "force" the ions through?
     
  16. Sep 21, 2008 #15

    atyy

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    I haven't the faintest idea, except in the broadest sense. The extracellular medium is essentially Na+Cl-, the intracellular medium essentially K+A-, where K is potassium, and A is mysterious stuff like proteins to make sure we start with no net charge (ignoring their biochemical importance for simplicity). If we now open a channel that only permits Na to pass, Na+ will diffuse down its concentration gradient into the cell, causing Na+ to accumulate in the cell, and leaving Cl- outside, ie. a voltage develops across the membrane. The Cl- left outside has negative charge which will attract Na+ back out, ie. the voltage opposes the inward flow of Na+. So the movement of Na+ into the cell continues until the voltage becomes large enough to balance the tendency of Na+ to diffuse into the cell. It turns out that we get a large voltage with only relatively few Na+ moving in, so we can treat the Na+ concentration on both sides of the membrane as essentially unchanged throughout this process. Using only these considerations, we can calculate the equilibrium voltage. From this point of view, the movement is statistical, and a Na+ ion that has moved in could randomly move back out.

    As usual, considerations of equilibrium don't tell us how we actually get there, only that we eventually get there. Like if you drop a ball from the sky, it will get to the ground, but exactly how it gets to the ground depends on what sort of buildings, trees and animals lie below. Now, obviously Hodgkin and Huxley needed to know something about the dynamics of the Na+ movement, and they deduced enough by measuring how fast Na+ got in under various conditions. It's a bit like watching the ball fall, noting that it speeded up and slowed down 4 times, and guessing that there were 4 branches in its way. Amazingly, they did this well enough to give us their remarkable model of the action potential. The actual biophysical details only came much later, and are still being figured out (I think). The most famous recent breakthrough was the determination of the potassium channel structure and the mechanism of its selective permeability by Rod MacKinnon, for which he won the Nobel Prize in 2003.
     
    Last edited: Sep 21, 2008
  17. Sep 21, 2008 #16

    Dale

    Staff: Mentor

    Hi bunburryist, I agree with atyy's description, and just wanted to add a little bit that may help understand, at a microscopic level, the statistics and forces involved.

    Let's say that a membrane has some "channels" in it. These channels are passive, but selective. In other words, they do not use any energy, but they only allow one kind of ion through e.g. K+. Since it does not use any energy it must be reversible, meaning that K+ must be able to go both directions through the channel equally well. So the amount of flux depends only on the number of times that a K+ ion randomly bumps into a channel. If the concentrations are the same then the flux is the same both directions, there is no net flux, and it is in equilibrium. However, if there are more K+ ions on one side then they will randomly bump into the channels more often and there will be a net flux from the high concentration side to the low. There will still be individual K+ ions randomly going from the low concentration side to the high, but those will be few compared to the number going the other way.

    Now, let's start with an equal concentration, and apply a voltage across the membrane. In this case the collisions are no longer entirely random. There is still the random jostling, but on top of that there is now the electrostatic force pulling the K+ towards the negative side. Because of this, even though the concentration is equal, there are more K+ ions that cross from positive to negative than vice versa, resulting in a net flux towards the negative side. Thus the negative side winds up with a greater concentration of K+ than the positive side. At some point, however, the increased concentration on the negative side leads to a situation where the extra "random jostling" on the negative side exactly balances the electrostatic attraction and you reach equilibrium.

    The voltage required to electrostatically balance a given concentration gradient is called the Nernst potential. I hope that helps.
     
  18. Sep 23, 2008 #17

    somasimple

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    Is the transversal resistance of a node higher than the transversal resistance of an unmyelinated axon?
     
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