Myelin conduction

My physiological psych textbook (Foundations of Physiological Psychology, 5th ed., Neil R Carlson says that the Myelin sheath that surrounds portions of axons allows the neuron’s signal to be conducted “the way an electrical signal is conducted through an insulated cable.”

My knowledge of physics says that in order for there to be conduction there needs to be a complete circuit (the Loop rule, and all that.) How is the conduction in the Myelin physically started, how does the Myelin conduct a current, and what, if anything, is completing the circuit?

And no, this isn’t homework, just curiosity.

Myelin doesn’t conduct current; it’s an insulator. That’s what your book was saying. The current is conducted in the nerve fiber, as always.

I suppose the return path for the current could be the salty interstitial fluid that fills the spaces between cells. Just a guess, as I’m not that conversant with neurological physiology.

As Nametag says, the myelin doesn’t conduct the current, it’s an insulator. However the current isn’t conducted in the fibre. A ‘normal’ nerve signal is ‘conducted’ by the neuron itself depolarising in a wave, from the source to the synapse. That’s fairly slow because it relies on physically changing the structure and position of various proteins and moving salt ions in and out, all along the nerve. A myelinated fibre on the other hand nerve is mostly covered in insulation, with breaks or nodes in the insulation occasionally. The myelinated sections can’t propagate a nerve signal. The ‘current’ is generated at the source by depolarisation and propagates by depolarisation for a short length. Then when it hits the insulation it jumps from there to the next uninsulated node, which depolarises. Once depolarisation is complete at that node it again jumps to the next node and so on. The current is ‘generated’ in the fibre, but it jumps from one break in the myelin coating to the next, rather than propagating along/in the fibre itself. This make transmission much faster because 9/10th of the transmission doesn’t require the fibre to depolarise, and depolarisation s a relatively slow process whereas jumping moves at an appreciable fraction of the speed of light.

That’s what your book meant. The signal is transmitted by electrical conduction and the flow of electrons/ions. That’s analogous to transmission in an electrical wire. Compare that to transmission in an unmyelinated fibre which is conducted by depolarisation and by shunting ions and moving proteins. That’s not analogous to an electrical wire at all. It’s more like a Mexican wave.

You’re right, in order for there to be conduction there needs to be a complete circuit, and there is. At the polarised node the outside is strongly positive. At the depolarised node the inside and outside of the neuron are relatively neutral. Given the normal ionic makeup of your body that makes the outside at the depolarised node negative relative to the polarised. The circuit is complete when the current jumps from the negative depolarised node to the more positive polarised node. The process of jumping causes the polarised node to depolarised, which again strengthens the signal so it can jump to the next node.

The conduction is physically started when any section of unmyelinated neuron depolarises sufficiently to allow the signal to jump to the next node. How that occurs depends on the neuron. Often various molecules bind to the neuron making it leaky so that the ionic concentrations inside and out equalise. Physically stretching the neuron also works, since it stretches the membrane, making it leaky. There are squillions of other ways as well.

Depends what you mean by return path. The current runs via the normal interstitial fluid. The neuron itself acts as an electrical cell, building up an ion gradient between the inside and the outside. When the current jumps it’s analogous to connecting the negative terminal of one galvanic cell to the positive of another. In those circumstances there is no return as such, except that the ions/electrons inside the cells move from anode to cathode or Vicki Vercoe.

As for how a signal is conducted within a nerve fiber, that’s a lengthy subject which should be taught in your course (Phys. Psych.? Well, maybe not). Briefly, a nerve fiber “at rest” has a constant potential difference across it — -70 mV, IIRC. A propagating nerve impulse triggers proteins in the nerve cell membrane to transport sodium ions across the membrane, resulting in a voltage change of +100 mV (again, IIRC). This local voltage spike triggers more proteins further along the nerve, causing more sodium transport down the line, and the signal continues to propagate. Behind the signal, other transport proteins work to return the nerve cell to its pre-signal condition.

Myelin makes this process more efficient by (a) reducing electric field loss, like any insulator, and (b) preventing ions from flowing anywhere but at breaks (nodes) in the myelin sheath. Between nodes, the signal is propagated as an electric field ripple, which moves much faster than the ion-propagated signal.

On Preview, Blake said most of this, but I still think mine is more user-friendly.

I agree.

I think this has been explained quite well, but no one seems to have mentioned the name for conduction of this type: saltatory conduction, from the Latin saltare ‘to jump’.

I think this has been explained quite well, but no one seems to have mentioned the name for conduction of this type: saltatory conduction, from the Latin saltare ‘to jump’.

Technically speaking, I think this is actually a form of “induction” rather than “conduction”.

Sorry if this has already explained, but what function do the nodes serve? That is, why not cover the whole axon in myelin?

Because there is a limit to how far a current will jump. Ever set the gap in your spark plugs too wide? The current just isn’t powerful enough to make the jump between the points. Same situation here. The nodes are spaced as far apart as possible while still allowing the curent to jump. There’s no way it could ever jump the whole way from one end of a neuron to another. There has to be a few breaks in between.