What would happen if myelinated neuron wouldn't have nodes of ranvier?

[Greentalk]

Hey,it would be great to know what would happen to action potential in axon if there would be no nodes of ranvier?Thanks

[Ramzes]

Народ подскажите пожалуйста, как или где можно быстро и выгодно продать свой Mitsubishi Lancer 2005г.в. на автомате в отличном состоянии. Мне деньги понадобились, а взять не где

[CPG]

Myelination on a nerve serves to increase the conduction velocity of the action potential; in other words, the action potential travels faster down the length of the axon because the nerve has been myelinated. This is dependent upon the properties of both the Nodes of Ranvier where ions cross the cell membrane as well as the internodes (the segment of axon between the Nodes of Ranvier) which are the portion insulated by the myelin sheath.

I’ll assume you know the basics of how an action potential works and try specifically to address action potentials in a myelinated axon but if I need to backtrack let me know.
Let’s say a cell’s resting membrane potential is -60 mV. If the membrane potential at one specific
point is increased by +10 mV, so it is now -50 mV. By electrotonic “conduction” a voltage change will spread immediately throughout the cell, but the magnitude of the voltage change will decay with distance. So maybe at a point close to where we started it will increase to -51 mV, and at another point a bit farther away it will be -55 mV and so on.
Any membrane will have a length constant associated with it that describes how this voltage change decays with distance from the site of depolarization. The beautiful thing that myelination accomplishes becomes apparent when you examine what determines the membrane length constant: the factors involved are the resistance of the membrane to current flow and the resistance of the axoplasm inside the cell to current flow. This is expressed mathematically as

Length constant = sqrt(Membrane Resistance) / sqrt(axial resistance)

 

So a higher membrane resistance and a lower resistance in the axoplasm will allow a change in voltage to spread farther before it decays by 37% (that's just the amount of decay that is used as a convention to define the length constant). Myelination wraps the membrane with additional layers of
insulation, thereby increasing membrane resistance, while the diameter of the axon underneath the myelin will expand and decrease axial resistance. Most textbooks will do a poor job of showing the latter. So a myelinated nerve will see a voltage change at one point in the axon more greatly affect the voltage at a point further down the axon, but note that these factors only prevent decay of the signal but do not amplify or actively propagate it. Voltage-gated ion channels are clustered at the nodes of Ranvier, and normally depolarization at one node is then able to bring the next node (or even several nodes away) to the voltage threshold for these ion channels, but not all the way down a very long axon. Without any nodes of Ranvier, then, your myelination only serves to dampen the inevitable attenuation of the signal and it is not propagated down the length of the axon to a synapse.

Edited February 22, 2013 by CPG