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Conceptual problem on myelination and conductance


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#1 litos

litos

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Posted 11 August 2010 - 07:35 AM

Hello,

I know this is resourceful and sage forum so I hope to clarify my doubts once for all ;)

I've recently started to do some research on electrophysiology and the concept of myelination and saltatory conduction still baffles me.

Needless to say, I know perfectly the mechanics behind AP conduction in non-myelinated fibers. I also know how myelin seems to speed up the propagation velocity of the AP but here come my doubts:

- Myelin increases transmembrane resistance, thus increasing the "length constant" (i.e. avoids the weakening of the passive currents that spread along the axon).

- Myelin decreases the capacitance of the internodal regions, reducing the "time constant" and enabling a faster depolarization of the nodal regions. But ¿WHY? Is not the capacitance of the nodal regions the same as in a "normal" (non myelinated) axons? If the internodal regions are "sealed" (myelin is an insulator), and no AP is generated here, what's the role of a lowered capacitance in this zone?

Note: I understand that a lowered capacitance means that fewer charges need to be moved in order to produce a given potential difference.

I'd be very grateful If anyone could help me with this complicated topic.

Thanks in advance,

L.




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