On 30 Oct 1996, Matt Jones wrote:
> In article <leipzjn9-301096000943 at rts0107.ppp.wfu.edu> Jeremy Leipzig,
>leipzjn9 at wfu.edu writes:
> >Does anyone know exactly how passive current spreads down a dendrite or
> >myelinated axon. One of my professors says the depolarization moves in
> >successive collisions of repelling cations, in a manner not unlike the
> >propagation of sound waves. Another one says that the electric field
> >created by incoming cations is enough to depolarize adjoining regions,
> >implying that passive current spreads close to the speed of light. I have
> >also heard in intro courses that simple diffusion of the cations is
> >responsible. Which, if any, is the correct explanation?
> <much deleted>
> In a myelinated axon, the membrane
> resistance is huge, so hardly any charge leaks out. Thus, the
> depolarization falls off very slowly with distance, and the next node
> down the axon sees almost the same voltage as the initial site of current
> injection. It sees that voltage in the time it takes for the electric
> field to propagate (at almost the speed of light) <deleted>
I've been curious about this issue for a long time, ever since I noticed
that textbooks often disagree, some describing conduction between nodes
in a myelinated axon as electrical and at the speed of light; others at a
more stately flow-of-ions pace. These statements are never
referenced, and when I query the authors, they typically tell me they
don't know. Refreshing candour, but unhelpful. The above explanation
falls into the "speed-of-light" (well, "almost") category.
Theoretical explanations notwithstanding, I think what we need is
empirical data. I've poked around in the early literature on saltatory
conduction without coming up with anything, but I find it hard to believe
the information isn't out there somewhere. What we need is a study in
which someone actually measured the speed of conduction of an action
potential _between_ two nodes of Ranvier. This may sound too technically
difficult, but I'd imagine that techniques exist to estimate the time
taken to cross the node and, subtracting a suitable number of such
values, to get a reasonable estimate for conduction between two nodes.
I'm betting it's at _substantially_ less than the speed of light but
that's only to provoke people. Anyone have some evidence? Or, if not, is
anyone looking for a thesis topic?
-Stephen
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Stephen Black, Ph.D. tel: (819) 822-9600 ext 2470
Department of Psychology fax: (819) 822-9661
Bishop's University e-mail: sblack at ubishops.ca
Lennoxville, Quebec
J1M 1Z7
Canada
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