[Neuroscience] Re: Large Fibers have a lower threshold than small fibers?

r norman via neur-sci%40net.bio.net (by r_s_norman At _comcast.net)
Sun Nov 12 19:10:15 EST 2006


On 12 Nov 2006 15:48:32 -0800, "Bill" <connelly.bill At gmail.com> wrote:

>Hi,
>
>So I know the dogma, that with extracellular stimulation, you recruit
>large fibers before your recruit thin ones. Why is that? and what would
>I cite when I say that?

It is so well known that you probably don't have to cite anything.
Certainly the original research goes way back, probably to the
original work of Erlanger and Gasser in the '20s and '30s.

 Large fibers have larger cross sectional area and therefore lower
longitudinal resistance so that stimulus current more easily travels
down the cell.  That produces an easy pathway for current to flow
through the cell.  This term varies with the square of the diameter.

It is also true that large fibers have larger membrane surface area
and therefore lower membrane resistance (and greater membrane
capacitance) so that the stimulus current (and current change, in the
case of capacitative current) can more easily enter the cell.  This is
countered by the fact that the lower resistance (and larger
capacitance) exactly compensates for the increased current entry to
produce no change in membrane potential with diameter.  This term
varies directly proportional to diameter.

Combining the two types of factors produces a situation where the
total path from extracellular stimulating electrode (cathode), in
through the membrane, down the interior of the cell, out through the
membrane, and into the indifferent stimulating electrode (anode) is
much more effect for large diameter axons than for small.






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