How is passive current propagated?

G K GRAY gord at homostudy.win-uk.net
Thu Oct 31 04:18:44 EST 1996


 
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?
>
>-- 
>Jeremy Leipzig
>
This is a question that should be addressed in depth, yet seems to
be a blind spot in all the standard Intro texts that adhere to the
view of myelin as an insulator with very low capacitance, e.g.
"From Neuron to Brain". The observable facts are:
a)      that the neural impulse propagates faster along myelinated
fibres than in bare fibres.
b)      that in both types of fibre the impulse propagation velocity
is many orders of magnitude less than the velocity of light - C, as
in Einstsein's well-remembered but little understood equation 
E = MC\2. 
        A point to be taken into account is that there is a short
delay between the pulse that triggers depolarisation and the
actual depolarisation per-se. Whatever the mechanism may be it is
this delay which limits the impulse velocity in both types of fibre.
        A charged particle, e.g. an ion of Ca, K or Na in relative 
motion to its  surroundings inevitably generates a magnetic field
that propagates at velocity C. (J. Clerk Maxwell) A burst of ions
at a Node of Ranvier in a myelinated fibre will send a magnetic
wave to the next node in line which may be strong enough to
generate a trigger pulse at that position, But the delay between
trigger and depolarisation limits the signal velocity.
        
Cheers! Gord         




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