In article <Pine.SUN.3.94.961213091037.28904A-100000 at hermes>, Ralph Leonhardt (leo at neuroinformatik.ruhr-uni-bochum.de) writes:
>>On Thu, 12 Dec 1996, Jeff Wilson wrote:
>>>>> Briefly, then, between nodes the potential travels at a speed
>> approaching the speed of light ...
>>>>Frankly, this is nonsense. Although, on the whole Jeff's explanation of
>action potential conduction is pretty convincing.
>>All the best, Leo
>>----------------------------------------------
>Dipl-.Biol.
>Ralph Leonhardt
>Institut f. Neuroinformatik, Geb. ND 04 / 297
>Ruhr-Universitaet-Bochum
>D-44780 Bochum, Germany
>>>Tel.: +49 (0)234 700 5559
>Fax: +49 (0)234 709 4209
>E-Mail: leo at neuroinformatik.ruhr-uni-bochum.de
This whole discussion exposes rather slipshod teaching of electrics
to undergraduates in neuroscience, and - dare I say it? - slipshod
presentation of certain aspects of the subject in even some of the
best introductory textbooks, e.g.; "From Neuron to Brain", 2nd and
3rd Edns.
While it is perfectly true that the *neural impulse* does not
"travel at the speed of light", in myelinated axons the only
physical means available to increase the *effective* velocity must
reside in the involvement of electromagnetism and its transmission
velocity.
Leaving aside for the present the question of whether or not
each internodal stretch of myelin has its lamellae connected in
series or parallel, it is clear from observation that axonal
internodes are free from ion pores of any kind, hence whatever
action increases pulse velocity must begin at the nodes where the
pores are concentrated. The usually assumed "culprit" is the Action
Potential, but what is the real agent is the *ion pulse* arising
at that point. The structure of the node, especially in mammals,
where the microvilli further constrict the nodal gap, deflects the
ion surge into the space between axon and sheath which shows clearly
in electron-micrographs.
This surge, parallel to the axis of the Nodal Uncollapsed
Spiral (NUS), induces a current within it, and the resulting
magnetic field is transmitted at the velocity of light to the
corresponding efferent NUS, generating a trigger pulse at the
efferent node. There is the usual *delay* between trigger and Action
Potential/Current, which repeats the process, passing the signal on
to the next node in the line.
The highest signal velocities occur in motor neurons where
Schwann cells have the highest number of lamellae, therefore the
same large number of turns in the 2 NUSs. The axons are
correspondingly thicker to provide more powerful magnetic-surges
reaching the successive nodes. This accounts for the nearly constant
ratio of lamellae to Internode length in the 2 kinds of myelin.
Cheers! Gord