current in an axon

Alan Saul saul+ at pitt.edu
Sun Apr 23 00:48:54 EST 1995


In article
<Pine.A32.3.91i.950422150520.15322A-100000 at homer09.u.washington.edu>, "C.
Wigren" <cwig at u.washington.edu> wrote:

> Can anyone out there provide me (a high school teacher) with a simple 
> analogy or explanation on the propagation of current in an axon bringing 
> into play concepts such as resistance, voltage, current, and the way the 
> membrane (channels), intracellular and extracellular fluid come into 
> play.  The only one I can come up with is a line of dominoes in which the 
> effect is felt at the beginning and subsequently at the end but in which 
> other than the dominoes falling over, no other displacement has occurred

Cable theory provides the main analogy. John Horn used several more
familiar analogies in teaching medical students. One was the sort of
gardent hose that has holes along it for watering a garden, the leaky hose
analogy. Another is the episode with the transatlantic cable where they
found out how small leaks really added up over such a long distance. Note
that you didn't explicitly mention capacitance above. The garden hose
helps students understand that concept.

Once this idea of passive conduction is understood, it will be clearer to
the students why active propagation is so important. There are lots of
examples of reaction-diffustion processes that are variably analogous to
action potential propagation. You can probably set up a demonstration of
the iodate-arsenous acid reaction if your students are patient enough to
follow its relatively slow propagation. However, it isn't analogous in
that the state changes permanently. It's like flame propagation. If you
can, get some BZ reaction equipment. You could also set up an electronic
or opto-electronic model, for instance with photodiodes.

Although not particularly enlightening, there's also the commonly
available toy where momentum is transferred through a series of suspended
metal balls.

-- 
Alan Saul
saul+ at pitt.edu



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