current in an axon

Matt Jones jonesmat at
Tue May 2 14:58:36 EST 1995

In article <799364933snz at> David Longley,
David at writes:
>Without wishing to sound like *Mr Angry*, can I suggest that (IMHO), one
>the worst things  you can teach  a young scientific mind is that
>concepts can be learned by analogy  or metaphor.  The essence of
>method and the value of scientific language  is that  it gets away from
>that mumbo jumbo.
>Those who can't accept it described in the terms it actually  happens 
>not learn it anyway, and those who do want to understand it  properly 
>just have excess baggage.

I respectfully disagree, for two main reasons: 
1)	Analogies are obviously useful tools for teaching students *what to
expect* from a certain process, if not what is actually happening. For
example, students can use fluid-flow analogies to make predictions about
changes in membrane potential (pressure) when the membrane resistance
(leakiness) of an axon (hose) is changed by the opening of ion channels
(valves). This type of analogy allows students to get an idea for how
things are expected to behave, if not how they actually are described in
the scientific literature. At the high school level, this is probably
much more important than being able to solve systems of differential
equations  or run sophisticated compartmental modelling programs.
2)	It is presumptuous of us to believe that *we* understand the way !it
actually happens!. All we, as scientists, have at our disposal are
theories (analogies) and models (metaphors). We do not actually know how
voltage-gated channels respond to depolarization (in any real molecular
sense), so we *hypothesize* that there must be a !gate!.  In fact, we
know beyond a doubt that our best equations represent only approximations
to the way !it really happens!, and that!s why we have statistics to tell
us whether our equations are close enough approximations to be taken
seriously or not. 

Sorry for the tirade. In fact, I happen to agree with David Longley!s
sentiments in the case of action potential propagation. The concepts of
resistance, capacitance, voltage and current should not be too difficult
for high school students to understand if they are really interested,
even including the simple math of Ohm!s law.  The only remaining aspects
of action potential propagation are voltage-gated channels, which might
take a little more explanation. But these can be considered as variable
resistors that have built in voltage sensitive switches.

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