Action potentials (was: Plant communication/sensing re)

Edwin Barkdoll barkdoll at cattell.psych.upenn.edu
Wed Dec 23 17:28:38 EST 1992


In article <1992Dec23.191118.7933 at mail.cornell.edu> Thomas_Bjorkman at cornell.edu (Thomas Bjorkman) writes:
[...]

>I brought up channels because the ion specificity of the channels that
>generate the action potential pretty much determines which ions are
>involved.

	Well, no.  The Goldman-Hodgkin-Katz constant field equation
explicitly include permeability (related to the channel conductance)
and concentration of the ion.  If one were to go simply by ion
specificity determined by patch clamp experiments, one would have to
assume that Li+ is an equal contributor in the squid axon action
potential and contributes more than sodium in the vertebrate
photoreceptor dark current, both of which are false.

>The patch clamp is the tool that answers that question most
>directly.

	Again I disagree.  The voltage clamp techniques developed by
Cole, Curtis, Hodgkin... proved equally direct information about
membrane currents.  The currents may be macroscopic but they can
equally direct measurements of ionic contribution.

>  Dainty, Hope and Walker had a pretty good picture of action
>potentials in Chara by the early 1960's.

	Actually I think Gaffey and Mullins made that comparison in
1958.

>Bruce Scott makes an explicit
>comparison between Chara and squid axons in his ca.1962 Scientific
>Ameican article.  The abstact begins "Electrical disturbances similar to
>the nerve impule are associated with a number of plant life processes." 
>Did I miss a posting where someone actually suggested that Na was involed
>in plant action potentials?  

	I think so -- that is the issue which started this thread:

In article <1992Dec18.145440.19868 at pixel.kodak.com> young at serum.kodak.com writes:

>  Clearly, there is a fundamental difference of opinion regarding the action
>  of Na+ and K+ in this area...either that, or my ignorance is showing, which
>  is entirely possible.  :-)

>  This is interesting...apparently there is some disagreement in this area?
>  I lifted the following from a different newsgroup written by Bill Williams
>  of St. Mary's College of Maryland in message <13812 at umd5.umd.edu> in
>  reference to mimosa pudica:
>
>~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>Sensing of environment:  I'm not too sure what the actual sensing mechanism is,
>but the signal is an action potential, just like in an animal's
> neurons.
> A
>charge separation propagates along the cells, causing sodium and potassium
>channels in their membranes to open and close, which alters the membrane
>potential and thus passes the signal along to the next patch of membrane. 
>Plant action potentials are usually much, much slower to propagate than animal
>action potentials.

	Bill quite explicitly refers Na and K channels
without mention of Cl or other channels.


Edwin Barkdoll
barkdoll at cattell.psych.upenn.edu
eb3 at world.std.com



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