IUBio

question about activated channels

r norman rsnorman_ at _comcast.net
Tue Jun 10 18:57:55 EST 2003


On Tue, 10 Jun 2003 23:29:01 GMT, "KP_PC"
<k.p.collins at worldnet.att.net> wrote:

>"David Capelle" <dcapelle at gmx.de> wrote in message
>news:754d51cf.0306100507.42c984d5 at posting.google.com...
>| hi,
>|
>| i have got a question about so called activated membrane channels.
>| having read in "from neuron to brain" that membrane channels can be
>| activated by extracellular application of ACh I was wondering what
>| exactly this means. i suppose it refers to the fact that activated
>| channels are constantly switching between an open and a closed
>state
>| whereas unactivated channels are most likely to be closed at any
>time.
>| however in the mentioned book it is stated (in box 1 in chapter 2)
>| that if all channels were activated there would not be any noise in
>| the overall membrane current to be expected.
>| therefore it is argued the formula relating variance, mean current
>and
>| the current  through an individual channel ought to be
>|
>| c=Var/I(1-p) rather than c=Var/I; where c is the individual
>current, I
>| the mean current and p the fraction of activated channels.
>| i would appreciate if anyone could explain this to me as it does
>not
>| make sense at the moment (possibly becos i am not getting the
>meaning
>| of "activated"). besides i fear the book which is supposedly so
>| brilliantly elucidating just does not suit the level of medical
>| undegraduates like me .........
>|
>| thanks a heap in advance,
>| david
>
>The view I'll discuss is not [yet] accepted
>by others, so do not 'quote' it as if it is.
>
>It requires a =lot= of discussion, but in my
>view, the "noise" to which the Author refers
>probably occurs as a function of detector-
>response characteristics, rather than any-
>thing in the Biology.
>
>It's a problem through all of Science where
>'detector' probes are employed.
>
>The detectors have their response character-
>istics, and experimenters attribute their
>data as 'capturing everything' that's going
>on within the experimental set-up.
>
>But this's =never= the case.
>
>In the view I hold, although an ion gate
>can be "open" or "closed", it can also be
>"in-between" - that is, the gates' func-
>tionality is not 'instantaneous', but the
>detectors' responses give the illusion
>that they are 'instantaneous'.
>
>The difference is important because it's
>important to see the overall Continuity,
>which is in the "in-between" stuff.
>
>Which leads me to 'lean' toward the
>formulation that the book 'denies'.
>
>ken [K. P. Collins]
>
Sorry, Ken, the patch clamp data on this is quite unambiguous.
The current detection in the patch clamp is exquisitely sensitive, but
is not at the quantal level nor even at the level of single ions.  The
currents measured are very small, but "macro"scopic with really little
possibility of being instrumental or conceptual artifacts.

Nothing is literally "instantaneous", but the time course of opening
and closing of the channel is so rapid as to be, for all practical
purposes, instantaneous.  That is, it is much shorter than the
duration of opening of the channel.  As far as can be told, all the
experimental evidence indicates that gates should be considered either
"open" or "closed" with no in-between.  When you put into effect the
spatial averaging that occurs by summing over the number of channels
found within a space constant and the temporal averaging that occurs
by summing over a time constant (using just ballpark figures that are
physiologically reasonable) then you do get "continuously" graded
signals.




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