IUBio

question about activated channels

KP_PC k.p.collins at worldnet.att.net
Tue Jun 10 20:45:02 EST 2003


"r norman" <rsnorman_ at _comcast.net> wrote in message
news:jmrcevgi57v18ll782virvflh8gm4amek4 at 4ax.com...
| 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.

Hi Ron.

I'm Sorry, too, because I've got to stand on what I've posted.

All 'voltage clamp' studies are done in-vitro, which 'just'
eliminates the substrate for the extremely-important 3-D
energydynamics that I've discussed.

"Of course", if one uses a leaky-pail, the pail will leak :-]

K. P. COllins





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