question about activated channels

Kim kn.green at nospamvirgin.net
Tue Jun 10 09:07:01 EST 2003


David,
An activated acetylcholine receptor channel is one which is open and 
therefore through which selective ions can pass, depending upon the 
electrochemical gradients for those ions.

For example the nicotinic acetylcholine receptor becomes "open" when 
acetylcholine binds to it. This causes a physical shift in the 5 helices 
that comprise the channel allowing the "emergence" of the pore. The pore 
is selective for both Na+ and K+ ions and so when opened Na+ ions flow 
into the cell, whilst K+ ions flow out of the cell as dictated by their 
electrochemical gradients.

Of course some channels may spontaneously open without acetylcholine 
binding, and so current would flow through these - but the probability of 
a channel openning without that binding is very small. Likewise a channel 
may not "open" upon acetylcholine binding but it is likely to.

An inactive channel is one which has recently been activated by 
acetylcholine binding and has been "open" but then desensitises to the 
acetylcholine binding thus preventing further ion flow through that 
channel. Its a kind of refractory period for the channel when it can no 
longer be opened. 



dcapelle at gmx.de (David Capelle) wrote in 
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
> 




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