> As I'm not the most knowledgeable in the field of molecular cell
> biology, I wonder if some one could help me understand a little about
> how a brain cell stores its set firing threshold.
>
> What determines at what potential it will fire?
> How is that value modified?
That is a very good question; I'm not sure I can give an equally good
answer since it depends on a lot of parameters.
Basically, the neurons contain more potassium ions than sodium ions
and outside the membrance the concentrations are reversed. This sets
up a membrane potential of around -70 mV. In the membrane there are
ion channels that can allow the ions to flow in and out; when they open
the cell is depolarized and the membrane potential increases (the cell
fires). Many of these channels are voltage dependent: they open if the
membrance potential is above a certain threshold, set by the shape
and properties of the protein molecule making up the channel. So if the
cell is depolarized above their threshold, they will open, ions will
start to flow, more channels will open and a chain reaction sets in,
and the cell fires (as it becomes maximally depolarized other voltage-
dependent channels set in, and the cell starts to hyperpolarize again).
So the exact threshold is set by:
* The ion concentrations in the cell and surrounding liquid.
* What ion channels are present
* If various chemicals have linked to the ion channels
* Cell geometry
* Passive membrance properties
+ a lot of other things we have not yet discovered.
The most common way of modifying the threshold is not to change it,
but to change the resting potential of the cell. If the cell contains
much calcium, its potential will be lower and it is harder to fire;
many regulating systems in the neurons depend on changing the calcium
levels (it also acts as a primitive internal messenger signal for
a variety of purposes).
-- ----------------------------------------------------------------------- Anders Sandberg Towards Ascension! asa@nada.kth.se http://www.nada.kth.se/~asa/ GCS/M/S/O d++ -p+ c++++ !l u+ e++ m++ s+/+ n--- h+/* f+ g+ w++ t+ r+ !y