Austin P. So (Hae-Jin) wrote in message
<381BC7F1.358970BC at netinfo.ubc.caX>...
(snipping relevant questions from the message...)
Q. could you explain in what way are leak currents responsible for
electrotonic potentials?
A. Electrotonic potentials are explained by the passive (resting) RC
properties of the membrane.
The resting resistance (conductance) is, in fact, what I have called the
"leak".
Q. Are you saying that if you make a I-V plot of leak current you will see a
straight line?
A. Yes. See Matt Jones' response where he talks about the relatively ohmic
I-V relation, that is, a straight line. Some channels rectify but most show
a reasonably linear I-V relation when open.
(skipping sections relating to the relation between leaks and active
transport)
Q. You seem to have the belief that there is a specific channel that gives
rise to the "leak"...but by definition these leak currents are undefined.
A. The Hodgkin-Huxley "leak" is what is left after Na and K currents are
accounted for. That is undefined. I am referring specifically to the
resting "sodium leak" and "potassium leak" which contribute to both the
resting potential, to the resting permeability/conductance of the membrane,
and which contribute to the need for the Na/K pump. The resting
permeability is necessarily due to the presence of ion channels in the
membrane and these must necessarily be "specific" channels. No doubt the
primary cause is K, but Cl probably also contributes.
Q. Okay...semantics...ion channels when they are closed do not leak ... at
any given membrane potential a subpopulation will be open and will give rise
to a current of particular magnitude. You can call this "leaky" if you
like...
A. Exactly. My original question can be rephrased: Is the resting ion
permeability of cells due to specific ion channels that are not otherwise
involved in action or synaptic potentials? Or are they just a subpopulation
of chemically or electrically gated channels that happen to remain open at
rest. (Of course, if the latter, it need not be specific individual
channels always open, but a statistical average of channels winking on and
off). I argue for the former based on the fact that cells that are not
electrically or chemically excitable also have resting ion permeabilities.
But that is just a guess. Does someone know of experimental evidence?
Q. Go figure...you are looking for a specific channel...current does not
equate to the existence of a channel...
A. Current through the membrane is either capacitative (CdV/dt) or ionic.
If it is ionic it must be some specific ion and therefore some specific
channel (or some statistical ensemble of specific channels)
must be involved.
