Guy C Brown wrote:
> Richard Kondo wrote:
> > Action potentials do not cause a change in organelle membrane
> >potentials.
> What evidence for this is there? I have not seen any evidence, and
> can not imagine how it would be measured. Can you give me some
> references?
I don't have references for this point, though the experiment could be
done with voltage sensitive dyes such as TMRE which preferentially
accumulate and fluoresce in the mitochondrial membrane when its
potential is highly negative. (It disperses when the membrane becomes
depolarized).
> >Action potentials do not make the intracellular media more positive,
> What evidence for this is there? Electrodes inserted into the
> intracellular medium of neurons and muscle cells show a more positive
> potential during an action potential relative to an earthed electrode.
> What else can this mean except that the cytosol has become more
> positive? The intracellular electrode is not on the membrane surface,
> it is in the bulk cytosol, and it measures a more positive potential
> relative to an extracellular electrode which is earthed. Fixed charges
> on the membrane are irrelevant to what these electrodes are measuring,
> which is that the cytosol becomes more postive relative to earth.
>
I am not suggesting that microelectrodes are directly measuring the
membrane potential or the fields created by the fixed charges.
The key points in electrophysiology are:
1. the bulk solutions remain electroneutral.
2. the membrane potential is the consequence of the unequal distribution of
ion species across the membrane
3. the membrane is selectively permeable to ion species.
4. microelectrodes measure ion current flow as the consequence of
ionic equilibrium potentials
I am stating what everyone knows, but...
so when a ion channel opens (say the sodium channel which opens
with the action potential), sodium moves in the direction of the
electrochemical gradient (inward). So there was a net movement
of positive charge into the cell. Since the sodium channel opens for
a few milliseconds, the excess positive charge in the cytoplasm
is miniscule, and the basic principle of electroneutrality is not
violated. So where are those charges? I don't know. Two possibilities
seem reasonable. Homogenous distribution or accumulation of the
charges along the membrane. Since the bulk solution is electroneutral,
there would be no energy cost to moving the charges away the membrane,
but I usually visualize the excess charges remaining near the membrane.
Why do I state that this has no effect on organelles. Because the
electrochemical gradients across their membranes has not been
altered.
However, you are suggesting that these excess charges are
altering the electric fields within the cell. I shall give an
argument as to why they do not.
The mitochondria has an excess negative charge and an associated
highly negative membrane potential (-200mV). Gauss's Law states
that the surface integral of the dot product of E (electric field) and da is
equal to the charge within the surface. Let's define that surface to
be the mitochondrial surface. During the action potential, there
is not net charge movement into the mitochondria. Therefore, the
charge within remains the same. The electric field across the
mitochondria could change and still give the same integral, but
the simplest solution is no change. How could that happen?
If the excess positive charge which moves into the cell during an
action potential, homogenously distributes itself along the
plasma membrane like a capacitor. Then the field created by these
charges would be restricted to the plasma membrane. Why do I
think that this happens. Well, if not, then the mitochondrial
membrane field would be highly dependent upon the geometry of the
cell, its location within the cell and the distribution of charges in the cell.
> >rather
> >they alter the charge distribution across the membrane (like a
> >capacitor).
> The action potential does not involve the movement of fixed charges
> from one side of the membrane to the other. Rather it involves the
> movement of bulk phase ions from one bulk phase to the other. The
> ions (and the potential) are not somehow localised to the membrane.
> >The point is that the electrical field is just across the membrane and
> >there is a quick
> >dropoff of the field away from the membrane. Therefore, it does not
> >affect
> >organelle membrane potentials.
> Of course the field in the plasma membrane can not affect the organelle
> membrane potential. But when the field reverses during an action potential
> the potential in the cytosol becomes more positive, and this inevitably
> changes the field across organelle membranes. If you think that the potential
> changes are entirely limited to the membrane with no changes in the bulk
> phase, what is the mechanism for this?
> >
So why can we measure currents, if the cytoplasm is electroneutral? Probably,
because the insertion of a microelectrode (nowadays, we use a patch clamp
electrode which seals to the membrane) into the cytoplasm slightly
violates electroneutrality. There would be a voltage difference between
the membrane and the electrode, but it would be extremely small, because
the conductivity of ionic solutions is very high. (just like a copper wire).
> >Finally, 2D voltage modeling has been done for numerous tissues - it is
> >most interesting in cardiac and neural tissue.
> But are there any models that deal with the spread of the potential away from
> the membrane?
>
I don't know any.
