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?
>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.
>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?
>>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?
Best wishes
Guy
Dr Guy C. Brown
Department of Biochemistry,
University of Cambridge,
Tennis Court Road,
Cambridge CB2 1QW,
UK
Tel: +44 (0)1223 333342
Fax: +44 (0) 1223 333345
