>> 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).
All the dyes that I know of that measure mitochondrial membrane potential
within cells work by accumulating in response to the potential difference
not directly responding to the field, thus none of them have response times
anywhere fast enough to see a change on the 1 millisecond scale.
>>> >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.
No, they are approximately electroneutral to an extremely high degree,
but a very, very small excess of net charge causes a huge electric field.
If all bulk solutions were perfectly electroneutral there would be no fields
between them. The fields are caused by the difference in net charge.
> 2. the membrane potential is the consequence of the unequal
>distribution of
> ion species across the membrane
The membrane potential is the consequence of the unequal sistribution of
net charge.
> 3. the membrane is selectively permeable to ion species.
> 4. microelectrodes measure ion current flow as the consequence of
> ionic equilibrium potentials
Microelectrodes can and do also measure differences in electric potential
between phases.
>>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.
Yes, the amount of charge moved is miniscule relative to the number
of ions already there, but it is not miniscule in terms of the electric
potential. The movement of ions causes a change in the net number
of charges in each of the bulk phases, and it is this change in the relative
number of net charges which causes the change in membrane potential.
Electroneutrality is not a fundamental law of physics. Approximate
electroneutrality is caused by the huge size of the electric force
at the distances that occur within cells. Whenever there is a net excess
of charge then there is a large field attracting opposite charges and repelling
like charges, thus the excess charges will rapidly redistribute throughout
a phase which is conductive to them. That is why the electric potential
of the whole cytoplasm changes during an action potential, as measured
by electrodes.
There is no point in argueing for reasons why the cytoplasm does not become
more positive during an action potential when thousands of measurements of
intracellular potential have shown that it does.
Best wishes
Guy Brown
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
