Richard Kondo wrote:>>>>>>>>>>>>>>>>>
Guy Brown wrote:
> Dear All,
> Can anyone help me with this problem?
> Nerve and muscle cell membranes depolarise and repolarise
> by about 120 mV up to 100 times per second.
> The depolarisation of the plasma membrane is very rapid
> (a few microseconds) and results in the extracellular
> medium gaining a more negative electrical potential,
> while the intracellular, cytosolic medium becomes more
> positive due to the entry of positive ions. The abrupt
> increase in the positive potential of the cytosol must
> immediately cause an increase in the transmembrane
> potential of intracellular organelles by up to 100 mV.
> For example the transmembrane potential of the mitochondria
> must increase by up to 100mV. This hyperpolarisation of
> intracellular organelles will disipate as ions move across
> the membranes in response to the increased potental. And
> the hyperpolarisation will be reversed (1 to 2 milliseconds
> later) when the plasma membrane is repolarised.
>> But if this is true there will inevitably be massive
> perturabtions of energy metabolism and intracellular
> ion fluxes up to 100 times per second!
> How large the perturbations will be depends on the relative
> electrical capacity and conductance of the extracellular and
> intracellular media.
> Does anyone know whether the spread of the intracellular
> potential change has been measured or modelled (other than
> in the squid axon)? Any references, ideas, or previous
> thought on this subject would be appreciated.
> Guy Brown
Action potentials do not cause a change in organelle membrane
potentials.
Action potentials do not make the intracellular media more positive,
rather
they alter the charge distribution across the membrane (like a
capacitor).
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.<<<<<<<<<<<<<<<<<<<<<<<<<<
I accept this answer. But then what is the potential away from the membrane,
where the organelles are? If it is not zero, how do we reconcile this with
the fact that it exerts no influence on organelles? I tried to discuss an
idealised model some time ago and think that found the solution, but fear that
the group becomes silent any time second-law problems are raised.
Best regards,
Pentcho
