IUBio

question on ACTION POTENTIALS!

Richard Norman rsnorman at mw.mediaone.net
Mon Jan 18 08:57:13 EST 1999


At last, some real neuroscience in the neuroscience newsgroup!

Mohanraj Narayanan wrote in message ...
>After the Na+ ions migrate into the cell in response to depolarization,
I
>was told that the ca2+ ions enter the cell so as to make K+ ions
migrate
>out of the cell to bring the voltage down. I was also told that the
>sodium-potasium pump kicks in and removes the Na+ to bring down the
>voltage to resting potential.  I am confused. Why does the ca2+ come in
if
>all the cell had to do was pump out the Na+ ions with the Na+/K+ pump?


I think you are confused as to the question of "why" things happen.
There are three separate steps: the forces that make ions move,
the control of ion flow by channels, and the factors that make channels
open and close.

Ions move across the membrane in response to physical forces (or the
equivalent, differences in energy levels) across the membrane.  Ions
tend to move from high concentration to low (diffusion).  Charged
particles
move in response to electrical potentials (Ohm's law).  The combination
of these two factors tends to cause Na+ and Ca++ to enter the cell and
K+ to leave the cell.

The ability of ions to flow is conditioned by whether they have a path
to
flow through.  If the membrane channels are open, they can flow, if not,
they can't.  So if a Na+ or a Ca++ channel opens, Na+ or Ca++ enter
the cell.  This carries positive charge into the cell and the cell
depolarizes.
If a K+ channel opens, K+ leaves the cell.  This carries positive charge
out of the cell and the cell hyperpolarizes.

The channels in electrically excitable membrane are sensitive to voltage
and time.  If the cell depolarizes, the Na+ channels open quickly
(activate)
and then close (inactivate).  If the cell depolarizes, the K+ channels
open
slowly and usually stay open until the cell repolarizes.

Put this all together:  Depolarize the cell, Na+ channels open, Na+
enters,
the cell depolarizes more causing more Na+ channels to open, etc.  This
positive feedback loop (the Hodgkin cycle) causes the rapid leading edge
of the action potential.  After a short while, the Na+ channels close
(inactivate) and the K+ channels open.  This causes the cell to
repolarize,
and also causes the hyperpolarization in your next question.  When the
cell repolarizes, the channels go back to their normal state and the
cell
is ready to make another action potential.

Complication number 1:  Calcium.  Some cells have Ca++ channels, others
do not.  The Ca++ channels work much like the sodium channels, and Ca++
and Na+ play similar roles in the production of the action potential.
The Ca++
does NOT enter the cell "to make something happen", it enters because
the
laws of physics make it move in that direction.  However, the effect of
Ca++
in the cell is very different from the effect of Na+.  The small amount
of Na+
that enters through the channels is negligible in comparison to the
amount
already inside the cell, so the Na+ concentration does not change
significantly.
However the small amount of Ca++ that enters is very large in comparison
to
the very tiny amount normally free inside the cell, so this extra Ca++
does
change the intracellular concentration considerably.  Calcium acts as an
intracellular "messenger" triggering all kinds of other phenomena such
as
muscle contraction and synatic transmitter release.  The calcium can
also
have secondary effects on opening ion channels.  However, you should
think of those processes as separate from the electrical events involved
in
the action potential.

Complication number 2: The Na-K pump.  The pump is (almost) completely
irrelevant.  Kill the pump and the cell can still make thousands of
action
potentials.  As I already said, the small amount of Na entering the cell
makes
a negligible chane in the internal Na concentration.  Similarly, the
small
amount of K leaving makes negligible changes in the external K
concentration.
However, after enough action potentials, these small changes accumulate.
The Na-K pump kicks in to restore the original balance.  Think of the
cell as
a flashlight running on rechargeable batteries.  The Na-K pump is the
battery
recharger.  You can use the flashlight quite a bit without the
recharger.  But
eventually, you have to recharge the system to continue operation.

>also, I am confused on the details of why there is a short
>hyperpolarization just after the voltage comes back down to resting
>potential.


This has to do with the details of the resting potential.  At rest,
there are some
open potassium channels, which keeps the resting potential inside
negative.
There are also a few open sodium channels (and chloride channels,
another
complication) which keep the resting potential a little above the
potassium
equilibrium potential level, the Nernst potential for Potassium.  During
the
repolarization phase of the action potential, there are so many open
potassium
channels that the ratio of potassium to sodium permeability (number of
open
channels) shifts in favor of potassium, as compared to the resting cell.
As a
result, the membrane voltage is even more hyperpolarized than the
resting cell.

I am sorry to be so long in answering.  However I know from years of
experience
in teaching this stuff that it is very tricky, indeed.








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