Mohanraj Narayanan <monst3+ at pitt.edu> writes:
>> 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?
>> also, I am confused on the details of why there is a short
> hyperpolarization just after the voltage comes back down to resting
> potential.
>> any input would be greatly appriciated.
> thank you
>
Well, you're certainly confused. I don't think I can cure the
confusion without writing at great length, but I can point out some of
the things you wrote that are incorrect:
1) Na+ ions don't migrate into the cell in *response* to
depolarization; the inflow of Na+ ions is what *causes*
depolarization.
2) Ca++ ions don't play a crucial part in the basic action
potential. (Some kinds of cells have Ca++ based action potentials,
though, which are much slower than Na+ based action potentials.)
3) K+ ions don't necessarily need to flow out of the cell to bring the
voltage down. Once the Na+ channels close, the voltage will come down
to rest automatically. Some types of cells have action potentials
that do not involve K+ channels at all.
4) Other types of cells have action potentials that do involve K+
channels. Their function is to bring the membrane back to rest more
quickly. They stay open for a little while after the Na+ channels
have all closed; that's the cause of the hyperpolarization.
5) The number of Na+ or K+ atoms that pass through the membrane during
an action potential is very small, so the Na+/K+ pump doesn't make a
very large difference during a single action potential. A typical
neuron can fire hundreds of action potentials before "running down"
even if the pump is completely disabled.
6) Ca++ has many functions inside the cell, and many action potentials
do cause an inflow of CA++ through voltage-dependent Ca++ channels,
but, as I said, this probably doesn't contribute much to the action
potential at the soma in most cases. Concerning what happens in the
dendrites, the story might be different; this is currently a subject
of very active research.
I hope this helps. If you want more information, you should consult a
textbook.
-- Bill
