question related to neuron spiking
xli6 at gmu.edu
Thu Mar 25 11:17:24 EST 2004
Matt Jones wrote:
> Your example of potassium chanel activation "brings the membrane
> potential back" is on the right track. If you think more along these
> lines, you'll be able to figure out AHPs and ADPs for yourself.
> Some hints:
> 1) Does K-channel activation really "bring the membrane potential
> back"? Or does it take the potential to somewher *different* than the
> resting potential? For example, a typical neuron may have a resting
> potential of
> -60 mV? Is this where the voltage goes immediately after a spike, or
> does it go somewhere else (i.e., where are the K-channels *trying* to
> take the membrane potential?)? How do you calculate this?
Suppose the reversal potential of K ion is -70mV, calculated from Nerst
Equation, mainly determined by the ratio of K ion concentrations inside
and outside of the neuron membrane. So opening of K-channels will try to
v-clamp to -70mV. It is even lower than the membrane resting
potential(say, -60mV). So Vmem will overshoot downwards a little bit.
This overshooting is called "afterhyperpolarization", or AHP.
More specifically, the curve of AHP starts from the point where Vmem
equal to Vrest when Vmem decreasing from the action potential peak, ends
at the point where Vmem is the most hyperpolarized. Correct?
> 2) If you answered the last question correctly, you should now know
> why K-channel opening causes an AHP. How would you instead cause an
> ADP? (you will have to think slightly outside of the HH equations to
> figure this out, but not very far outside).
Now I guess, following the discussion about afterhyperpolarization
above, the membrane potential will return(=rise) to its resting membrane
potential. The resting membrane potential is determined by the neuron's
membrane leakness and other a few opening channels. This part is called
More specifically, the curve of ADP starts at the end point of AHP, ends
at the point where Vmem backs to Vres. Compared to AHP, ADP takes much,
much longer time and the slope is much shallow. Correct?
If I am correct above, I know this phenomenon. I just took it for
granted. Never thought it has some significant meaning. I read in the
paper "Neurons showed spike after-depolarizations along with doublet or
burst firing...". I am confused. Any neuron doesn't show
> 3) Regarding the "monophasic" part, that means that there are no
> sudden jumps or inflections during the decay of the AHP.
Question: I have never seen any sudden jumps or infections happening
within a spike. Could you give me more information?
> cells, there are biphasic AHPs, that decay as the sum of two
> exponential components. Yes, this shape does tell you something
> significant about the underlying mechanisms. What would you have to do
> to the HH equations that would produce a biphasic AHP?
If I am correct about the curve of AHP above, fitting this curve with
one or two exponential functions determines if it is mono-phasic or
bi-phasic. But I don't see how pure Hodgkin-Huxley mechanism will
produce a mono-phasic AHP with time constant tao. And what this tao
correspond to which parameter in HH model? I don't know how to change
the model to have biphasic AHP.
Thank you very much.
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