Parameters for the Ih current

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Alberto59
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Joined: Wed Dec 12, 2018 3:23 am

Parameters for the Ih current

Post by Alberto59 »

I have a very basic question. I'm trying to model the Ih current in dendrites. The PARAMETER section of the h.mod file (e.g. in ModelDB # 154732) contains parameters describing the channel kinetics:

vhalft=-75 (mV)
a0t=0.011 (/ms)
zetat=2.2 (1)
gmt=.4 (1)

Could someone please recall me the physical meaning of these parameters?
Thank you for your time,

Alberto
rbdewell
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Re: Parameters for the Ih current

Post by rbdewell »

As it happens, I was working a modified version of that mod file last week. While its commonly used and it reproduces the measured current well, that mod file has some mistakes in the units. Confusion with the parameters is quite understandable as the terms are fairly specific to that file.
All 4 of these parameters influence the relationship between membrane potential and the h channel time constant.

"vhalft" roughly sets the membrane potential with the slowest tau (if gmt ==1, the slowest tau would be at vhalft; gmt<1 shifts the tau peak to more negative potentials)
"a0t" is a scalar gain term of the channel time constant with higher values producing a faster channel
"zetat" sets the steepness of tau's change with membrane potential. Higher values of zetat mean tau decreases more quickly with change from vhalft. (zetat=0 would make tau independent of Vm).
"gmt" is the ratio of the forward and backward rates. If gmt==1 then tau is symmetric around vhalft; if gmt<1 then tau decreases faster on the depolarized slope. This mimics an h channel that closes faster than it opens.
Alberto59
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Re: Parameters for the Ih current

Post by Alberto59 »

Many thanks for the exhaustive explanations. I'm still wondering if there is a "molecular" counterpart for some of these parameters. For instance, does zetat have anything to do with the "gating charge" of kinetic models?
Thanks again,

Alberto
ted
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Re: Parameters for the Ih current

Post by ted »

that mod file has some mistakes in the units
modlunit reports units inconsistencies in that particular file, but they are entirely benign and have no effect on numerical results.

If you want more information about the Borg-Graham approach to describing channel gating, start by reading the references cited in
viewtopic.php?t=2160#p8212. Unfortunately the link to Borg-Graham's masters thesis no longer works--if you can find this document elsewhere online, please post an updated URL as a reply.
Alberto59
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Re: Parameters for the Ih current

Post by Alberto59 »

ted
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Re: Parameters for the Ih current

Post by ted »

Excellent. Of the two URLs, the second one
http://lyle.neurophysics.eu/pdf/MIT-AITR-1161.pdf
returns the most legible document.

Chapter 4 should help resolve your questions. Similar discussions are presented in many standard textbooks on cellular neurophysiology.
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