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### Sangrey Model: Gating current

Posted: **Thu Apr 07, 2016 3:32 pm**

by **mganguly**

I am trying to reproduce the results of the Sangrey et. al. 2004 paper (Analysis of the Optimal Channel Density of the Squid Giant Axon Using a Reparametrized Hodgkin-Huxley Model). In addition to changing the exponent of potassium activation factor n from 4 to 6, they also incorporated a gating current (on top of the HH sodium, HH potassium and HH leak currents) as mentioned in eq. 4 of the paper [I_gating (t) = -Vm(t)*Cna_max*(dm/dt) + Cna_max*(1-m(t))*(dVm/dt]. I am not sure how can I incorporate this gating current. Can I declare a new current in the .mod file (I_g) and then evaluate I_g as mentioned in the paper? If yes, how do I evaluate the voltage derivative (dVm/dt) ?

Mohit

### Re: Sangrey Model: Gating current

Posted: **Sat Apr 09, 2016 10:21 am**

by **hines**

A mod file can modify the value of cm used by the

capacitance mechanism. (In a BEFORE_BREAKPOINT block in which the mod

file has a POINTER that can change cm). Note that if you have several mod files that change cm at the same location, it might be

administratively helpful to have a helper mod file with a name alphabetically less than any of the other gating current mod files

( channel mod files are called in alphabetical order) so that its INITIAL and BEFORE BREAKPOINT block can initialize cm

on every time step.

This should work for fixed and variable step methods. ( I'd definitely compare results, especially with regard to plotting current).

Perhaps not entirely to the point but worth knowing...

Although the current balance equation for membrane is usually expressed as

c*dv/dt + i_ion = i_stim

I believe the form when c is not constant is

d(c*v)/dt + i_ion = i_stim

or

c*dv/dt + (dc/dt)*v + i_ion = i_stim

so that some of effect of changing capacitance appears as an extra

conductance. This "conductance" can be handled with a normal channel

mod file. That mod file can also modify the value of c used by the

capacitance mechanism.