TITLE Ca R-type channel with medium threshold for activation
: used in somatic regions. It has lower threshold for activation/inactivation
: and slower activation time constant
: than the same mechanism in dendritic regions
: uses channel conductance (not permeability)
: written by Yiota Poirazi on 3/12/01 poirazi@LNC.usc.edu
NEURON {
SUFFIX somacar
USEION ca READ eca WRITE ica
RANGE gcabar, m, h
RANGE inf, fac, tau
}
UNITS {
(mA) = (milliamp)
(mV) = (millivolt)
}
INDEPENDENT {t FROM 0 TO 1 WITH 1 (ms)}
PARAMETER { : parameters that can be entered when function is called in cell-setup
v (mV)
dt (ms)
celsius = 34 (degC)
gcabar = 0 (mho/cm2) : initialized conductance
eca = 140 (mV) : Ca++ reversal potential
}
STATE { m h } : unknown activation and inactivation parameters to be solved in the DEs
ASSIGNED { : parameters needed to solve DE
ica (mA/cm2)
inf[2]
fac[2]
tau[2]
}
BREAKPOINT {
SOLVE states
ica = gcabar*m*m*m*h*(v - eca)
}
INITIAL {
m = 0 : initial activation parameter value
h = 1 : initial inactivation parameter value
states()
ica = gcabar*m*m*m*h*(v - eca) : initial Ca++ current value
}
PROCEDURE calcg() {
mhn(v*1(/mV))
m = m + fac[0]*(inf[0] - m)
h = h + fac[1]*(inf[1] - h)
}
PROCEDURE states() {
: exact when v held constant
calcg()
VERBATIM
return 0;
ENDVERBATIM
}
FUNCTION varss(v, i) {
if (i==0) {
varss = 1 / (1 + exp((v+60)/(-3))) :Ca activation
}
else if (i==1) {
varss = 1/ (1 + exp((v+62)/(1))) :Ca inactivation
}
}
FUNCTION vartau(v, i) {
if (i==0) {
vartau = 100 : activation variable time constant
}
else if (i==1) {
vartau = 5 : inactivation variable time constant
}
}
PROCEDURE mhn(v) {LOCAL a, b :rest = -70
:TABLE inf, fac DEPEND dt, celsius FROM -100 TO 100 WITH 200
FROM i=0 TO 1 {
tau[i] = vartau(v,i)
inf[i] = varss(v,i)
fac[i] = (1 - exp(-dt/tau[i]))
}
}