TITLE Ca Rtype 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)
}
PARAMETER {
eca = 140 (mV) : Ca++ reversal potentia
gcabar = 0 (mho/cm2) : initialized conductance
celsius =34 (degC)
}
ASSIGNED { : parameters needed to solve DE
v (mV)
ica (mA/cm2)
ecar (mV) : Ca++ reversal potential
inf[2]
fac[2]
tau[2]
: cai (mM)
:cao (mM)
}
STATE {
m
h
}
INITIAL {
m = 0 : initial activation parameter value
h = 1 : initial inactivation parameter value
rates(v)
ica = gcabar*m*m*m*h*(v  eca) : initial Ca++ current value
}
BREAKPOINT {
SOLVE states METHOD cnexp
:ecar = (1e3) * (R*(celsius+273.15))/(2*FARADAY) * log (cao/cai)
ica = gcabar*m*m*m*h*(v  eca)
}
DERIVATIVE states {
rates(v)
m' = (inf[0]m)/tau[0]
h' = (inf[1]h)/tau[1]
}
PROCEDURE rates(v(mV)) {
FROM i=0 TO 1 {
tau[i] = vartau(i)
inf[i] = varss(v,i)
}
}
FUNCTION varss(v(mV), 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(i) {
if (i==0) {
vartau = 100 : activation variable time constant
}
else if (i==1) {
vartau = 5 : inactivation variable time constant
}
}
