TITLE T-calcium channel
UNITS {
(mA) = (milliamp)
(mV) = (millivolt)
(molar) = (1/liter)
(mM) = (millimolar)
FARADAY = 96520 (coul)
R = 8.3134 (joule/degC)
KTOMV = .0853 (mV/degC)
}
NEURON {
SUFFIX cat
USEION ca READ cai,cao
USEION Ca WRITE iCa VALENCE 2
: The T-current does not activate calcium-dependent currents.
: The construction with dummy ion Ca prevents the updating of the
: internal calcium concentration.
RANGE gcatbar, hinf, minf, taum, tauh, iCa
}
PARAMETER {
v (mV)
tBase = 23.5 (degC)
celsius = 22 (degC)
gcatbar = 0 (mho/cm2) : initialized conductance
ki = 0.001 (mM)
cai = 5.e-5 (mM) : initial internal Ca++ concentration
cao = 2 (mM) : initial external Ca++ concentration
tfa = 1 : activation time constant scaling factor
: tfi = 0.68
tfi = 0.68 : inactivation time constant scaling factor
eca = 140 : Ca++ reversal potential
}
STATE {
m h
}
ASSIGNED {
iCa (mA/cm2)
gcat (mho/cm2)
hinf
tauh
minf
taum
}
INITIAL {
rates(v)
m = minf
h = hinf
gcat = gcatbar*m*m*h*h2(cai)
}
BREAKPOINT {
SOLVE states METHOD cnexp
gcat = gcatbar*m*m*h*h2(cai)
iCa = gcat*ghk(v,cai,cao)
}
DERIVATIVE states { : exact when v held constant
rates(v)
m' = (minf - m)/taum
h' = (hinf - h)/tauh
}
UNITSOFF
FUNCTION h2(cai(mM)) {
h2 = ki/(ki+cai)
}
FUNCTION ghk(v(mV), ci(mM), co(mM)) (mV) {
LOCAL nu,f
f = KTF(celsius)/2
nu = v/f
ghk=-f*(1. - (ci/co)*exp(nu))*efun(nu)
}
FUNCTION KTF(celsius (DegC)) (mV) {
KTF = ((25./293.15)*(celsius + 273.15))
}
FUNCTION efun(z) {
if (fabs(z) < 1e-4) {
efun = 1 - z/2
}else{
efun = z/(exp(z) - 1)
}
}
FUNCTION alph(v(mV)) {
TABLE FROM -150 TO 150 WITH 200
alph = 1.6e-4*exp(-(v+57)/19)
}
FUNCTION beth(v(mV)) {
TABLE FROM -150 TO 150 WITH 200
:beth = 1/(exp((-v+15)/10)+1.0)
beth = 1/(exp((-v+15)/10)+1.0)
}
FUNCTION alpm(v(mV)) {
TABLE FROM -150 TO 150 WITH 200
alpm = 0.1967*(-1.0*v+19.88)/(exp((-1.0*v+19.88)/10.0)-1.0)
}
FUNCTION betm(v(mV)) {
TABLE FROM -150 TO 150 WITH 200
betm = 0.046*exp(-v/22.73)
}
PROCEDURE rates(v (mV)) { :callable from hoc
LOCAL a
a = alpm(v)
taum = 1/(tfa*(a + betm(v))) : estimation of activation tau
minf = a/(a+betm(v)) : estimation of activation steady state
a = alph(v)
tauh = 1/(tfi*(a + beth(v))) : estimation of inactivation tau
hinf = a/(a+beth(v)) : estimation of inactivation steady state
} |