TITLE L-type calcium channel with low threshold for activation
: used in somatic and proximal dendritic regions
: it calculates I_Ca using channel permeability instead of conductance
UNITS {
(mA) = (milliamp)
(mV) = (millivolt)
FARADAY = 96520 (coul)
R = 8.3134 (joule/degK)
KTOMV = .0853 (mV/degC)
}
INDEPENDENT {t FROM 0 TO 1 WITH 1 (ms)}
PARAMETER { :parameters that can be entered when function is called in cell-setup
dt (ms)
v (mV)
celsius = 34 (degC)
gcalbar = 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 = 5 : time constant scaling factor
eca = 140 : Ca++ reversal potential
}
NEURON {
SUFFIX cal
USEION ca READ cai,cao WRITE ica
RANGE gcalbar, minf,taum
}
STATE { m } : unknown parameter to be solved in the DEs
ASSIGNED { : parameters needed to solve DE
ica (mA/cm2)
gcal (mho/cm2)
minf
taum
}
INITIAL { : initialize the following parameter using rates()
rates(v)
m = minf
gcal = gcalbar*m*h2(cai)
}
BREAKPOINT {
SOLVE states
gcal = gcalbar*m*h2(cai) : maximum channel permeability
ica = gcal*ghk(v,cai,cao): calcium current induced by this channel
}
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) { : temperature-dependent adjustment factor
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 alpm(v(mV)) {
TABLE FROM -150 TO 150 WITH 200
alpm = 0.055*(-27.01 - v)/(exp((-27.01-v)/3.8) - 1)
}
FUNCTION betm(v(mV)) {
TABLE FROM -150 TO 150 WITH 200
betm =0.94*exp((-63.01-v)/17)
}
UNITSON
LOCAL facm
:if state_cagk is called from hoc, garbage or segmentation violation will
:result because range variables won't have correct pointer. This is because
:only BREAKPOINT sets up the correct pointers to range variables.
PROCEDURE states() { : exact when v held constant; integrates over dt step
rates(v)
m = m + facm*(minf - m)
VERBATIM
return 0;
ENDVERBATIM
}
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 value
facm = (1 - exp(-dt/taum))
}