TITLE Low threshold calcium current
COMMENT
Ca++ current responsible for low threshold spikes (LTS)
THALAMOCORTICAL CELLS
Differential equations
Model based on the data of Huguenard & McCormick, J Neurophysiol
68: 1373-1383, 1992 and Huguenard & Prince, J Neurosci.
12: 3804-3817, 1992.
Features:
- kinetics described by Nernst equations using a m2h format
- activation considered at steady-state
- inactivation fit to Huguenard's data using a bi-exp function
- shift for screening charge, q10 of inactivation of 3
Written by Alain Destexhe, Salk Institute, 1993; modified 1995
ENDCOMMENT
INDEPENDENT {t FROM 0 TO 1 WITH 1 (ms)}
NEURON {
SUFFIX mcIt
USEION ca READ cai,cao WRITE ica
GLOBAL q10
RANGE gcabar, m_inf, tau_m, h_inf, tau_h, shift
}
UNITS {
(molar) = (1/liter)
(mV) = (millivolt)
(mA) = (milliamp)
(mM) = (millimolar)
FARADAY = (faraday) (coulomb)
R = (k-mole) (joule/degC)
}
PARAMETER {
v (mV)
celsius = 36 (degC)
gcabar = 0.002 (mho/cm2)
q10 = 3 : Q10 of inactivation
shift = 2 (mV) : corresponds to 2mM ext Ca++
cai = 2.4e-4 (mM) : adjusted for eca=120 mV
cao = 2 (mM)
}
STATE {
h
}
ASSIGNED {
ica (mA/cm2)
carev (mV)
m_inf
tau_m (ms) : dummy variable for compatibility
h_inf
tau_h (ms)
phi_h
}
BREAKPOINT {
SOLVE castate METHOD cnexp
carev = (1e3) * (R*(celsius+273.15))/(2*FARADAY) * log (cao/cai)
ica = gcabar * m_inf * m_inf * h * (v-carev)
}
DERIVATIVE castate {
evaluate_fct(v)
h' = (h_inf - h) / tau_h
}
UNITSOFF
INITIAL {
h = 0
:
: Transformation to 36 deg assuming Q10 of 3 for h
: (as in Coulter et al., J Physiol 414: 587, 1989)
:
phi_h = q10 ^ ((celsius-24 (degC) )/10 (degC) )
}
PROCEDURE evaluate_fct(v(mV)) { LOCAL Vm
Vm = v + shift
m_inf = 1.0 / ( 1 + exp(-(Vm+57)/6.2) )
h_inf = 1.0 / ( 1 + exp((Vm+81)/4.0) )
: if(Vm < -80) {
: tau_h = exp((Vm+467)/66.6) / phi_h
: } else {
: tau_h = ( 28 + exp(-(Vm+22)/10.5) ) / phi_h
: }
tau_h = 30.8 + (211.4 + exp((Vm+113.2)/5)) / (1 + exp((Vm+84)/3.2))
tau_h = tau_h / phi_h
}
UNITSON