TITLE Slow Ca-dependent cation current
:
: Ca++ dependent nonspecific cation current ICAN
: Differential equations
:
: Model based on a first order kinetic scheme
:
: + n cai <-> (alpha,beta)
:
: Following this model, the activation fct will be half-activated at
: a concentration of Cai = (beta/alpha)^(1/n) = cac (parameter)
:
: The mod file is here written for the case n=2 (2 binding sites)
: ---------------------------------------------
:
: Kinetics based on: Partridge & Swandulla, TINS 11: 69-72, 1988.
:
: This current has the following properties:
: - inward current (non specific for cations Na, K, Ca, ...)
: - activated by intracellular calcium
: - NOT voltage dependent
:
: A minimal value for the time constant has been added
:
: Ref: Destexhe et al., J. Neurophysiology 72: 803-818, 1994.
: See also: http://www.cnl.salk.edu/~alain , http://cns.fmed.ulaval.ca
:
INDEPENDENT {t FROM 0 TO 1 WITH 1 (ms)}
NEURON {
SUFFIX ican
USEION n READ en WRITE in VALENCE 1
USEION ca READ cai
RANGE gbar, m_inf, tau_m, in
GLOBAL beta, cac, taumin
}
UNITS {
(mA) = (milliamp)
(mV) = (millivolt)
(molar) = (1/liter)
(mM) = (millimolar)
}
PARAMETER {
v (mV)
celsius = 36 (degC)
en = -20 (mV) : reversal potential
cai (mM) : initial [Ca]i
gbar = 0.00025 (mho/cm2)
beta = 0.001 :0.002 (1/ms) : backward rate constant
cac = 0.01 (mM) : middle point of activation fct
taumin = 0.1 (ms) : minimal value of time constant
}
STATE {
m
}
ASSIGNED {
in (mA/cm2)
m_inf
tau_m (ms)
tadj
}
BREAKPOINT {
SOLVE states METHOD euler
in = gbar * m*m * (v - en)
}
DERIVATIVE states {
evaluate_fct(v,cai)
m' = (m_inf - m) / tau_m
}
UNITSOFF
INITIAL {
:
: activation kinetics are assumed to be at 22 deg. C
: Q10 is assumed to be 3
:
tadj = 3.0 ^ ((celsius-22.0)/10)
evaluate_fct(v,cai)
m = m_inf
}
PROCEDURE evaluate_fct(v(mV),cai(mM)) { LOCAL alpha2
alpha2 = beta * (cai/cac)^2
tau_m = 1 / (alpha2 + beta) / tadj
m_inf = alpha2 / (alpha2 + beta)
if(tau_m < taumin) { tau_m = taumin } : min value of time cst
}
UNITSON
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