TITLE Slow Cadependent cation current
:
: Ca++ dependent nonspecific cation current ICAN
: Differential equations
:
: Model based on a first order kinetic scheme
:
: <closed> + n cai <> <open> (alpha,beta)
:
: Following this model, the activation fct will be halfactivated 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: 6972, 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: 803818, 1994.
:
: Modifications by Arthur Houweling for use in MyFirstNEURON
:
: Some parameter changes by Paulo Aguiar (pauloaguiar@fc.up.pt):
: tau_factor = 40 => parameter beta changes from 2.0e3 to 5.0e5
: cac = 0.5e3 (before was 1.0e3)
INDEPENDENT {t FROM 0 TO 1 WITH 1 (ms)}
NEURON {
SUFFIX iCaAN
USEION can READ ecan WRITE ican VALENCE 1
USEION ca READ cai
RANGE gbar, m_inf, tau_m
RANGE ican
GLOBAL beta, cac, taumin
}
UNITS {
(mA) = (milliamp)
(mV) = (millivolt)
(molar) = (1/liter)
(mM) = (millimolar)
}
PARAMETER {
v (mV)
celsius (degC)
dt (ms)
ecan = 20 (mV) : reversal potential
cai (mM)
gbar = 0.00025 (mho/cm2)
beta = 2.0e3 (1/ms) : backward rate constant (original value)
tau_factor = 40 : scaling factor allowing tuning
:cac = 1.0e3 (mM) : middle point of activation fct (original value)
cac = 5e4 (mM) : middle point of activation fct
taumin = 0.1 (ms) : minimal value of time constant
:parameter tau_factor and cac were set to produce tau_m ~ 2000(ms) at cai=cac and celsius=36;
:implications of parameters change when cai=cac:
: > BEFORE (beta=2.0e3;tadj=4.66) => tau_m ~ 50 ms
: > AFTER (beta=5.0e5;tadj=4.66) => tau_m ~ 2000 ms
:
:also cac was reduced to half, from 1.0 uM to 0.5 uM
}
STATE {
m
}
ASSIGNED {
ican (mA/cm2)
m_inf
tau_m (ms)
tadj
}
BREAKPOINT {
SOLVE states :METHOD euler
ican = gbar * m*m * (v  ecan)
}
:DERIVATIVE states {
: evaluate_fct(v,cai)
:
: m'= (m_infm) / tau_m
:}
PROCEDURE states() {
evaluate_fct(v,cai)
m = m + ( 1exp(dt/tau_m) )*(m_infm)
:printf("\n iCAN tau_m=%g", tau_m)
}
UNITSOFF
INITIAL {
:
: activation kinetics are assumed to be at 22 deg. C
: Q10 is assumed to be 3
:
tadj = 3.0 ^ ((celsius22.0)/10)
evaluate_fct(v,cai)
m = m_inf
}
PROCEDURE evaluate_fct(v(mV),cai(mM)) { LOCAL alpha2
alpha2 = beta * (cai/cac)^2
tau_m = tau_factor / (alpha2 + beta) / tadj : tau_m = tau_factor / ( beta * (1 + (cai/cac)^2) ) / tadj
m_inf = alpha2 / (alpha2 + beta) : m_inf = (cai/cac)^2 / ( 1 + (cai/cac)^2 )
if(tau_m < taumin) { tau_m = taumin } : min value of time cst
}
UNITSON
