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: Fox-Lu algorithm for HH-style K-channel.
: Author: Ronald van Elburg

NEURON {
    SUFFIX HH_FL
    USEION k READ ek WRITE ik
    RANGE gk_single, KSingle 
}

UNITS {
    (S)     = (siemens)
    (mV)    = (millivolt)
    (mA)    = (milliamp)
}

PARAMETER { 
    gk_single = 0.00036 (S/cm2)         : Single channel conductance (here a 
                                        : fantasy value at present) reasonable
                                        : value is needed here.
                                        
    KSingle   = 100   (1)               : The number of potassium channels
}

ASSIGNED {
    v (mV)
    ek (mV)                
    ik (mA/cm2)
}

STATE {
        n
}

BREAKPOINT {
    SOLVE states METHOD cnexp
    ik  = gk_single* KSingle* n^4 * (v - ek)
}

INITIAL {
    : Assume v has been constant for a long time
    n = alpha(v)/(alpha(v) + beta(v))
}

DERIVATIVE states {
    : Computes state variable n at present v & t
    n' = (1-n)*alpha(v) - n*beta(v)+gn()
}

FUNCTION alpha(Vm (mV)) (/ms) {
    LOCAL x
    UNITSOFF
    x = (Vm+55)/10
    if (fabs(x) > 1e-6) {
        alpha = 0.1*x/(1 - exp(-x))
    }else{
        alpha = 0.1/(1 - 0.5*x)
    }
    UNITSON
}

FUNCTION beta(Vm (mV)) (/ms) {
    UNITSOFF
    beta = 0.125*exp(-(Vm+65)/80)
    UNITSON
}

FUNCTION varn() {
    UNITSOFF
    varn=(2/KSingle)*(alpha(v)*beta(v))/(alpha(v)+beta(v))
    UNITSON
}

FUNCTION gn() (/ms) {
    UNITSOFF
    gn=normrand(0,sqrt(varn()))
    UNITSON
}