kaprox.mod

TITLE K-A channel from Klee Ficker and Heinemann
: modified to account for Dax A Current --- M.Migliore Jun 1997
: modified to be used with cvode  M.Migliore 2001

UNITS {
	(mA) = (milliamp)
	(mV) = (millivolt)

}

PARAMETER {
	v (mV)
	celsius		(degC)
	gkabar=.008 (mho/cm2)
        vhalfn=11   (mV)
        vhalfl=-56   (mV)
        a0l=0.05      (/ms)
        a0n=0.05    (/ms)
        zetan=-1.5    (1)
        zetal=3    (1)
        gmn=0.55   (1)
        gml=1   (1)
	lmin=2  (mS)
	nmin=0.1  (mS)
	pw=-1    (1)
	tq=-40
	qq=5
	q10=5
	qtl=1
	ek
}


NEURON {
	SUFFIX kap
	USEION k READ ek WRITE ik
        RANGE gkabar,gka,vhalfn,vhalfl,i
        GLOBAL ninf,linf,taul,taun,lmin
}

STATE {
	n
        l
}

ASSIGNED {
	ik (mA/cm2)
	i (mA/cm2)
        ninf
        linf      
        taul
        taun
        gka
}

INITIAL {
	rates(v)
	n=ninf
	l=linf
}


BREAKPOINT {
	SOLVE states METHOD cnexp
	gka = gkabar*n*l
	i = gka*(v-ek)
	ik = i

}


FUNCTION alpn(v(mV)) {
    LOCAL zeta,Arg
    Arg=(v-tq)/qq
    
    if (Arg<-50) {zeta=zetan+pw}
    else if (Arg>50) {zeta=zetan}
    else {zeta=zetan+pw/(1+exp(Arg))}
    
    Arg=1.e-3*zeta*(v-vhalfn)*9.648e4/(8.315*(273.16+celsius))
    
    if (Arg<-50) {alpn=0}
    else if (Arg>50) {alpn=exp(50)}
    else {alpn=exp(Arg)}
}

FUNCTION betn(v(mV)) {
    LOCAL zeta,Arg
    Arg=(v-tq)/qq
    
    if (Arg<-50) {zeta=zetan+pw}
    else if (Arg>50) {zeta=zetan}
    else {zeta=zetan+pw/(1+exp(Arg))}
    
    Arg=1.e-3*zeta*gmn*(v-vhalfn)*9.648e4/(8.315*(273.16+celsius))
    
    if (Arg<-50) {betn=0}
    else if (Arg>50) {betn=exp(50)}
    else {betn=exp(Arg)}
}

FUNCTION alpl(v (mV)) {
    LOCAL Arg
    Arg=1.e-3*zetal*(v-vhalfl)*9.648e4/(8.315*(273.16+celsius))
    
    if (Arg<-50) {alpl=0}
    else if (Arg>50) {alpl=exp(50)}
    else {alpl=exp(Arg)}
}

FUNCTION betl(v(mV)) {
    LOCAL Arg
    Arg=1.e-3*zetal*gml*(v-vhalfl)*9.648e4/(8.315*(273.16+celsius))
    
    if (Arg<-50) {betl=0}
    else if (Arg>50) {betl=exp(50)}
    else {betl=exp(Arg)}
}

DERIVATIVE states {     : exact when v held constant; integrates over dt step
        rates(v)
        n' = (ninf - n)/taun
        l' =  (linf - l)/taul
}

PROCEDURE rates(v (mV)) { :callable from hoc
        LOCAL a,qt
        qt=q10^((celsius-24)/10)
        a = alpn(v)
        ninf = 1/(1 + a)
        taun = betn(v)/(qt*a0n*(1+a))
	if (taun<nmin) {taun=nmin}
        a = alpl(v)
        linf = 1/(1+ a)
	taul = 0.26*(v+50)/qtl
	if (taul<lmin/qtl) {taul=lmin/qtl}
}