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 {
	sh = 0
	v (mV)
	celsius		(degC)
	gkabar=.008 (mho/cm2)
        vhalfn=-5   (mV) :-15 (Hyun) -5 (Kim) 11 (Hemond)
        vhalfl=-65   (mV) :-70 (Hyun) -65 (Kim) -56 (Hemond)
        a0l=0.05      (/ms) 
        a0n=0.05    (/ms)
        zetan=-1.8    (1) :-3 (Hyun) -1.8 (Kim) -1.5 (Hemond)
        zetal=3.7    (1) :5 (Hyun) 3.7 (Kim) 3 (Hemond)
        gmn=0.55   (1)
        gml=1   (1)
	lmin=2  (mS) :6.7 (Hyun) 
	nmin=0.1  (mS)
	pw=-1    (1)
	tq=-40
	qq=5
	q10=5
	qtl=1 :0.5 (Hyun)
	ek
}


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

STATE {
	n
        l
}

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

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


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

}


FUNCTION alpn(v(mV)) {
LOCAL zeta
  zeta=zetan+pw/(1+exp((v-tq-sh)/qq))
  alpn = exp(1.e-3*zeta*(v-vhalfn-sh)*9.648e4/(8.315*(273.16+celsius))) 
}

FUNCTION betn(v(mV)) {
LOCAL zeta
  zeta=zetan+pw/(1+exp((v-tq-sh)/qq))
  betn = exp(1.e-3*zeta*gmn*(v-vhalfn-sh)*9.648e4/(8.315*(273.16+celsius))) 
}

FUNCTION alpl(v(mV)) {
  alpl = exp(1.e-3*zetal*(v-vhalfl-sh)*9.648e4/(8.315*(273.16+celsius))) 
}

FUNCTION betl(v(mV)) {
  betl = exp(1.e-3*zetal*gml*(v-vhalfl-sh)*9.648e4/(8.315*(273.16+celsius))) 
}

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-sh)/qtl
	if (taul<lmin/qtl) {taul=lmin/qtl}
}