TITLE CaGk
: Calcium activated K channel.
: Modified from Moczydlowski and Latorre (1983) J. Gen. Physiol. 82

UNITS {
	(molar) = (1/liter)
}

UNITS {
	(mV) =	(millivolt)
	(mA) =	(milliamp)
	(mM) =	(millimolar)
	FARADAY = 96485.332 (coul)
	R = 8.3144626 (joule/K)
    T = 273.15 (K)
	KTOMV = .0853 (mV/K)
}


NEURON {
	SUFFIX cagk
	USEION ca READ cai
	USEION k READ ki, ko WRITE ik
	RANGE gkca,ik, ggk, gbar
	GLOBAL oinf, tau
}


PARAMETER {
	celsius		(degC)
	v		(mV)
	gbar=.01	(mho/cm2)	: Maximum Permeability
	cai 		(mM)
	:ek		(mV)

	d1 = .84
	d2 = 1.
	k1 = .48e-3	(mM)
	k2 = .13e-6	(mM)
	abar = .28	(/ms)
	bbar = .48	(/ms)
	ggk
		ko0=5	(mM)
		ki0=150 (mM)
		ko=5	(mM)
		ki=150 (mM)
		nfact =1 (mV)
        st=1            (1)
}

ASSIGNED {
	ik		(mA/cm2)
	oinf
	tau		(ms)
        gkca          (mho/cm2)
}

INITIAL {
        rate(v,cai)
		nfact=-1000*R*(273.15+celsius)/FARADAY
		ggk=nfact*(log(ki/ko))
        o=oinf
}

STATE {	o }		: fraction of open channels

BREAKPOINT {
	SOLVE state METHOD cnexp
	gkca = gbar*o^st
	ggk=nfact*(log(ki/ko))
	ik = gkca*(v - ggk)
}

DERIVATIVE state {	: exact when v held constant; integrates over dt step
	rate(v, cai)
	
	o' = (oinf - o)/tau
}

FUNCTION alp(v (mV), c (mM)) (1/ms) { :callable from hoc
	alp = c*abar/(c + exp1(k1,d1,v))
}

FUNCTION bet(v (mV), c (mM)) (1/ms) { :callable from hoc
	bet = bbar/(1 + c/exp1(k2,d2,v))
}

FUNCTION exp1(k (mM), d, v (mV)) (mM) { :callable from hoc
	if (v>-9) {exp1 = k*exp(-2*d*FARADAY*v/R/(273.15 + celsius)) }else{exp1=exp(699)}
}

PROCEDURE rate(v (mV), c (mM)) { :callable from hoc
	LOCAL a
	a = alp(v,c)
	tau = 1/(a + bet(v, c))
	oinf = a*tau
}