COMMENT
Two state kinetic scheme synapse described by rise time taur,
and decay time constant taud. The normalized peak condunductance is 1.
Decay time MUST be greater than rise time.

The solution of A->G->bath with rate constants 1/taur and 1/taud is
 A = a*exp(-t/taur) and
 G = a*taud/(taud-taur)*(-exp(-t/taur) + exp(-t/taud))
	where taur < taud

If taud-taur -> 0 then we have a alphasynapse.
and if taur -> 0 then we have just single exponential decay.

The factor is evaluated in the
initial block such that an event of weight 1 generates a
peak conductance of 1.

Because the solution is a sum of exponentials, the
coupled equations can be solved as a pair of independent equations
by the more efficient cnexp method.

ENDCOMMENT

NEURON {
	POINT_PROCESS ghknmda
	USEION na WRITE ina
	USEION k WRITE ik
	USEION ca READ cai, cao WRITE ica
	
	RANGE taur, taud
	RANGE inmda

	RANGE P, mg, Pmax :AreaFactor
	GLOBAL  mgb
}

UNITS {
	(nA) = (nanoamp)
	(mV) = (millivolt)
	(uS) = (microsiemens)
	(molar) = (1/liter)
	(mM) = (millimolar)
	FARADAY = (faraday) (coulomb)
	R = (k-mole) (joule/degC)
  	(um)    = (micron)
  	PI      = (pi)       (1)

}

PARAMETER {
	taur=5 (ms) <1e-9,1e9>
	taud = 50 (ms) <1e-9,1e9>
	cai = 100e-6(mM)	: 100nM
	cao = 2		(mM)
	nai = 18	(mM)	: Set for a reversal pot of +55mV
	nao = 140	(mM)
	ki = 140	(mM)	: Set for a reversal pot of -90mV
	ko = 5		(mM)
	celsius		(degC)
	mg = 2		(mM)
	Pmax=1e-6   (cm/s)	: According to Canavier, PNMDA's default value is
						: 1e-6 for 10uM, 1.4e-6 cm/s for 30uM of NMDA
	:AreaFactor =250 (cm2) : Computed from PI*Diam*1e2
}

ASSIGNED {
	ina     (mA/cm2)
	ik      (mA/cm2)
	ica     (mA/cm2)
	v (mV)
	P (cm/s)
	factor
	mgb	(1)
	inmda	(mA/cm2)

	diam (um)
}

STATE {
	A (cm/s)
	B (cm/s)
}

INITIAL {
	LOCAL tp
	if (taur/taud > .9999) {
		taur = .9999*taud
	}
	A = 0
	B = 0
	tp = (taur*taud)/(taud - taur) * log(taud/taur)
	factor = -exp(-tp/taur) + exp(-tp/taud)
	factor = 1/factor
}

BREAKPOINT {
	SOLVE state METHOD cnexp
	P=B-A
	mgb = mgblock(v)

: Area is for unit conversion from nA to mA/cm2 which is what ica, ina and ik use.
 
	ina = P*mgb*ghk(v, nai, nao,1)     :/AreaFactor	
	ica = P*10.6*mgb*ghk(v, cai, cao,2)  :/AreaFactor
	ik = P*mgb*ghk(v, ki, ko,1)      :/AreaFactor
	inmda = ica + ik + ina
}

DERIVATIVE state {
	A' = -A/taur
	B' = -B/taud
}

FUNCTION ghk(v(mV), ci(mM), co(mM),z) (0.001 coul/cm3) {
	LOCAL arg, eci, eco
	arg = (0.001)*z*FARADAY*v/(R*(celsius+273.15))
	eco = co*efun(arg)
	eci = ci*efun(-arg)
	ghk = (0.001)*z*FARADAY*(eci - eco)
}

FUNCTION efun(z) {
	if (fabs(z) < 1e-4) {
		efun = 1 - z/2
	}else{
		efun = z/(exp(z) - 1)
	}
}

FUNCTION mgblock(v(mV)) (1){
	TABLE 
	DEPEND mg
	FROM -140 TO 80 WITH 1000 

	: from Jahr & Stevens, JNS, 1990

	mgblock = 1 / (1 + exp(0.062 (/mV) * -v) * (mg / 3.57 (mM)))
}

NET_RECEIVE(weight (uS)) { 	: No use to weight, can be used instead of Pmax,
							: if you want NetCon access to the synaptic
							: conductance.
	state_discontinuity(A, A + Pmax*factor)
	state_discontinuity(B, B + Pmax*factor)
}