TITLE decay of internal calcium concentration
:
: Internal calcium concentration due to calcium currents and pump.
: Differential equations.
:
: Simple model of ATPase pump with 3 kinetic constants (Destexhe 92)
:     Cai + P <-> CaP -> Cao + P  (k1,k2,k3)
: A Michaelis-Menten approximation is assumed, which reduces the complexity
: of the system to 2 parameters: 
:       kt = <tot enzyme concentration> * k3  -> TIME CONSTANT OF THE PUMP
:	kd = k2/k1 (dissociation constant)    -> EQUILIBRIUM CALCIUM VALUE
: The values of these parameters are chosen assuming a high affinity of 
: the pump to calcium and a low transport capacity (cfr. Blaustein, 
: TINS, 11: 438, 1988, and references therein).  
:
: Units checked using "modlunit" -> factor 10000 needed in ca entry
:
: VERSION OF PUMP + DECAY (decay can be viewed as simplified buffering)
:
: All variables are range variables
:
:
: This mechanism was published in:  Destexhe, A. Babloyantz, A. and 
: Sejnowski, TJ.  Ionic mechanisms for intrinsic slow oscillations in
: thalamic relay neurons. Biophys. J. 65: 1538-1552, 1993)
:
: Written by Alain Destexhe, Salk Institute, Nov 12, 1992
:

INDEPENDENT {t FROM 0 TO 1 WITH 1 (ms)}

NEURON {
	SUFFIX cad
	USEION ca READ ica, cai WRITE cai
	RANGE ca, depth
	GLOBAL cainf,taur
}

UNITS {
	(molar) = (1/liter)      :moles do not appear in units
	(mM)	= (millimolar)
	(um)	= (micron)
	(mA)	= (milliamp)
	(msM)   = (ms mM)
}

PARAMETER {
	depth = .2	(um)     : depth of shell
	taur  = 0.1     (ms)     : time constant of calcium decay
	cainf = 4e-5	(mM)
	cai		(mM)
}

STATE {
	ca   (mM)
}

INITIAL {
	ca = cainf
	cai = ca
}

ASSIGNED{
	ica		(mA/cm2)
	drive_channel   (mM/ms)
}

BREAKPOINT{
	SOLVE state METHOD cnexp
}

DERIVATIVE state {

	drive_channel = -(10000)*ica/(2*96494*depth)

	if(drive_channel <= 0.) {drive_channel = 0.}:cannot pump inward

	ca' = drive_channel + (cainf-ca)/taur
	cai = ca
}