TITLE Fast mechanism for submembranal Ca++ concentration (cai)
:
: Takes into account:
:
:	- increase of cai due to calcium currents
:	- extrusion of calcium with a simple first order equation
:
: This mechanism is compatible with the calcium pump "cad" and has the 
: same name and parameters; however the parameters specific to the pump
: are dummy here.
:
: Parameters:
:
:	- depth: depth of the shell just beneath the membran (in um)
:	- cainf: equilibrium concentration of calcium (2e-4 mM)
:	- taur: time constant of calcium extrusion (must be fast)
:	- kt,kd: dummy parameters
:
: Written by Alain Destexhe, Salk Institute, 1995
:

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

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

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

CONSTANT {
	FARADAY = 96489		(coul)		: moles do not appear in units
:	FARADAY = 96.489	(k-coul)	: moles do not appear in units
}

PARAMETER {
	depth	= .1	(um)		: depth of shell
	taur	= 5	(ms)		: rate of calcium removal
	cainf	= 2e-4	(mM)
	kt	= 0	(mM/ms)		: dummy
	kd	= 0	(mM)		: dummy
}

STATE {
	cai		(mM) 
}

INITIAL {
	cai = cainf
}

ASSIGNED {
	ica		(mA/cm2)
	drive_channel	(mM/ms)
}
	
BREAKPOINT {
	SOLVE state METHOD euler
}

DERIVATIVE state { 

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

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

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