Object Type:	Ca_concen

Description:	Single pool model for Ca concentration.

Author:		M. Wilson, Caltech (2/89)

------------------------------------------------------------------------------

ELEMENT PARAMETERS

DataStructure:	Ca_concen_type  [in src/segment/seg_struct.h]

Size:		112 bytes

Fields:		tau		time constant of decay
		Ca_base		resting (base level) concentration
		B		equals 1/(ion_charge * Faraday * volume)
		Ca		Resulting concentraton of Ca++ ions
		C		Intermediate result, Ca - Ca_base
		thick		Holds thickness of shell (used by readcell)

------------------------------------------------------------------------------

SIMULATION PARAMETERS

Function:	CaConcen  [in src/segment/Ca_concen.c]

Classes:	concentration segment

Actions:	INIT
		PROCESS
		RESET
		CHECK
		SAVE2
		RESTORE2

Messages:       I_Ca  Ik              Receives ionic current Ik from a channel
                BASE  Ca_base         Sets Ca_base with a message
                fI_Ca Ik fraction     Receives fractional current Ik*fraction
                INCREASE  delta_Ik    Add delta_Ik to incoming current
                DECREASE  delta_Ik    Subtract delta_Ik from incoming current

------------------------------------------------------------------------------

Notes:		Single shell model for Ca concentration.
		Solves  dC/dt = B*Ik - C/tau.
		Ca = Ca_base + C.

		In SI units, where concentration is moles/m^3
		(milli-moles/liter) and current is in amperes, theory gives B
		= 5.2e-6/(shell volume).  In practice, B is a parameter to be
		fitted or estimated from experiment, as buffering, non-uniform
		distribution of Ca, etc., will modify this value.  If thick =
		0, the readcell routine calculates B by dividing the "density"
		parameter in the cell parameter file by the volume of the
		compartment.  Otherwise, it scales as a true shell, with the
		volume of a shell having thickness thick.  A negative value of
		the "density" parameter may be used to indicate that it should
		be taken as an absolute value of B, without scaling.  

Example:	see Scripts/burster

See also:	readcell