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 = * 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
:
: Modiefied by B Kampa Oct 2005: use diam/4 instead of depth to calculate [Ca]
INDEPENDENT {t FROM 0 TO 1 WITH 1 (ms)}
NEURON {
SUFFIX cad
USEION ca READ ica, cai WRITE cai
RANGE ca
GLOBAL depth,cainf,taur
}
UNITS {
(molar) = (1/liter) : moles do not appear in units
(mM) = (millimolar)
(um) = (micron)
(mA) = (milliamp)
(msM) = (ms mM)
FARADAY = (faraday) (coulomb)
}
PARAMETER {
diam (um)
depth = .1 (um)
taur = 10 (ms) : rate of calcium removal was 100 or 200
cainf = 100e-6(mM)
cai (mM)
}
STATE {
ca (mM) <1e-5>
}
INITIAL {
ca = cainf
cai = ca
}
ASSIGNED {
ica (mA/cm2)
drive_channel (mM/ms)
}
BREAKPOINT {
SOLVE state METHOD derivimplicit
}
DERIVATIVE state {
depth = diam/4
drive_channel = - (10000) * ica / (2 * FARADAY * depth)
if (drive_channel <= 0.) { drive_channel = 0. } : cannot pump inward
ca' = drive_channel + (cainf-ca)/taur
cai = ca
}