/
chan_CaKdyn.mod
121 lines (86 loc) · 3.21 KB
/
chan_CaKdyn.mod
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TITLE Submembrane calcium dynamics for N, P/Q, R calcium pool for NAcb cell
INDEPENDENT {t FROM 0 TO 1 WITH 1 (ms)}
NEURON {
SUFFIX CaKdyn
USEION ca READ ica, cai WRITE cai
RANGE pump, cainf, taur, drive
}
UNITS {
(molar) = (1/liter)
(mM) = (millimolar)
(um) = (micron)
(mA) = (milliamp)
(msM) = (ms mM)
}
CONSTANT {
FARADAY = 96489 (coul) : moles do not appear in units
}
PARAMETER {
drive = 10000 (1)
depth = 0.1 (um) : depth of shell
cainf = 1e-5 (mM) : gives eca = 108 mV
taur = 43 (ms) :
kt = 1e-4 (mM/ms) : left over from Destexhe
kd = 1e-4 (mM)
pump = 0.02 : turn pump up/down
}
STATE {
cai (mM)
}
INITIAL {
cai = cainf
}
ASSIGNED {
ica (mA/cm2)
drive_channel (mM/ms)
drive_pump (mM/ms)
}
BREAKPOINT {
SOLVE state METHOD derivimplicit
}
DERIVATIVE state {
drive_channel = - drive * ica / (2 * FARADAY * depth)
: this part converts the incoming calcium (from channels) into
: a corresponding change in internal concentration
if (drive_channel <= 0.) { drive_channel = 0. } : cannot pump inward
drive_pump = -kt * cai / (cai + kd ) : Michaelis-Menten
: this accounts for calcium being pumped back out - M-M
: represents mechanism that is rate-limited by low ion conc.
: at one end and max pumping rate at high end
cai' = ( drive_channel + pump*drive_pump + (cainf-cai)/taur )
: (cainf-cai)/taur represents exponential decay towards cainf
: at a time constant of taur from diffusive processe
}
COMMENT
Internal calcium concentration due to calcium currents and pump.
Differential equations.
This file contains two mechanisms:
1. Simple model of ATPase pump with 3 kinetic constants (Destexhe 1992)
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).
For further information about this this mechanism, see Destexhe, A.
Babloyantz, A. and Sejnowski, TJ. Ionic mechanisms for intrinsic slow
oscillations in thalamic relay neurons. Biophys. J. 65: 1538-1552, 1993.
2. Simple first-order decay or buffering:
Cai + B <-> ...
which can be written as:
dCai/dt = (cainf - Cai) / taur
where cainf is the equilibrium intracellular calcium value (usually
in the range of 200-300 nM) and taur is the time constant of calcium
removal.
All variables are range variables
Written by Alain Destexhe, Salk Institute, Nov 12, 1992
Citations:
Destexhe, A. Babloyantz, A. and Sejnowski, TJ. Ionic mechanisms for
intrinsic slow oscillations in thalamic relay neurons. Biophys. J. 65:
1538-1552, 1993.
Jackson MB, Redman SJ (2003) Calcium dynamics, buffering, and buffer
saturation in the boutons of dentate granule-cell axons in the hilus. J
Neurosci 23:1612-1621.
ENDCOMMENT