TITLE Slow Ca-dependent potassium current
:
: Ca++ dependent K+ current IC responsible for slow AHP
: Differential equations
:
: Model based on a first order kinetic scheme
:
: + n cai <-> (alpha,beta)
:
: Following this model, the activation fct will be half-activated at
: a concentration of Cai = (beta/alpha)^(1/n) = cac (parameter)
:
: The mod file is here written for the case n=2 (2 binding sites)
: ---------------------------------------------
:
: This current models the "slow" IK[Ca] (IAHP):
: - potassium current
: - activated by intracellular calcium
: - NOT voltage dependent
:
: A minimal value for the time constant has been added
:
: Ref: Destexhe et al., J. Neurophysiology 72: 803-818, 1994.
: See also: http://www.cnl.salk.edu/~alain , http://cns.fmed.ulaval.ca
: modifications by Yiota Poirazi 2001 (poirazi@LNC.usc.edu)
: taumin = 0.5 ms instead of 0.1 ms
NEURON {
SUFFIX kca
USEION k READ ek WRITE ik
USEION ca READ cai
RANGE gk, gbar, m_inf, tau_m,ik
GLOBAL beta, cac
}
UNITS {
(mA) = (milliamp)
(mV) = (millivolt)
(molar) = (1/liter)
(mM) = (millimolar)
}
PARAMETER {
v (mV)
celsius = 36 (degC)
ek = -80 (mV)
cai = 2.4e-5 (mM) : initial [Ca]i
gbar = 0.01 (mho/cm2)
beta = 0.03 (1/ms) : backward rate constant
cac = 0.00035 (mM) : middle point of activation fct
taumin = 0.5 (ms) : minimal value of the time cst
gk
}
STATE {m} : activation variable to be solved in the DEs
ASSIGNED { : parameters needed to solve DE
ik (mA/cm2)
m_inf
tau_m (ms)
tadj
}
BREAKPOINT {
SOLVE states METHOD derivimplicit
gk = gbar*m*m*m : maximum channel conductance
ik = gk*(v - ek) : potassium current induced by this channel
}
DERIVATIVE states {
evaluate_fct(v,cai)
m' = (m_inf - m) / tau_m
}
UNITSOFF
INITIAL {
:
: activation kinetics are assumed to be at 22 deg. C
: Q10 is assumed to be 3
:
tadj = 3 ^ ((celsius-22.0)/10) : temperature-dependent adjastment factor
evaluate_fct(v,cai)
m = m_inf
}
PROCEDURE evaluate_fct(v(mV),cai(mM)) { LOCAL car
car = (cai/cac)^4
m_inf = car / ( 1 + car ) : activation steady state value
tau_m = 1 / beta / (1 + car) / tadj
if(tau_m < taumin) { tau_m = taumin } : activation min value of time cst
}
UNITSON
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