TITLE hyperpolarization-activated current (H-current)
COMMENT
Two distinct activation gates are assumed with the same asymptotic
opening values, a fast gate (F) and a slow gate (S). The following
kinetic scheme is assumed
s0 --(Alpha)--> s1 + n Cai --(k1)--> s2
<--(Beta)--- <--(k2)--
f0 --(Alpha)--> f1 + n Cai --(k1)--> f2
<--(Beta)--- <--(k2)--
where s0/f0, s1/f1, and s2/f2 are resp. fraction of closed slow/fast
gates, fraction of open unbound slow/fast gates, and fraction of open
calcium-bound slow/fast gates, n is taken 2, and k1 = k2*C where
C = (cai/cac)^n and cac is the critical value at which Ca2+ binding
is half-activated.
The total current is computed according
ih = ghbar * (s1+s2) * (f1+f2) * (v-eh)
*********************************************
reference: Destexhe, Babloyantz & Sejnowski (1993)
Biophys.J. 65, 1538-1552
found in: thalamocortical neurons
*********************************************
Maxim Bazhenov's first mod file
Rewritten for MyFirstNEURON by Arthur Houweling
ENDCOMMENT
INDEPENDENT {t FROM 0 TO 1 WITH 1 (ms)}
NEURON {
SUFFIX iH
USEION h READ eh WRITE ih VALENCE 1
USEION ca READ cai
RANGE ghbar, tau_s, tau_f, tau_c, ih
GLOBAL cac
}
UNITS {
(mA) = (milliamp)
(mV) = (millivolt)
(molar) = (1/liter)
(mM) = (millimolar)
}
PARAMETER {
v (mV)
cai (mM)
celsius (degC)
eh = -43 (mV)
ghbar = 4e-5 (mho/cm2)
cac = 5e-4 (mM)
}
STATE {
s1 : fraction of open unbound slow gates
s2 : fraction of open calcium-bound slow gates
f1 : fraction of open unbound fast gates
f2 : fraction of open calcium-bound fast gates
}
ASSIGNED {
ih (mA/cm2)
h_inf
tau_s (ms) : time constant slow gate
tau_f (ms) : time constant fast gate
tau_c (ms) : time constant calcium binding
alpha_s (1/ms)
alpha_f (1/ms)
beta_s (1/ms)
beta_f (1/ms)
C
k2 (1/ms)
tadj
s0 : fraction of closed slow gates
f0 : fraction of closed fast gates
}
BREAKPOINT {
SOLVE states METHOD cnexp
ih = ghbar * (s1+s2) * (f1+f2) * (v-eh)
}
UNITSOFF
DERIVATIVE states {
evaluate_fct(v,cai)
s1' = alpha_s*s0 - beta_s*s1 + k2*(s2-C*s1)
f1' = alpha_f*f0 - beta_f*f1 + k2*(f2-C*f1)
s2' = -k2*(s2-C*s1)
f2' = -k2*(f2-C*f1)
s0 = 1-s1-s2
f0 = 1-f1-f2
}
INITIAL {
: Q10 assumed to be 3
tadj = 3^((celsius-35.5)/10)
evaluate_fct(v,cai)
s1 = alpha_s / (beta_s+alpha_s*(1+C))
s2 = alpha_s*C / (beta_s+alpha_s*(1+C))
s0 = 1-s1-s2
f1 = alpha_f / (beta_f+alpha_f*(1+C))
f2 = alpha_f*C / (beta_f+alpha_f*(1+C))
f0 = 1-f1-f2
tau_c = 1 / (1+C) / k2 : for plotting purposes
}
PROCEDURE evaluate_fct( v(mV), cai(mM)) {
h_inf = 1 / (1+exp((v+68.9)/6.5))
tau_s = exp((v+183.6)/15.24) / tadj
tau_f = exp((v+158.6)/11.2) / (1+exp((v+75)/5.5)) / tadj
alpha_s = h_inf / tau_s
alpha_f = h_inf / tau_f
beta_s = (1-h_inf) / tau_s
beta_f = (1-h_inf) / tau_f
C = cai*cai/(cac*cac)
k2 = 4e-4 * tadj
}
UNITSON