TITLE CaGk
: Calcium activated mAHP K channel.
: From Moczydlowski and Latorre (1983) J. Gen. Physiol. 82
UNITS {
(molar) = (1/liter)
}
UNITS {
(mV) = (millivolt)
(mA) = (milliamp)
(mM) = (millimolar)
}
INDEPENDENT {t FROM 0 TO 1 WITH 100 (ms)}
NEURON {
SUFFIX mykca
USEION ca READ cai
USEION k READ ek WRITE ik
RANGE gkbar, ik
GLOBAL oinf, tau
}
UNITS {
FARADAY = (faraday) (kilocoulombs)
R = 8.313424 (joule/degC)
}
PARAMETER {
v (mV)
dt (ms)
ek (mV)
celsius = 20 (degC)
gkbar = 0.01 (mho/cm2) : Maximum Permeability
cai = 1e-3 (mM)
d1 = 0.84
d2 = 1.0
k1 = 0.18 (mM)
k2 = 0.011 (mM)
bbar = 0.28 (/ms)
abar = 0.48 (/ms)
}
COMMENT
the preceding two numbers were switched on 8/19/92 in response to a bug
report by Bartlett Mel. In the paper the kinetic scheme is
C <-> CCa (K1)
CCa <-> OCa (beta2,alpha2)
OCa <-> OCa2 (K4)
In this model abar = beta2 and bbar = alpha2 and K4 comes from d2 and k2
I was forcing things into a nomenclature where alpha is the rate from
closed to open. Unfortunately I didn't switch the numbers.
ENDCOMMENT
ASSIGNED {
ik (mA/cm2)
oinf
tau (ms)
}
STATE { o } : fraction of open channels
BREAKPOINT {
SOLVE state
ik = gkbar*o*(v - ek) : potassium current induced by this channel
}
LOCAL fac
:if state_cagk is called from hoc, garbage or segmentation violation will
:result because range variables won't have correct pointer. This is because
:only BREAKPOINT sets up the correct pointers to range variables.
PROCEDURE state() { : exact when v held constant; integrates over dt step
rate(v, cai)
o = o + fac*(oinf - o)
VERBATIM
return 0;
ENDVERBATIM
}
INITIAL { : initialize the following parameter using rate()
rate(v, cai)
o = oinf
}
FUNCTION alp(v (mV), ca (mM)) (1/ms) { :callable from hoc
alp = abar/(1 + exp1(k1,d1,v)/ca)
}
FUNCTION bet(v (mV), ca (mM)) (1/ms) { :callable from hoc
bet = bbar/(1 + ca/exp1(k2,d2,v))
}
FUNCTION exp1(k (mM), d, v (mV)) (mM) { :callable from hoc
exp1 = k*exp(-2*d*FARADAY*v/R/(273.15 + celsius))
}
PROCEDURE rate(v (mV), ca (mM)) { :callable from hoc
LOCAL a
a = alp(v,ca)
tau = 1/(a + bet(v, ca)) : estimation of activation tau
oinf = a*tau : estimation of activation steady state value
fac = (1 - exp(-dt/tau))
}