% Source: Sherman, Keizer, and Rinzel, Domain model for 
% calcium-inactivation of calcium channels at low 
% channel density, Biophysical Journal 58:985-995, 1990.
% (implemented for xpp by Gerda de Vries, 12/21/95)

%
% Initial values of the variables.
%
init v=-64, pc=1, po=0, n=0.0, cai=0.9

%
% Values of the model parameters.
%
params cm=5310, vcell=1150, kca=0.03, f=0.001
params r=6.5, farad=96.487
%%%%%% for ica:
params gca=7, vm=-5, sm=10, taumbar=1.25
params kminus=0.018, kplus=0.036, a=-0.0007
params cao=10, cain=0, p=14, nca=200, ratio=13.35
%%%%%% for ik:
params gk=2500, vk=-75, vn=-10, sn=5.6
params c=60, lambda=2.4, vbar=-75, sa=65, sb=20
%%%%%% for ikca:
params gkca=30000, kd=100

%
% Activation functions.
%
minf(v)   = 1/(1+exp((vm-v)/sm))
alpham(v) = minf(v)/taumbar
betam(v)  = (1-minf(v))/taumbar
ninf(v)   = 1/(1+exp((vn-v)/sn))
taun(v)   = c/lambda/(exp((v-vbar)/sa)+exp((vbar-v)/sb))

%
% Currents.
%
i(v)        = gca*p*v/ratio*(cao-cain*exp(v/ratio))/(1-exp(v/ratio))
ica(v,po)   = nca*po*i(v)
ik(v,n)     = gk*n*(v-vk)
ikca(v,cai) = gkca*cai*(v-vk)/(kd+cai)

%
% Differential equations.
%
v'   = -(ica(v,po)+ik(v,n)+ikca(v,cai))/cm
pc'  = -alpham(v)*pc + betam(v)*po
po'  = kminus*(1-po-pc) - kplus*a*i(v)*po + alpham(v)*pc - betam(v)*po
n'   = (ninf(v)-n)/taun(v)
cai' = f*(-ica(v,po)/2/vcell/farad - kca*cai)

%
% Check the instantaneous fraction of calcium channels 
% that are inactivated, pb.  
%
aux pb = 1-po-pc

done