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# Entering an equation

Here we will enter in a differential equation and use AUTO to compute the bifurcation diagram. The equations that we want to study is the famous Hodgkin-Huxley model:

The first thing you should do to analyze this is to create an ODE file which is a text file that XPPAUT use to solve differential equations. Here is the file

#  hhh.ode
par i=0
par ena=50  ek=-77  el=-54.4  gna=120  gk=36  gl=0.3  c=1  phi=1
am(v)=phi*.1*(v+40)/(1-exp(-(v+40)/10))
bm(v)=phi*4*exp(-(v+65)/18)
ah(v)=phi*.07*exp(-(v+65)/20)
bh(v)=phi*1/(1+exp(-(v+35)/10))
an(v)=phi*.01*(v+55)/(1-exp(-(v+55)/10))
bn(v)=phi*.125*exp(-(v+65)/80)
v'=(I - gna*h*m^3*(v-ena)-gk*n^4*(v-ek)-gl*(v-el))/c
m'=am(v)*(1-m)-bm(v)*m
h'=ah(v)*(1-h)-bh(v)*h
n'=an(v)*(1-n)-bn(v)*n
init v=-65  m=.05  h=0.6  n=.317
done

The file is typed in pretty much the same way you would if you were writing the equations; XPP is too stupid to recognize spaces for multiplication, so you have to put the *'' sign between quantities. Functions such as am(V) are defined in the obvious fashion. Parameters are declared with the par statement. They can be separated by spaces or commas. The ='' sign, the parameter name, and its starting value must have NO spaces between them.

THIS IS WRONG:

par gl = .3


THIS IS RIGHT:

par gl=.3


Initial data can be defined with the init statement. Each file must end with a done statement. Any line that starts with a # is a comment and ignored.

G. Bard Ermentrout
1999-09-15