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Programming Euler's Method into Mathematica

I am trying to use Euler's Update Method to Numerically Integrate a
Function (I know how to use NDSolve for this case, but I want to
demonstrate an approximation method as well).  For those familiar with
neuroscience, I am trying to model the Hodgkin Huxley Equations.  I
have four ordinary differential equations (ODE) that I have written in
discrete form  using Euler's Method.  I am trying to solve for one of
these discrete equations, V, which is a function of the other three
differential equations (n, m, and h). All four equations are also a
function of time. Essentially, the command for NDSolve for this case
is in the following form: NDSolve[{ODE1, ODE2, ODE3, ODE4,
V[0]==0,n[0]==1, m[0]==2, h[0]==3},{V,n,m,h},{t,0,50},MaxSteps->1000]

Instead of the above ODE input, I want to use Euler's Method. (i.e. V(t
+delta_t)=V(t)+delta_t("stuff").  Now, the code below works for a
function of x and y. MY QUESTION IS: how would I change this code to
apply to my situation in which I want to solve for V[t] but it is a
function of n[t], m[t], and h[t].

I would appreciate any hints/clues/suggested programs.  I know that
Mathematica has a built-in "Explicit Euler" Method, but I would like
to develop the program myself. I believe I am making a small mistake
that is preventing my success.

Thank you,

The following program code works for a function f(x,y):

Block[{	xold=x0,


      Do[ xnew=xold+h;


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