plot piecewise 3D function

Greetings. I need to plot a piecewise 3D function. I'm familiar with the 3D plot commands, but I seem to be having some difficulty using the proper syntax to define the function.

The function itself is:

n(a,t) = B(t-a)*exp(-int(mu(i),i=0..a)) if a <= t

n(a,t) = n(a-t,0)*exp(-int(mu(i),i=a-t..t)) if a > t

n(0,t) = int(B(a)n(a,t),a=0..inf)

This is the solution to the simple McKendrick von Foerster PDE.

My question is simply: how do I input the above piecewise function to properly define n in Matlab?