Mathematically modelling neurons, basically. We were using some horrendous sketchy PDE solver that had by far the most hostile UI of any program I've ever used, and that's quite an achievement.

It was taught by this fellow, who is one of the loveliest folks I've ever met in academia: https://uwaterloo.ca/applied-mathematics/profiles/brian-inga...

Edit: Looks like the course is still around 20 years later, but has expanded quite a bit in scope since I took it. https://uwaterloo.ca/applied-mathematics/current-undergradua...