It's 4. Biological neurons are much more complex than they appear: they're sensitive to many, many different chemical signals, they have internal structure that

It's 4. Biological neurons are much more complex than they appear: they're sensitive to many, many different chemical signals, they have internal structure that's non-trivial, and they can have not just excitatory/inhibitory outputs, but modulatory as well. It's hard to know how to even start extracting subcomponents.