My understanding is that they don't just differ between species, but they differ substantially between different types within the same individual.

My understanding is extremely limited.

That being said, my impression is that some of the axiis they can vary on include things like : how they respond to some other chemicals ("neurotransmitters"? not sure if that's the right word) in their environment, whethether they produce/emit those other chemicals, the types of conditions under which they fire (e.g. from how much they "leak" activation over time, counteracting the signals they get sent from others), things like uh, I think(?) whether they tend to fire at a particular rate by default which can be increased or decreased by signals they receive(?) vs whether they by default do not fire unless they receive enough other signals,

I'm unsure of basically all the things I just listed, but my impression is that there are lots of ways they can differ, and they definitely aren't all the same.