Almost every other cell in the worm can be simulated with known biophysics. But we don't have a clue how any individual nematode neuron actually works. I don't have the link but there are a few teams in China working on visualizing brain activity in living C. elegans, but it's difficult to get good measurements without affecting the behavior of the worm (e.g. reacting to the dye).
Summary (my paraphrasing):
They partially figured out how two neurons (AVA, AVB) control forward and backward movement, previous theories assumed one neuron controlled forward and one controlled backward, but that didn't correctly model actual movement.
They found that AVA+AVB combine in a complex mechanism with two different signaling/control methods acting at different timescales to produce a graded shifting between forward+backward when switching directions, as opposed to an on/off type switch (that previous models used but didn't match actual movements).
Interesting learnings from this paper (at least for me):
1-Most neurons in worm are non-spiking (I had no idea, I've read about this stuff a lot and wasn't aware)
2-Non-spiking neurons can have multiple resting states at different voltages
3-Neurons AVA and AVB are different, they each have different resting state characteristics and respond differently to inputs