I read a paper long ago (so there's no chance of my recalling the source!) and one of the takeaways was that in a cockroach one of the neural ganglia basically had a binary "run!" mode that was flipped on instantly if sense nerves very close to it were triggered. So when researchers tapped or blew air on the rears of the roaches the roach in question would sprint away, its powerful legs being efficiently driven at full tilt by this little sprinting circuit without needing any input or interaction from the more complex main brain. Imagine getting used to that effect! "Ahhh! Why am I suddenly running and where am I going to steer this runaway body?"
Humans have that too. Startle response, withdrawing from pain (hot stove), blink response upon incoming object - all these happen without involving the higher brainstem at all. I think some of them barely even connect with the brain.
