You can't raise a finger without your brain directing muscle cells, and scientists have figured out another reason that usually works so well.
A neuron sends a message, or neurotransmitter, to the muscle cell to tell it what to do. To get the message, the receiving cell must have a receptor. Oddly, the unstable protein rapsyn is responsible for anchoring the receptor so it's properly positioned to catch the message.
Medical College of Georgia scientists have found what keeps rapsyn in proper conformation.