Apparently it is a myth that muscles are “paralyzed” by anaesthesia. Tone remains! A resting muscle shrinks 20% when cut. “There is a constant battle to relax the muscles during some procedures.” Doubtless I know some people who will now say, “I could have told you that. If you’d asked.” Too bad I didn’t ask!
The tone is mediated by the CNS. Some believe there is some intrinsic regulation of tone — that is, the muscle sets its own tone — but Dr. Steven Levin directly refutes this with some pretty sound logic: “Curare works at the neuro-muscular synapse, so it is the CNS that maintains the muscle tone, including the resting muscle tone (RMT). In my many years of doing surgery, I have never cut a muscle that did not retract unless it was curare-ized (and even then there is some contraction), so it is a primitive function, maybe some of it spinal, present even in deeply anesthetized creatures.” That’s from this page, a bit hard on the eyes and heavy reading, but neat stuff.