During REM sleep, there is a marked reduction in the tone of muscles in:

Tone of most voluntary muscles is reduced temporarily. The tone of the neck muscles is reduced markedly during REM sleep. The loss of tone is due to an active inhibition of motor neurons mediated by a group of neurons in the locus ceruleus in the brain stem. The reduction of tone is NOT observed in: Muscles of respiration, Extraocular muscles (which contract rhythmically to produce the rapid eye movements), and Muscles of middle ear (which protect the inner ear).