What prevents the body from acting out a dream?
During REM sleep, skeletal muscle movements are inhibited by neural structures in the brain stem so that dreamed movements are not acted out which could lead to severe injuries (Hobson et al., 2000; Jouvet, 1965; Pace-Schott, 2003).
What parts of the sleeping body are not paralyzed?
However, the sleeping body is not completely paralyzed. Obviously, life preserving muscles like the heart muscle are still active, but of special interest in this thesis are movements of the eyes. According to the Scanning Hypothesis by Ladd (1892), eye movements show the same patterns during dreams as in wake life. There seems to be evidence for this hypothesis; it is however still unclear, how strong the relationship between dream eye movements and real eye movements is (Schredl, 2006; Arnulf, 2011).
Apart from eye movement, there are also other correlations between dreamed actions and physiological measurements of the sleeping body: for brain activity measured by EEG (e.g., Hong et al., 1996), limb twitches measured by electromyography (e.g., Gardner et al., 1975), and cardiovascular parameters (e.g., Baust & Engel, 1971).