Which disorder is characterized by the absence of nerve pathways that prevent muscle movement during REM sleep?

The disorder characterized by the absence of nerve pathways that prevent muscle movement during REM sleep is REM Sleep Behavior Disorder. This condition occurs when the usual muscle paralysis that occurs during REM sleep is incomplete or absent, allowing individuals to physically act out their dreams. As a result, they may exhibit movements such as kicking, punching, or even getting out of bed, which can be potentially harmful to themselves or their bed partner.

In contrast, sleepwalking disorder involves walking or performing complex behaviors while in a sleep state, but this does not specifically relate to the mechanisms of REM sleep and the absence of muscle atonia. Night terrors occur during deep non-REM sleep and involve extreme fear or panic, without the associated muscle movements characteristic of REM sleep. Sleep paralysis is a temporary inability to move or speak while falling asleep or waking up, which can happen during transitions between sleep stages but does not involve acting out dreams. Thus, REM Sleep Behavior Disorder is the one that fits the criteria of lacking the protective mechanism that normally inhibits movement during REM sleep.