A 62-year-old man presents to the sleep clinic with a 3-year history of violent dream enactment during sleep—his wife reports he punches and kicks during nightmares, sometimes injuring himself. Polysomnography shows loss of normal muscle atonia during the stage marked **C** in the diagram. Which of the following best explains the pathophysiology of his condition?

Explanation

## Why "Loss of pontine inhibition of spinal motor neurons during REM sleep, resulting in REM Sleep Behavior Disorder (RBD)" is right REM sleep (stage **C**) is normally characterized by skeletal muscle atonia—a state of paralysis that prevents enactment of dreams. This atonia is maintained by active inhibition of spinal motor neurons via pontine cholinergic and GABAergic pathways. When this inhibitory mechanism fails, patients lose the protective atonia and act out their dreams violently—the defining feature of REM Sleep Behavior Disorder (RBD). The polysomnographic finding of preserved or increased muscle tone during REM (loss of atonia) is pathognomonic for RBD. Guyton & Hall (14e, Ch 60) and Harrison (21e, Ch 27) both emphasize that REM atonia is an active, pontine-mediated process; its loss is the core pathophysiology of RBD. ## Why each distractor is wrong - **Excessive K-complex generation during Stage 2 NREM, leading to motor disinhibition**: K-complexes are normal EEG features of Stage 2 NREM (stage **A**), not REM. They are not associated with motor disinhibition or dream enactment. This confuses NREM Stage 2 physiology with REM pathology. - **Failure of thalamocortical synchronization during slow-wave sleep, causing arousal-related movements**: Slow-wave sleep (stage **B**) involves high-amplitude delta waves and thalamocortical synchronization. This stage is not associated with RBD or dream enactment; RBD is specific to REM sleep disruption. - **Increased theta wave activity during Stage N1, preventing normal sleep-to-wake transition**: Stage N1 (stage **D**) is the drowsy transition stage characterized by theta waves. Theta activity does not cause RBD; RBD is a REM-specific disorder involving loss of atonia, not theta dysregulation. **High-Yield:** REM Sleep Behavior Disorder = loss of REM atonia due to pontine inhibitory failure; strongly associated with α-synucleinopathies (Parkinson's, Lewy body dementia, MSA) and may precede motor symptoms by years. Treat with clonazepam or melatonin. [cite: Guyton & Hall 14e Ch 60; Harrison 21e Ch 27]