A polysomnography report shows recurrent episodes of loss of skeletal muscle tone (except diaphragm and extraocular muscles) with REMs and desynchronised EEG. This sleep stage is characterised physiologically by:

• A Predominant delta waves (0.5–4 Hz) with high amplitude
• B Highest arousal threshold among all sleep stages
• C Active glycine/GABA-mediated inhibition of anterior horn motor neurons causing atonia ✓
• D Predominance of sleep spindles (12–14 Hz) and K-complexes

Explanation

The described stage is REM sleep. Atonia during REM is produced by active glycine- and GABA-mediated post-synaptic inhibition of spinal anterior horn motor neurons, originating from brainstem centres (sublaterodorsal nucleus/ventromedial medulla). This prevents acting out of dreams. The EEG is desynchronised (low-amplitude, mixed-frequency — resembling wakefulness). Delta waves characterise NREM stage 3; sleep spindles are in NREM stage 2; REM has a relatively low arousal threshold (easily wakened).

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.