Final answer:
During REM sleep, specific brainstem neurons release serotonin and norepinephrine as neurotransmitters.
Step-by-step explanation:
During REM sleep, there is reduced activity in specific brainstem neurons. These neurons release serotonin and norepinephrine as neurotransmitters. Serotonin and norepinephrine are both involved in regulating sleep-wake cycles, mood, and arousal.