Pain and sleep: from reaction to action

Sleep disruption by painful stimuli is frequently observed both in clinical and experimental conditions. Nociceptive stimuli produce significantly more arousals (30% of stimuli) than non-nociceptive ones. However, even if they do not interrupt sleep, they can trigger a variety of other reactions. Reflex behaviours in response to nociceptive stimuli can be observed during all sleep stages, and are more likely to occur in association with an arousal than alone. Cardiac activation represents a robust sympathetically driven effect preserved whatever the state of vigilance, even if its magnitude can be modulated by a concomitant cortical arousal. Not withstanding these reactions, incorporation of nociceptive stimuli into dream content remains limited. At cortical level, laser-evoked potential studies demonstrate that the processing of nociceptive stimulations is partly conserved during all sleep stages. Furthermore, when nociceptive stimulations interrupt sleep, the cortical response presents a late component suggesting that the stimulation has to be cognitively processed in order to produce a subsequent arousal. More complex reactions to nociceptive stimulations were occasionally reported. In this context, an epileptic patient with intracerebral electrodes implanted for therapeutic purposes allowed us extending these observations. This patient exhibited finger lifts in response to stimulations delivered during paradoxical (REM) sleep. This motor reaction was previously used during wakefulness to indicate that the stimulation had been perceived. When these finger lifts occurred a systematic re-activation of the anterior cingulate preceded each movement. This observation suggests that during PS, not only the processing of sensory inputs but also the capacity for the sleeper to intentionally indicate his perception could be preserved under particular circumstances.