In this article parallels are drawn between sleep and anaesthesia. Both can be described as 'behavioural states' in which stimulation and inhibition of certain neuronal groups will give rise to specific psychological states. The neuronal mechanisms leading to these states are reviewed and compared. Sensory information flow through the thalamus is reduced in sleep and anaesthesia, the thalamic gating being controlled by other areas of the brain including the sensory cortex. Thus a feedback loop may exist with the brain determining its own level of arousal. Anaesthesia increases gating at the thalamus and thus specifically reduces arousal. Consciousness is a specific attribute of the brain and is not required for the processing of sensory stimuli and learning, both of which have been shown to occur during sleep and anaesthesia. The implications of these findings for monitoring awareness during anaesthesia are discussed.
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