Sleepwalking: survival mechanism gone awry

‘Recent research from Stanford University shows that up to 4 per cent of adults [sleepwalk]. In fact, sleepwalking is on the rise, in part due to increased use of pharmacologically based sleep aids – notably Ambien.

Often, the episodes are harmless. Sometimes, of course, sleepwalking is dangerous. Somnambulists are in an irrational state during which they could harm themselves or others…

Why do some enter into such a potentially harmful state during sleep? One answer comes from studies suggesting that ‘sleepwalking’ might not be an appropriate term for what is going on; rather, primitive brain regions involved in emotional response (in the limbic system) and complex motor activity (within the cortex) remain in ‘active’ states that are difficult to distinguish from wakefulness. Such activity is characterised by ‘alpha wave’ patterns detected during electroencephalogram (EEG) recordings. At the same time, regions in the frontal cortex and hippocampus that control rationality and memory remain essentially dormant and unable to carry out their typical functions, manifesting a ‘delta wave’ pattern seen during classic sleep. It’s as though sleepwalking results when the brain doesn’t completely transition from sleep to wakefulness – it’s essentially stuck in a sleep-wake limbo.

‘The rational part of the brain is in a sleep-like state and does not exert its normal control over the limbic system and the motor system,’ explains the Italian neuroscientist Lino Nobili, a sleep researcher at Niguarda Hospital in Milan. ‘So behaviour is regulated by a kind of archaic survival system like the one that is activated during fight-or-flight.’..

Scientists now agree that bouts of localised wakeful-like activity in motor-related areas and the limbic system can occur without concurrent sleepwalking. In fact, these areas have been shown to have low arousal thresholds for activation. Surprisingly, despite their association with sleepwalking, these low thresholds have been considered an adaptive trait – a boon to survival. Throughout most of our extensive ancestry, this trait may have been selected for its survival value.

‘During sleep, we can have an activation of the motor system, so although you are sleeping and not moving, the motor cortex can be in a wake-like state – ready to go,’ explains Nobili, who led the team that conducted the work. ‘If something really goes wrong and endangers you, you don’t need your frontal lobe’s rationality to escape. You need a motor system that is ready.’ In sleepwalking, however, this adaptive system has gone awry. ‘An external trigger that would normally produce a small arousal triggers a full-blown episode.’ …’

Source: Aeon Ideas

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