What is it about the nature of slow-wave sleep that makes it difficult to awaken the sleeper? In humans, delta wave sleep ("slow wave" sleep, stages III and IV by AASM criteria) is more common in children and becomes only a small percentage of total sleep time in adults.

Cortical EEG waves have notably low frequencies (sometimes as low as 1Hz) during these stages.  I've always modeled this in my own understanding as the brain "polling" the environment at a lower rate, but why do stimuli that coincide directly with the peaks of the cortical waves still not seem to arouse or awaken the person sleeping?  Clearly there are some physiological "shunts" occurring in the brainstem and cortex to dampen the effects of external stimuli, but why see any activity at all, then?
Slow EEG waves reflect a slow oscillation in cortical neurons, between a depolarized state (UP-State) and hyper polarized state (DOWN-state; Steriade et al., 1993a, 1993b). During the down-state, neurons are disfacilitated (they can't really be activated by external stimuli or depolarized; Contreras et al., 1996). As the brain progress into deep sleep (lots of large-amplitude slow waves), not only cortical but also thalamic neurons becomes more and more hyper polarized (Amzica &amp; Steriade, 2001). Steriade (2006) proposed that this progressive hyper polarization that is occurring during deep sleep might protect the brain from interference from the external world and facilitate memory consolidation, because the brain becomes "non-responsive" to the external environment. Indeed, this thalamo-cortical hyper polarization occuring during the down-state might "gate" the brain from external influence.

Recent neuroimaging studies in humans (Schabus et al., 2012) indeed showed that during the DOWN state, there was no cortical response to auditory stimuli in fMRI.  Thus we think that those waves, either in N2 (K-Complex) or N3 (SW), protect the sleep brain from external influence.

Sources :


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