K-complex, a reactive EEG graphoelement of NREM sleep:: an old chap in a new garment

This review summarises data gathered on the KC phenomenon over the past 70 yr. The following issues are discussed: definitions, morphology and topography of KC, the regular participation in NREM steep, elicitability features of evoked KC, autonomic and motor concomitants, relationship of KC with information processing during NREM steep, relationship of KC and deltas of NREM steep, and relationship of KC with steep cyclicity. KC is a complex multifunctional phenomenon of the sleeping brain involved in information processing and defence against the arousal effect of sensory stimuli. To put the old chap in a new garment, the relationship of KC with synchronisation-type and desynchronisation-type microarousals, and the 'cyclic alternating pattern', wilt be discussed, with an emphasis on the steep-protecting role of KC and synchronisation-type answers in sleep regulation executed by phasic events. Lastly, the role of KC providing gating functions in idiopathic generalized epilepsies and other, different, steep disorders are characterised. A 'theoretical epilogue' is appended to show some system theoretical and regulational aspects. (C) 2005 Elsevier Ltd. All rights reserved.