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This issue includes an article by Amzica and Steriade1 that makes an interesting and important observation: K-complexes, one of the hallmarks of nonrapid eye movement (NREM) sleep are generated by the cortex by a synchronized cortical network that periodically excites or inhibits cortical neurons. These waveforms are generated by a slow (<1 Hz) oscillation due to a sequence of depolarizing and hyperpolarizing events that occur synchronously in cortical neurons. This finding, made possible by sophisticated electrophysiologic monitoring techniques, has important clinical implications, enhancing our understanding of important functions and activities of the human brain: sleep, dreaming, and consciousness.
The immediate importance of this finding is that it solves the long-standing mystery of the origin of K-complexes-they arise from the cortex. In the past, K-complexes have been thought to emanate from the thalamus or cortex and were variably viewed as signs of arousal or considered to serve as a "defensive" sleep-protective function.2 More importantly, this finding identifies the presence of yet another spontaneous oscillatory rhythm present during NREM sleep, further establishing the existence of oscillatory activity during all states of being-wakefulness, NREM sleep, and REM sleep.
There are three major oscillatory NREM sleep rhythms, and another is present during both wakefulness and REM sleep. The NREM sleep rhythms are (1) slow (<1 Hz), (2) delta (1 to 4 Hz), and (3) spindle (7 to 14 Hz) frequencies.3 The slow and the delta rhythms are generated by the cortex, and …
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