Activation of the brainstem precedes and outlasts the K-complex in humans

A K-complex (KC) in the electroencephalographs (EEGs) indicates a moderate depth of slow-wave sleep (SWS) in humans and animals. The cortical activities are upregulated during the periods between the KCs ("up state"), and it is proposed that temporarily stored memories during wakeful periods will be consolidated during these periods. Although the underlying mechanism for KCs is proposed to be in the cortex itself, the involvement of the brainstem has not been explored. Here we investigate the excitability changes of the brainstem preceding, during, and after KCs in humans. We simultaneously recorded brainstem auditory evoked potentials (BAEPs) with EEGs, and sequentially analyzed BAEPs around the KCs. The results showed a transient activation of the ventral brainstem preceding the KC and a sustained activation of the dorsal brainstem outlasting the KC. Thus, it is suggested that KCs are triggered by the activation of the brainstem and that the "up state" is maintained by the sustained activation of the brainstem.