Delta frequency 14 Hz EEG activity during NREM sleep

The cellular basis of delta waves (0.5-4 Hz) originating in thalamocortical neurons is that a hyperpolar-ized membrane potential in cortically projecting thalamic neurons permits the occurrence of delta waves in thalamocortical circuits. In both the thalamus and cortex, a phasic input to the hyperpolarized membrane causes a calcium-mediated depolarization (low threshold spike, LTS) on which a burst of sodium-mediated spikes occur (low threshold burst). The slow time course of delta waves is determined by the time course of the LTS and the subsequent hyper-polarization to re-enable another LTS. Any factors persistently depolarizing the membrane in either thal-amic or cortical neurons will block delta waves through inactivation of the calcium channels mediating the LTS, and these are the wake-active systems illustrated in Fig. 2.1b. Cholinergic input from BF to the cortex, and from LDT/PPT to the thalamus, is thought to be especially important in arresting delta waves. Also, brainstem noradrenergic and serotoner-gic projections may disrupt delta activity in waking, although they are inactive during REM sleep. Thus, delta waves during sleep may be seen to represent thalamocortical oscillations occurring in the absence of activating inputs. For a more detailed review of delta frequency EEG activity during NREM sleep, the reader is referred to [11].

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