Sleep EEG Phenomena

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In the neurophysiology of sleep two classic EEG phenomena have been established: the spindles or waves between 7 and 14 Hz, also called sleep or sigma spindles, which appear at sleep onset, and the delta waves (1-4 Hz), which are paradigmatic of deeper stages of sleep. Recently, the work of Mircea Steriade and coworkers in Quebec described in animals another very slow oscillation (0.6-1 Hz) that is able to modulate the occurrence of different typical EEG sleep events, such as delta waves, sleep spindles, and even short high-frequency bursts. This very slow oscillation was recently revealed in the human EEG by Peter Acherman and A. Borbely and in the MEG by our group.

The basic membrane events that are responsible for the occurrence of sleep spindles and delta waves were described previously. The sleep spindles are generated in the thalamocortical circuits and result from the interplay between intrinsic membrane properties of the thalamocortical relay neurons (TCR) and the GA-BAergic neurons of the reticular nucleus, and the properties of the circuits to which these neurons belong. It is clear that the spindles are a collective property of the neuronal populations. Experimental evidence has demonstrated that the sleep spindle oscillations are generated in the thalamus since they can be recorded in this brain area after decortication and high brain stem transection. The very slow rhythm (0.6-1 Hz), on the contrary, is generated intracortically since it survives thalamic lesions, but it is disrupted by intracortical lesions. Interestingly, note that the rhyth-micity of the very slow oscillation appears to be reflected in that of the typical K complexes of human EEG during non-REM sleep.

How are these oscillations controlled by modulating systems? It is well-known that sleep spindles are under brain stem control. It is a classic neurophysiological phenomenon that electrical stimulation of the brain stem can block thalamocortical oscillations causing the so-called ''EEG desynchronization,'' as shown in studies by Moruzzi and Magoun. This desynchronization is caused by the activation of cholinergic inputs (Fig. 4) arising from the mesopontine cholinergic nuclei, namely, the pedunculopontine tegmental and the laterodorsal tegmental areas. Indeed, both the reticular nucleus and the TCR neurons receive cholinergic muscarinic synapses. Cholinergic activation of the reticular nucleus neurons elicits hyperpolarization with a K+ conductance increase that is mediated by an increase in a muscarinic-activated potassium current. In contrast, it causes depolarization of TCR neurons.

Figure 4 Basic thalamic networkresponsible for the generation of spindles at 7-14 Hz. A thalamocortical (TH-cx) neuron and two neurons of the reticular nucleus (RE), which are interconnected by mutual inhibitory synapses, are shown. This network is under the modulating influence of mesopontine afferents arising from the cholinergic (Ch5) neurons of the pedunculopontine tegmental nucleus. The stimulation of Ch5 neurons causes depolarization of TH-cx neurons and hyperpolarization of RE neurons. In this way, the occurrence of spindles is blocked (i.e., desynchronization of the corresponding oscillation takes place) (reproduced with permission from Steriade et al., Electroencephalogr. Clin. Neurophysiol. 76, 481-508, 1990).

Figure 4 Basic thalamic networkresponsible for the generation of spindles at 7-14 Hz. A thalamocortical (TH-cx) neuron and two neurons of the reticular nucleus (RE), which are interconnected by mutual inhibitory synapses, are shown. This network is under the modulating influence of mesopontine afferents arising from the cholinergic (Ch5) neurons of the pedunculopontine tegmental nucleus. The stimulation of Ch5 neurons causes depolarization of TH-cx neurons and hyperpolarization of RE neurons. In this way, the occurrence of spindles is blocked (i.e., desynchronization of the corresponding oscillation takes place) (reproduced with permission from Steriade et al., Electroencephalogr. Clin. Neurophysiol. 76, 481-508, 1990).

Furthermore, the reticular nucleus receives inputs from the basal forebrain that may be GABAergic and can also exert a strong inhibition of the reticular neurons leading to the subsequent suppression of spindle oscillations. In addition, monoaminergic inputs from the brain stem, namely those arising at the mesopontine junction (i.e., from the noradrenergic neurons of the locus coeruleus and the serotoninergic neurons of the dorsal raphe nuclei), also modulate the rhythmic activities of the forebrain. These neuronal systems have only a weak thalamic projection but have a diffuse projection to the cortex. Metabotropic glutamate receptors also appear to exert a modulating influence on the activation of thalamic circuits by descending corticothalamic systems.

Because this point is often misunderstood, it is emphasized that slow-wave sleep, characterized by typical EEG delta activity, does not correspond to a state in which cortical neurons are inactive. On the contrary, in this sleep state cortical neurons can display mean rates of firing similar to those during wakefulness and/or REM sleep. Regarding the neuronal firing patterns, the main difference between delta sleep and wakefulness and REM sleep is that in the former the neurons tend to display long bursts of spikes with relatively prolonged interburst periods of silence, whereas in the latter the firing pattern is more continuous. The functional meaning of these peculiar firing pattern of delta sleep has not been determined.

In general, EEG signals covary strongly with different levels of arousal and consciousness. The changes of EEG with increasing levels of anesthesia are typical examples of this property.

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