Thalamus
The thalamus is the final common path for most of the information reaching the cortex from receptors, apart from olfaction, and from the brainstem. The thalamus modifies these inputs, in effect acting as a gate determining which of the stimuli reach the cortex and in what form, although thalamic activity is itself regulated by the cerebral cortex. As the homeostatic drive to sleep wanes, the ability of the thalamus to block transmission to the cortex weakens and arousals occur.

The thalamic reticular nucleus is especially important in interacting with the cerebral cortex and determining the state of arousal. It has groups of excitatory neurones, which release glutamate, and inhibitory neurones, which release GABA at their synapses.

Intrathalamic relay neurones modify the thalamic activity whereas other neurones form the thalamocortical projection fibres. There is reciprocal activity between the thalamus and cerebral cortex through these fibres and thalamic excitation is followed by inhibition by corticothalamic impulses. Their timing determines the frequency of the bursts of thalamic stimulation of the cortex which varies from 0.6 to 1 Hz _[22].

The thalamus filters and modifies activity from the ascending reticular activating system and other systems passing through the midbrain. It modifies midbrain spindle activity and through its widespread projections to the cerebral cortex is able to integrate and synchronize cortical activity. Synchronization of cortical activity initiates and maintains the state of NREM sleep. It effectively disconnects the cerebral cortex from brainstem and other influences, although this is reversible through the mechanism of arousal.

The thalamus not only protects the cortex from other incoming electrical impulses, so that it is deafferented or sensorily deprived, but also, through its GABA-secreting neurones, it inhibits brainstem centres that are capable of leading to an arousal. It also influences REM sleep through its projections to the LDT/PPT.

Although the thalamic arousal inhibiting mechanism responsible for NREM sleep has a global projection to the cerebral cortex there are also specific thalamic effects during NREM sleep, for instance on primary sensory areas _[19].

Cerebral cortex
The cerebral cortex is an extensive heterogeneous structure, which has correspondingly complex effects on the initiation, maintenance and cessation of sleep.

The cerebral cortex is responsible for most aspects of the adaptive sleep–wake drive, and it enables the homeostatic and circadian influences on sleep to be more precisely adjusted to optimize responses to environmental changes. Cortical synchronization during NREM sleep is essential for the maintenance of this state, but its activity during REM sleep is more similar to that during wakefulness than to that during NREM sleep.

References