This is page is an unedited draft from Marcello’s manuscript. It needs citations, figures, cross-links, and other work to be done. It will take a while!
It is here so you can see the general gist of Marcello’s argument across the whole essay.
Feel free to visit from time to time for updates.
In the living brain in addition to the massive inputs arriving to the cerebral cortex via the thalamus, from the basal ganglia and from the cerebellum, the neocortex receives also inputs from more primordial centers in the brain stem. The early ideas of inputs from subcortical structures activate the cortex were proposed by Moruzzi and Magoun with their suggested ascending reticular activating system (ARAS) from the brain stem. The idea of the ARAS as an organised hierarchy of the cerebral “waking centres” distributed along the entire cerebral axis is now well established.
Current evidence indicates that awakening from sleep is preceded by a surge in the firing of wake-promoting neurons not only in the brainstem, but also in basal forebrain and hypothalamus.
In early studies, a significant change of brain activity was detected in the transition from regular slow waves, travelling across the cortex during deep sleep, to awakeness with associated ‘desynchronisation’ the slow brain waves. The apparent lack of synchrony, with the disappearance of the of slow waves across the activated neocortex in awake states is likely to be associated with a higher degree of synchrony between smaller areas of the neocortex at higher frequency (gamma waves at ~40Hz). The term desynchronisation should therefore be replaced by ‘activation’, a term that was already proposed by Moruzzi and Magoun (1949).
During sleep, the REM phase is characterised by interruption of slow waves replaced by faster frequencies, similar to those observed in awake ‘desynchronised’ states. The underlying mechanism involves activation of ponto-geniculate-occipital pathways and is associated with dreaming.
Awake animals have depolarised thalamocortical cells that operate strictly in tonic mode and thus reliably relay information to cortex. However, there are no studies yet that describe fuller spatio-temporal maps of transition from sleep to awakeness. In most studies there have been insufficient numbers of recording points to construct proper spatio-temporal maps.
A recent study published by Chen Song, a brain researcher at Cardiff University who participated in the study while a postdoc at the University of Wisconsin-Madison, found that the frontal cortical regions of the brain may be among the first to initiate the waking up from sleep.
Recovery from anaesthetic-induced unconsciousness appears to require restoration of bidirectional communication across the brainstem, thalamus, and cortex.
Upon discontinuation of the anaesthetic, brainstem arousal centres are reactivated and stimulate the thalamic reticular nuclei, which in turn excite the thalamocortical projections reactivating the cortex contributing to restoration of pyramidal neuron function in the neocortex.
The fact that cholinergic stimulation of the prefrontal cortex (PFC), but not parietal cortex, can induce wakefulness even during general anaesthesia, suggests that the medial PFC can promotes arousal.