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Whether it is mechanical (trauma), chemical (anaesthesia) or electrical - which part of the brain is shut down to cause loss of consciousness?

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I am not sure but I think it is reticular formation. –  biogirl Sep 12 '13 at 16:53
    
This is a very tricky question, since it is difficult to measure consciousness. –  Memming Sep 12 '13 at 16:57
    
Related: biology.stackexchange.com/questions/10351/… –  Amory Sep 12 '13 at 17:06
    
I agree with biochick, it's reticular formation The reticular formation has projections to the thalamus and cerebral cortex that allow it to exert some control over which sensory signals reach the cerebrum and come to our conscious attention. It plays a central role in states of consciousness like alertness and sleep. Injury to the reticular formation can result in irreversible coma. en.wikipedia.org/wiki/Reticular_formation#Functions BUt again I guess there are a lot of other control mechanisms in the brain on the cellular and bigger levels. –  Derfder Sep 12 '13 at 17:57

2 Answers 2

There is no widely-accepted neurological structure that mediates 'consciousness.' Even if some structures have been shown to be necessary for consciousness, they have not been shown to be sufficient. This is true with anesthetic mechanisms as well -- their ability to paralyze and block pain signals is fairly well-understood, but the mechanism of loss-of-consciousness is still unknown.

Still, 'consciousness' has to be there, somewhere between being awake and being dead, states which anesthetics can readily bridge (review):

Nevertheless, at some level of anesthesia between behavioral unresponsiveness and the induction of a flat EEG [indicating the cessation of the brain’s electrical activity, one of the criteria for brain death (22)], consciousness must vanish.

Later in the same review:

The evidence from anesthesia and sleep states (Fig. 2–3) converges to suggest that loss of consciousness is associated with a breakdown of cortical connectivity and thus of integration, or with a collapse of the repertoire of cortical activity patterns and thus of information (Fig. 2). Why should this be the case? A recent theory suggests a principled reason: information and integration may be the very essence of consciousness (52).

This is consistent with my own take. Consciousness itself is the subjective experience of 'brain,' so it can't be lost, just poorly integrated.

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@Rayan, very useful reference (Consciousness and Anesthesia). Thanks –  Ram Manohar M Sep 13 '13 at 3:23

Two structures are required for consciousness: the brain stem reticular activating system (RAS) and one cerebral hemisphere. e.g. a lesion in the RAS or bicerebral injury will induce loss of consciousness.

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