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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    $\begingroup$ I am not sure but I think it is reticular formation. $\endgroup$ – biogirl Sep 12 '13 at 16:53
  • $\begingroup$ This is a very tricky question, since it is difficult to measure consciousness. $\endgroup$ – Memming Sep 12 '13 at 16:57
  • $\begingroup$ Related: biology.stackexchange.com/questions/10351/… $\endgroup$ – Amory Sep 12 '13 at 17:06
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    $\begingroup$ 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. $\endgroup$ – Derfder Sep 12 '13 at 17:57
  • $\begingroup$ Do u regard speep as non-conscious state? $\endgroup$ – Anixx Oct 28 '14 at 17:12

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.

  • $\begingroup$ @Rayan, very useful reference (Consciousness and Anesthesia). Thanks $\endgroup$ – Ram Manohar M Sep 13 '13 at 3:23
  • $\begingroup$ "somewhere between being awake and being dead". I think that the gap between the 'dream' and the 'sleep without dreams' is narrower, so the study of sleep and dreams may be closer to the answer than the use of anesthetics. $\endgroup$ – Rodrigo Aug 9 '16 at 5:06

Consciousness is a difficult term to grasp in a short answer, and the question is as interesting as it is challenging. The reticular formation as mentioned by others before me is crucial, as the thalamus is the gateway of information to and from the brain. Therefore, subcortical structures such as the thalamus, when 'shut down', induce unconsciousness. Mircea Steriade (2005) has an excellent review on this and he is a leader in the field the field of sleep and absence epilepsy, thalamo-cortical interactions and consciousness. He describes there the cortical responses seen in the EEG during sleep: the sleep spindles, or related spike-and-wave discharges (SWDs) seen in the EEG during absence epileptic seizures, where patients temporarily loose consciousness. Sleep spindles and SWDs are highly synchronized responses that basically block out sensory input and cortical output. The blocking of sensory input and blocking of output to the body may be regarded as a loss of consciousness, as one is not able to sense the environment or undertake action in response to environmental stimuli (e.g., Kostopoulus (2000)).

However, since the question is what part of the brain needs to shut down, it is technically the neocortex, as the neocortex holds the key to being aware of one's surroundings, i.e., processing of sensory input as well as driving motor neurons to move, talk and respond to stimuli (see wiki page on neocortex). So the end organ mainly associated with consciousness is the neocortex - as described in a review paper by Nobel laureate sir Francis Crick (1990). The authors also indicate the thalamus and other structures as possible sites involved, but principally it is the neocortex. I hope this helps you a bit further.

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    $\begingroup$ It would be great if yo added some references to support your response! As it stands this is more similar to a comment! $\endgroup$ – Bez Oct 28 '14 at 19:55
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    $\begingroup$ This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post - you can always comment on your own posts, and once you have sufficient reputation you will be able to comment on any post. $\endgroup$ – Bez Oct 28 '14 at 19:55
  • $\begingroup$ Hi Bez, I am a newbie in Stack Exchange. I will try to improve my answers! $\endgroup$ – AliceD Oct 28 '14 at 20:40
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    $\begingroup$ Hello new Chris and welcome to the Biology StackExchange. Yes, that would be fantastic! It simply requires some references added, which are supportive of the response :) $\endgroup$ – Bez Oct 28 '14 at 21:30
  • $\begingroup$ I hope my edits help and please excuse me for my inexperience. $\endgroup$ – AliceD Oct 29 '14 at 1:57

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

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    $\begingroup$ Some references would be nice. $\endgroup$ – Chris Oct 28 '14 at 13:40
  • $\begingroup$ As mentioned by @Chros (The old Chris that is :) ), this response requires references! As it stands, this is more similar to a comment! $\endgroup$ – Bez Oct 28 '14 at 19:57

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