Unconsciousness is produced in one of three ways: diffuse bilateral involvement of the cerebral hemispheres, injury to the brainstem reticular activating system, or a combination of focal injuries to the cortex and brainstem. Herniation is a special case of insult to the reticular activating system caused by a cerebral space-occupying lesion which produces a mechanical shift of brainstem structures.
Consciousness has two necessary components: arousal and content. Specific structures in the central nervous system modulate arousal; lesions in these areas render the patient unable to respond despite intact sensory afferents. Unconsciousness is the condition in which the patient makes no appropriate responses to stimuli, either external (e.g. pain) or internal (e.g. thirst). This state does not exclude posturing and other reflex movements.
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