The anatomy of consciousness

Two major neuroanatomic structures are necessary for consciousness: the reticular activating system and the cerebral hemispheres. The reticular activating system is primarily responsible for arousal mechanisms, and the hemispheres influence the content of consciousness ( Bio£k...§..n.d Ble.ck...199!.5).

The reticular activating system receives input from all major afferent tracts, and projects widely to the thalamus, the basal forebrain, and the cerebral hemispheres. The crucial segment of the reticular activating system for arousal is between the rostral midbrain and the midpons. Isolated lesions to this portion of the reticular activating system produce coma, whereas lower lesions do not. Damage to the thalamus or hypothalamus can also alter consciousness, which is understandable given the large interconnection between these structures and the reticular activating system, but bilateral involvement is usually required.

Focal lesions in the cerebral cortex tend not to alter arousal but instead affect the content of consciousness. To affect arousal, large areas of both hemispheres need to be involved, on either a structural or a metabolic basis. Large focal processes such as tumor or infarction may alter the contralateral hemisphere by pressure effects, or by disrupting circulation or metabolism. Dominant hemisphere lesions may be more significant in affecting arousal than lesions in the non-dominant hemisphere.

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