ABSTRACT
Disturbances in the level of consciousness reflect dysfunction
of either the ascending reticular activating system (ARAS)
(Parvizi and Damasio 2003), the thalamus, or the cortex. The
ARAS is a midline brainstem structure which extends from
the rostral pontine tegmentum up into the mesencephalic
tegmentum, where it lies ventral to the periaqueductal gray
matter. Fibers from the ARAS then extend up into the
thalamus, specifically to the thalamic intralaminar nuclei
and the reticular nucleus, and subsequently fibers from the
thalamus project to the cortex. In addition to this efferent
pathway of the ARAS, there is also a subsidiary, extratha-
lamic pathway, which ascends from the ARAS, and then
passes on through the hypothalamus toward the frontal
cortex.