Figure 61.1.— The arousing system. A: brain; B: brainstem; C: pons; D: medulla; 1: cerebral cortex; 2: ascending reticular activating system efferent projection (curved arrows); 3: diencephalom; 4: ascending reticular activating system; 5: ascending reticular activating system afferent projection (straight arrows).
Coma may be produced by involvement of the ARAS, its projection, or the cerebral cortex. Coma due to ARAS involvement may occur with a small midline lesion in the central core of the midbrain or the upper half of the pons where the ARAS forms a single midline structure. Coma due to involvement of the smaller ARAS columns in the diencephalon, the ARAS projection fibers, or the cerebral cortex occur with extensive and diffuse bilateral diencephalic and brain hemispheric lesions.