Why can a small lesion in the brainstem cause a coma when a large one in the cortex does not? · Grey Matter

Wakefulness depends on a system of nuclei in the upper brainstem (the reticular activating system with the neuromodulator nuclei near it) that project through the thalamus and the basal forebrain to the whole cortex.

Why can a small lesion in the brainstem cause a coma when a large one in the cortex does not?

Wakefulness depends on a system of nuclei in the upper brainstem (the reticular activating system with the neuromodulator nuclei near it) that project through the thalamus and the basal forebrain to the whole cortex. All those fibres funnel through a few cubic centimetres of the upper pons and midbrain, so a small stroke or bleed there removes the drive for the entire cortex at once. A cortical lesion, however large, leaves the rest of the cortex awake; in a series of brainstem strokes studied by Parvizi and Damasio, coma followed lesions of the upper pontine and midbrain tegmentum.