(4) Why the brainstem is usually less susceptible to damage than
the cerebral cortex
Blood flow and metabolism are higher in the inferior colliculi than anywhere else in the
brain [48-52]. Figure 13 shows the high level of blood flow in nuclei of the brainstem
auditory pathway, demonstrated by rapid distribution of a radioactive tracer. Sokoloff
(1981) commented, "The inferior colliculus is clearly the most metabolically active
structure in the brain" .
Despite its high metabolic activity, the inferior colliculi are not predictably affected by
oxygen insufficiency. Under hypoxic conditions, protective mechanisms like (a)
vasodilation go into action as a means of sparing this metabolically important nucleus
[53, 54]. Also,
(b) hemoglobin delivers oxygen more readily to the most metabolically active tissues,
those producing the highest levels of carbon dioxide (the Bohr Effect) .
Damage restricted to the brainstem only occurs when total interference with aerobic
metabolism occurs; Janzer and Friede (1980) referred to brainstem pathology as
"cardiac arrest encephalopathy" ; this was the case in monkeys subjected to
several minutes of total asphyxia at birth [11, 12].