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Ventral View of the Brainstem and Apparent (or Superficial) Origin of Cranial Nerves

Surgical Correlation


Ventral view of the brainstem and apparent (or superficial) origin of cranial nerves. The olfactory tracts carry afferent fibers from the olfactory bulb to olfactory centers in the cerebrum including the entorhinal cortex, piriform cortex, and amygdala. The olfactory tracts divide into medial, lateral, and intermediate olfactory stria anterior to the anterior perforated substance.  The latter is a series of pinpoint holes on the ventral surface of the forebrain formed by the penetration of many ganglionic lenticulostriate arteries. The visual pathway revealed here includes the optic nerves (sectioned), optic chiasm, and optic tracts on their way to the lateral geniculate nuclei of the thalamus. The oculomotor nerve exits the midbrain medial to the cerebral peduncle and lateral to the posterior perforated substance. Note the close proximity of the parahippocampal gyrus and uncus to the oculomotor nerve. This anatomical relationship explains why uncal transtentorial herniations cause anisocoria and oculomotor nerve palsies. The trigeminal nerve enters the mid-pons through the middle cerebellar peduncle and has two roots: a sensory root (portio major) and a motor root (portio minor) which is more medial and superior. At the pontomedullary junction, within the cerebellopontine angle and medial to the flocculus, are attached the vestibulocochlear and facial nerves.  Closer to midline lies the abducens nerve, which exits the pontomedullary junction just cranial to the pyramid. The olives (inferior olivary nucleus) are located lateral to the pyramids and are good anatomical landmarks for finding the apparent origins of the lower cranial nerves. The hypoglossal nerve exits the medulla oblongata anteromedially (in the preolivary sulcus), while the glossopharyngeal, vagus, and accessory nerves exit the medulla oblongata more posterolaterally, in the postolivary sulcus. (Image courtesy of AL Rhoton, Jr.)