The Midbrain

Basal and Alar Plates

The immature neurons of the alar plate give rise to the quadrigeminal plate, from which the superior and inferior colliculi arise ( Fig. 13.2 A, upper arrow ). In humans, the superior colliculus consists of alternating layers of cells and fibers, whereas the inferior colliculus appears more homogeneous. Immature alar plate neurons also migrate into anterior areas of the developing midbrain to form the red nucleus and the substantia nigra ( Fig. 13.2 A, lower arrow ).

Immature neurons of the basal plate give rise to the somatic efferent (SE) neurons of the oculomotor and trochlear nuclei. In addition, the visceral efferent (VE) preganglionic parasympathetic cells associated with the oculomotor complex also arise from the basal plate ( Fig. 13.2 ).

As the basal and alar plates differentiate, the marginal layer is invaded by axons originating from cells located outside the midbrain. These fibers collect in the anterolateral area of the developing mesencephalon to form an especially prominent bundle, the crus cerebri ( Fig. 13.2 B ).

Anterior (Ventral) Midbrain

The presence of a pair of large axon bundles, the crura cerebri, is a characteristic feature of the anterior aspect of the midbrain. These bundles emerge from the cerebral hemispheres caudal to the optic tracts, converge slightly toward the midline as they course through the midbrain, and disappear into the basilar pons ( Fig. 13.3 ). The oculomotor nerves exit the medial edge of each crus and pass through the space between the crura: the interpeduncular fossa ( Fig. 13.3 ). Anteriorly, the rostral limit of the midbrain is marked by the exit of the crura cerebri from the cerebral hemispheres and by the caudal edge of the mammillary bodies. The caudal border of the midbrain is formed where each crus enters the basilar pons.

The subarachnoid space of the interpeduncular fossa is called the interpeduncular cistern. This cistern contains the oculomotor nerves and the upper part of the basilar artery, including its bifurcation and proximal branches. Numerous vessels penetrate the roof of this fossa and create many small perforations ( Fig. 13.3 B ). This area is frequently called the posterior perforated substance.

Posterior (Dorsal) Midbrain

The posterior surface of the adult midbrain is characterized by four elevations collectively called the corpora quadrigemina ( Fig. 13.4 ). The rostral two elevations are the superior colliculi, and the caudal two are the inferior colliculi. Just caudal to the inferior colliculus, the exit of the trochlear nerve marks the pons-midbrain junction on the posterior surface of the brainstem, whereas the midbrain-diencephalic boundary is formed by the posterior commissure ( Fig. 13.5 ).

Rostrolaterally, the inferior colliculus is joined to the medial geniculate body of the diencephalon by a fiber bundle called the brachium of the inferior colliculus ( Fig. 13.4 ). The inferior colliculus and the medial geniculate body are part of the auditory system. The brachium of the superior colliculus extends from the optic tract to the superior colliculus in a groove located between the medial geniculate body and pulvinar of the diencephalon (see Figs. 13.4 and 13.14 ). The superior colliculus, pulvinar, and lateral geniculate body are parts of the visual and visual-motor systems.

On the midline, the pineal gland, a diencephalic structure, extends posteriorly above and between the superior colliculi ( Fig. 13.5 ). Tumors of the pineal may produce noncommunicating (obstructive) hydrocephalus because of compression of the colliculi of the midbrain and resulting occlusion of the cerebral aqueduct.

The subarachnoid space immediately posterior (dorsal) to the colliculi is the quadrigeminal cistern. This cistern contains the exiting trochlear nerves, the great vein of Galen, and distal branches of the posterior cerebral arteries. The ambient cistern is located at the lateral aspect of the midbrain and contains segments P ₂ to P ₃ and the superior cerebellar and quadrigeminal arteries ( Fig. 13.6 ). The crural cistern is located between the crus cerebri and the immediately adjacent parahippocampal gyrus; it contains medial posterior choroidal (see Fig. 8.13 ) and anterior choroidal (see Fig. 8.6 ) arteries and the basal vein ( of Rosenthal ) (see Fig. 8.24 ). The interpeduncular cistern is that part of the subarachnoid space that occupies the interpeduncular fossa ( Fig. 13.6 ).