The mesencephalon, or midbrain, is the most rostral portion of the brainstem. It gives rise to cranial nerves III and IV, conducts ascending and descending tracts, and contains nuclei that are essential to motor function. Caudally the midbrain is continuous with the pons, and rostrally it joins the diencephalon. The cerebral aqueduct, the cavity of the midbrain, is continuous rostrally with the third ventricle and caudally with the fourth ventricle. The blood supply to the mesencephalon is primarily from proximal branches of the posterior cerebral arteries (P₁ or P₂) and from penetrating branches of the posterior communicating artery.

External Features

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. The oculomotor nerves exit the medial edge of each crus and pass through the space between the crura: the interpeduncular fossa. 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. 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. 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.

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. 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. 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. 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 P₂ to P₃ and the superior cerebellar and quadrigeminal arteries. The crural cistern is located between the crus cerebri and the immediately adjacent parahippocampal gyrus; it contains medial posterior choroidal and anterior choroidal arteries and the basal vein (of Rosenthal). The interpeduncular cistern is that part of the subarachnoid space that occupies the interpeduncular fossa.

Vasculature of the Midbrain

The primary blood supply to the mesencephalon arises via branches of the basilar artery, with smaller branches from the superior cerebellar, anterior choroidal, medial posterior choroidal, and posterior communicating arteries. An important source of blood to the posterior portion of the midbrain is the quadrigeminal artery, a branch of the posterior cerebral artery (P1 segment). Also, the superior cerebellar artery gives rise to branches that serve caudal parts of the posterior midbrain and adjacent regions of the pons.