Chapter 13
The Midbrain
Internal Anatomy of the Midbrain
General Regions: Tectum, Tegmentum, and Basis Pedunculi
Summary of Descending Pathways
Diencephalon Junction-Midbrain
Internal Vasculature of the Midbrain
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.
DEVELOPMENT
The mesencephalon (Fig. 13-1) arises early in development as one of the three primary brain vesicles. Immature neurons that arise from the ventricular zone form the intermediate zone and give rise to alar and basal plates, which are continuous with the alar and basal plates of the rhombencephalon (Fig. 13-2). These cell groups are the rostral continuations of the same primitive cell columns described for the metencephalon. The surrounding marginal layer contains the developing axons of cells located in other levels of the neuraxis. The cerebral aqueduct is narrow relative to the fourth ventricle; therefore the basal and alar plates lie anterior and posterior to it, respectively, as in the spinal cord and caudal medulla.
Figure 13-1. Lateral view of the human brain at about 7 weeks of gestation. The midbrain is highlighted.
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. 13-2A, 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. 13-2A, 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. 13-2B).
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 (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.
Figure 13-3. Anterior (ventral) views (A, undissected; B, dissected) of the brainstem with emphasis on the midbrain.
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. 13-3B). 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).
Figure 13-5. Sagittal view of the brainstem with emphasis on midbrain structures.
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 P2 to P3 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 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 (Fig. 13-6).
Figure 13-6. Major subdivisions of the midbrain and the general positions of cisterns related to the midbrain.
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.
INTERNAL ANATOMY OF THE MIDBRAIN
General Regions: Tectum, Tegmentum, and Basis Pedunculi
The midbrain is divisible into three regions, which can be appreciated best in cross section. Posterior to the cerebral aqueduct is the tectum (roof) of the midbrain (Figs. 13-5 and 13-6). The characteristic structures of this area are the superior and inferior colliculi. The periaqueductal gray (central gray) is a sleeve of neuron cell bodies that completely surrounds the cerebral aqueduct. The tegmentum of the midbrain extends from the base of the tectum to, but does not include, the substantia nigra. The anterolateral portion of the midbrain on either side is formed by the basis pedunculi, which consists of the substantia nigra and the crus cerebri (Fig. 13-6). In turn, the crus cerebri is composed primarily of descending fibers. The term cerebral peduncle is sometimes used for the crus cerebri, but it actually represents the entire midbrain below the tectum (tegmentum and basis pedunculi).
Summary of Ascending Pathways
The long ascending pathways that traverse the pons and medulla continue through the midbrain (Fig. 13-7). These include the medial lemniscus, the anterolateral system, and the posterior and anterior trigeminothalamic tracts. The lateral lemniscus is also prominent in the caudal midbrain. Other, smaller bundles, such as the medial longitudinal fasciculus, are also present and occupy positions comparable to those seen in lower levels of brainstem.
Figure 13-7. Diagrammatic representation of the brain showing the location and trajectory of three important pathways and the trigeminal nuclei. The color coding for each is continued in Figures 13-8, 13-12, and 13-14.
Summary of Descending Pathways
Descending fibers from the cerebral cortex pass through the midbrain and, as we have seen, are also prominent features of the pons, medulla, and spinal cord (Fig. 13-7). At midbrain levels, these corticospinal, corticonuclear (corticobulbar), and corticopontine fibers are parts of the crus cerebri. Some of the descending fiber bundles that were discussed in the pons and medulla, such as the tectobulbospinal system and the rubrospinal and central tegmental tracts, originate from nuclei of the midbrain.
The following sections describe the anatomy of the midbrain at caudal and rostral brain levels and at the level of the midbrain-diencephalon junction. The anatomy is presented from posterior to anterior at each level, starting with the tectum and proceeding through the tegmentum to the basis pedunculi.
Caudal Midbrain Levels
Transverse sections through the caudal midbrain are characterized by the presence of the inferior colliculus, trochlear nucleus, and decussation of the superior cerebellar peduncle (Figs. 13-8 to 13-10). The inferior colliculus is composed of a large central nucleus bordered posteriorly by a smaller pericentral (dorsal) nucleus and laterally by an external (lateral) nucleus. Lateral lemniscus fibers enter the inferior colliculus from an anterior direction and give this area a goblet-like appearance (Figs. 13-8 and 13-9). The lateral lemniscus provides auditory input to the nuclei of the inferior colliculus, which then transmit this information to the medial geniculate body via fibers of the brachium of the inferior colliculus. A complete tonotopic representation (frequency map) of the cochlea is found in several of the central auditory nuclei (see Chapter 21).
Figure 13-8. Cross section of the midbrain at the level of the inferior colliculus. Correlate with Figure 13-7. The anatomic orientation is flipped to illustrate internal structures in a clinical orientation; the clinically important tracts and nuclei are shown on a T2-weighted magnetic resonance image at a comparable level of the inferior colliculus.
