Midbrain & Pons Gross Anatomy
In addition to the medulla oblongata, the pons and midbrain form the middle and proximal parts of the brainstem, respectively. Together, they are responsible for housing various primary and accessory cranial nerve nuclei, providing a pathway for the forebrain, hindbrain, spinal cord and peripheral nervous system to communicate, and housing several life sustaining relay centres. This article will focus on the gross anatomy and blood supply of the midbrain and pons. Additionally, it will review the location of different nuclei and their associated tracts.
Gross Anatomy of the Midbrain
The midbrain is a mesencephalic structure that extends from the superior medullary velum of the pons (roof of the fourth ventricle) to the posterior commissure of the third ventricle. This 2 cm long structure is made up of a left and a right pedunculus cerebri or cerebral peduncles anteriorly, and four rounded structures posteriorly (corpora quadrigemini).
The cerebral peduncles are caudal continuations of the internal capsule (a white matter structure that separates the thalamus and caudate nucleus from the lentiform nucleus [globus pallidus internus and externus, and the putamen combined]). The medial surface of each cerebral peduncle borders the mammillary bodies of the hypothalamus and the posterior perforated substance. The latter is an important access point for blood vessels supplying the midbrain, which will be discussed later. The space between the peduncles where the posterior perforated substance resides is known as the interpeduncular fossa. The optic tracts (post-chiasmatic fibers of the optic nerves) are related to the cerebral peduncles anteriorly proximal to the optic chiasm and laterally as the tracts travel distal to the chiasm. The uncus of the temporal lobe, lateral geniculate body (LGB) and medial geniculate body (MGB) are all in lateral relations to the cerebral peduncles; with the former being most anterior and the latter being most posterior of the structures. The anterior surface of the midbrain also features the emergence of CN III (oculomotor) from the superior pontine sulcus (border between the midbrain and pons).
The posterior midbrain surface boasts several remarkable structures that are involved in the auditory and visual pathways. The most prominent structures located here are the corpora quadrigemini. It is comprised of paired superior colliculi and inferior colliculi. The left colliculus of both groups is separated from its homologue on the right by a continuation of the frenulum of the superior medullary velum. The frenulum is also the sight of emergence for CN IV (trochlear), which exits the brainstem posteriorly and courses around the lateral part of the cerebral peduncle to gain access to the cavernous sinus of the middle cranial fossa. The LGB receives visual input from the optic tract and subsequently relays this information to the superior colliculi via the superior brachium (running inferior to the pulvinar of the thalamus).
The pons, like the cerebellum, is another metencephalic derivative of the rhombencephalon. This 2.5 cm long structure is aptly named for its resemblance of a bridge, as it forms a communication pathway between the left and right hemispheres of the cerebellum. The anterior surface of the pons has a striated appearance. This is a result of corticopontocerebellar fibers running horizontally, bringing information from the cerebral cortices and the pontine nuclei to the dentate nucleus of the cerebellum. The transition from the pons to the midbrain is accentuated by the superior pontine sulcus. Similarly, an inferior pontine sulcus also marks the change from the pons to the medulla oblongata inferiorly. Emerging from the inferior pontine sulcus are three paired cranial nerves:
- CN VI (abducent) is most medial
- CN VII (facial) and its intermediate nerve of Wrisberg are in the middle, just above the olive of the medulla
- CN VIII (vestibulocochlear) is most lateral and adjacent to the flocculus of the cerebellum
There is a vertical depression along the anterior pons known as the basilar groove. This impression is home to the basilar artery, which was formed from the two vertebral arteries. Lastly, two fibers associated with CN V (trigeminal) arise bilaterally, from the anterolateral surface of the pons. The larger fibre is the sensory component, while the smaller fibre is the motor component.
The posterior surface of the pons is obscured by the cerebellum. However, when the cerebellum is removed, it can be appreciated that the posterior pons corresponds to the superior half of the rhomboid fossa (floor of fourth ventricle). This region features two protuberances known as the median eminence separated vertically by the dorsal median sulcus. The lower extremity of the median eminence is significantly more pronounced than the upper part, due to the internal genu of CN VII as it winds around the nucleus of CN VI. It is known as the facial colliculus. The lateral part of each median eminence is bordered by the sulcus limitans (embryological landmark separating basal & alar plates). The striae medullaris (carrying fibers of the arcuate nucleus) marks the inferior limit of the posterior pons. Closer to the superior apex, on either side of the median eminence is a blue-grey area of melanin containing cells known as the locus coeruleus (or substantia ferruginea). The associated nucleus has the responsibility of producing catecholamines in the brain. The vestibular area (corresponding to the vestibular nuclei) is also observable in this area. Communication between the pons and the cerebellum is permitted by the middle and inferior cerebellar peduncles. The former carries pontocerebellar fibers from the pontine nucleus to the dentate nucleus, while the latter carries both efferent and afferent fibers between the cerebellum and the vestibular nuclei.
Blood Supply of the Midbrain and Pons
The basilar component of the vertebrobasilar system spans the length of the pons. It commences at the inferior pontine sulcus, where the left and right vertebral arteries unite, and bifurcates into the left and right posterior cerebral arteries at the superior pontine sulcus. It is equally noteworthy that branches of the basilar artery clasp two cranial nerves bilaterally at its commencement and termination. The posterior cerebral and the superior cerebellar arteries clasp the emerging CN III and the labyrinthine and anterior inferior cerebellar arteries clasp CN VI.
Along its course, the basilar artery gives off short pontine branches that pierce the substance of the pons. The posterior cerebral artery travels lateral to the cerebral peduncle to supply deep brain structures, such as the temporal and occipital lobes. On its journey, the artery gives off thalamogeniculate and posteromedial central perforating arteries that go on to supply the midbrain.
Venous drainage of the midbrain and pons is achieved by both the vertebrobasilar veins and the venous sinus systems. Lateral and posterior mesencephalic veins drain the upper pons and the midbrain to the great cerebral vein of Galen, which drains to the confluence of sinuses by way of the straight sinus. The lateral pontine vein receives venous blood from the pons. It joins the transverse pontine veins and the vein of the lateral recess of the fourth ventricle, along with tributaries from the cerebellum to form the petrosal vein, which drains to the superior petrosal sinus. The anterior pontomesencephalic vein courses from the cerebral peduncles where it communicates with the basal vein of Rosenthal. It continues caudally along the anterior surface of the pons, while receiving venous tributaries from its substance. It becomes the anteromedian medullary vein after passing the pontomedullary junction.