Mesencephalon [midbrain]

The mesencephalon (also called the “midbrain”) is the second of three vesicles that arise from the neural tube that forms the brain of developing animals. Caudally, the midbrain joins the metencephalon and rostrally the diencephalon. In the brain of a fully developed human, the midbrain becomes the least developed, both in appearance and in its own structure, of the three vesicles. The midbrain is considered part of the brainstem.

Its substantia nigra is associated with the motor pathways of the basal nuclei. The human midbrain is considered an archipallium from an evolutionary perspective, as it shares its general architecture with that of the most ancient vertebrates. Dopamine produced in the substantia nigra plays a role in the development of motivation and habits in many species, from humans to more basic ones such as insects.

External structure

The midbrain is a nerve structure interposed between the forebrain, which is anterior and superior to it, the cerebellum, posterior, and the pons of Varolius, posterior-inferior. It is the shortest portion of the brainstem being on average 2 cm long and is contained like the rest of the brainstem in the posterior cranial fossa. In the midbrain two parts are conventionally distinguished, the cerebral peduncles, lateral and the roof of the midbrain between them. The cerebral peduncles are further divided by the black substance into an anterior part called the foot and a posterior part called the tegmental part.

Dividing the peduncles is the interpeduncular fossa, within which cerebrospinal fluid flows. The oculomotor nerve exits between the peduncles, and the trochlear nerve is visible coiled around the outside of the peduncles. In contrast, the roof of the midbrain is divided into the inferior and superior colliculi, which together are called the quadrigeminal bodies or tubercles, and by the pretectal area. They help decussate several optic nerve fibers, and because of their contralateral arrangement the optic chiasm is formed. However, some fibers also show an ipsilateral arrangement, that is, they run parallel on the same side without decussating.

The superior tubercle is involved in saccadic movements of the eye, while the inferior tubercle is a synapse point for sound information. The trochlear nerve exits from the posterior surface of the midbrain, below the inferior tubercle. Anterior to the roof of the midbrain runs the narrow canal called Silvio’s aqueduct, an antero-superior continuation of the fourth ventricle, surrounded by periacqueductal gray matter.

Internal structure

For simplicity of description, we imagine dissecting the midbrain first at mid-height and then at the upper portion.

Middle portion

In a section of the midbrain, it is possible to distinguish very well the two parts of which it is composed: two cerebral peduncles, one on each side, in anterior position, and a more voluminous roof located posteriorly. In the cerebral peduncles run the corticospinal bundle, the cortibulbar bundle and the frontopontine bundle. Postero-medially to them there is still some pontine nucleus. Behind them extends a clearly visible grayish stripe, the black substance. Between the two black substances are received the interpeduncular nuclei while behind them runs the medial lemniscus. The superior cerebellar peduncle is posterior to the interpeduncular nuclei; behind it runs the medial longitudinal fasciculus and behind this is the trochlear nucleus. Behind the trochlear nucleus, surrounded by the periacqueductal gray, runs the aqueduct of Silvio. In the medial-lateral direction starting from the periacqueductal gray is the reticular formation, the inferior colliculus, the lateral lemniscus and the spinothalamic and spinoreticular bundles. Behind the inferior colliculus runs the arm of the inferior colliculus.

Superior portion

The superior portion of the midbrain is elongated transversely. The anterior portion is again largely formed by the cerebral peduncles, medial to these is the black substance, and then, still more medial, the voluminous red nucleus. Interposed between the two red nuclei is the medial tegmental area, which is separated from them by the fibers of the oculomotor nerve, whose nucleus is located posteriorly and medially to the red nucleus. Posterior to each red nucleus runs the medial longitudinal fasciculus. Behind the oculomotor nucleus there is the Edinger-Westphal nucleus and posterior to it the periacqueductal gray with Silvio’s aqueduct in the center. In the medio-lateral direction starting from the periacqueductal gray there are the spinothalamic and spinoreticular bundle, the medial lemniscus, the medial geniculate body and the lateral geniculate body. Posterior to the periacqueductal gray is the superior colliculus.

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