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In biological anatomy, the mesencephalon (or midbrain) is the middle of three vesicles that arise from the neural tube that forms the brain of developing animals. Caudally the mesencephalon adjoins the pons and rostrally it adjoins the diencephalon. In mature human brains, the mesencephalon becomes the least differentiated, from both its developmental form and within its own structure, among the three vesicles. The mesencephalon is considered part of the brain stem. Its substantia nigra is closely associated with motor system pathways of the basal ganglia. The human mesencephalon is archipallian in origin, meaning its general architecture is shared with the most ancient of vertebrates. Dopamine produced in the substantia nigra plays a role in motivation and habituation of species from humans to the most elementary animals such as insects.
Gross structures on the midbrain On the posterior (back) surface are located the superior colliculus and the inferior colliculus. The superior colliculus is involved with saccadic eye movements; the inferior is a synapsing point for sound information. The trochlear nerve comes out of the posterior surface of the midbrain, below the inferior colliculus. On the anterior surface the cerebral peduncles are prominent. These contain the corticospinal tract fibres, from the internal capsule, as well as the substantia nigra. Between the peduncles is the interpeduncular fossa, which is a cistern filled with cerebrospinal fluid. The oculomotor nerve comes out between the peduncles, and the trochlear nerve is visible wrapping around the outside of the peduncles. Cross-section through the midbrain The midbrain is usually sectioned at the level of the superior and inferior colliculi. A cross-section through the superior colliculus shows the red nucleus, the nuclei of the oculomotor nerve (and associated Edinger-Westphal nucleus), as well as the substantia nigra. The substantia nigra is still present at inferior colliculus level. Also apparent are the trochlear nerve nucleus, and the decussation of the superior cerebellar peduncles. The cerebral aqueduct runs through the midbrain, and is the communication between the third and fourth ventricle. As a mnemonic the mesencephalic cross-section resembles a bear (or teddybear) upside down with the two red nuclei as the eyes and the crus cerebri as the ears. Organization Related topic | |||||||||||||||||||||||||||||||||
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