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 (metencephalon) and rostrally it adjoins the diencephalon (Thalamus, hypothalamus, et al).
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
The mesencephalon forms the upper part of the brain stem. It carries corpora quadrigemina (also called as optic lobes or tectum) on the dorsal side and cerebral peduncles (also called as crura cerebrii) on the ventral side of the cerebral aqueduct.
It consists of four solid optic lobes on the dorsal side of cerebral aqueduct, where the anterior (front) pair called as the superior colliculus and the posterior (back) pair is called as inferior colliculus. The 4 solid optic lobes help to decussate several fibres of the optic nerve, and by their contra lateral arrangement, the optic chiasma is formed. However some fibres also show homolateral or ipsilateral arrangement i.e they run parallel on the same side without decussating. The superior colliculus is involved with saccadic eye movements;while 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.
These are paired structures, present on the ventral side of cerebral aqueduct, and they further carry tegmentum on the dorsal side and cresta or pes on the ventral side, and both of them accommodate the corticospinal tract fibres, from the internal capsule (i.e ascending + descending tracts = longitudinal tract.) the middle part of cerebral peduncles carry substantia nigra (also called "Black Matter") which is a type of basal nucleus. It is the only part of the brain that carries melanin pigment.
Between the peduncles is the interpeduncular fossa, which is a cistern filled with cerebrospinal fluid. The occulomotor 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.
Diagram of the midbrain, sectioned at the level of the superior colliculus
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