From Academic Kids
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. The mesencephalon caudally adjoins the pons and rostrally 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 or the midbrain. The substantia nigra is closely associated with motor system pathways of the basal ganglia.
The mesencephalon is archipallian in origin, meaning its general architecture is shared with the most ancient of vertebrates. Dopamine produced in the subtantia 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, there are structures called the superior colliculus and the inferior colliculus. The superior colliculus is involved with the pupil's response to light, 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.
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.
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.