The midbrain or mesencephalon (from the Greek mesos - middle, and enkephalos - brain) is a portion of the central nervous system associated with vision, hearing, motor control, sleep/wake, arousal (alertness), and temperature regulation. Anatomically, it comprises the tectum (or corpora quadrigemina), tegmentum, ventricular mesocoelia (or "iter"), and the cerebral peduncles, as well as several nuclei and fasciculi. Caudally the mesencephalon adjoins the pons (metencephalon) and rostrally it adjoins the diencephalon (Thalamus, hypothalamus, etc.). The midbrain is located below the cerebral cortex and above the hindbrain placing it near the center of the brain.
During embryonic development, the midbrain arises from the second vesicle, also known as the mesencephalon, of the neural tube. Unlike the other two vesicles, the prosencephalon and rhombencephalon, the mesencephalon remains undivided for the remainder of neural development. It does not split into other brain areas while the prosencephalon, for example, divides into the telencephalon and the diencephalon.
Throughout embryonic development, the cells within the midbrain continually multiply and compress the still-forming Aqueduct of Sylvius or cerebral aqueduct. Partial or total obstruction of the cerebral aqueduct during development can lead to congenital hydrocephalus.
The mesencephalon is considered part of the brainstem. 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. The midbrain is the smallest region in the brain and helps to relay information for vision and hearing.