Midbrain

Mesencephalon

NervousOrgansHead

View in 3D

What it is
The top of the brainstem, passing through the notch of the tentorium. In front, two cerebral peduncles diverge upward with the interpeduncular fossa between them; behind, the tectum carries four bumps, the superior and inferior colliculi, and the cerebral aqueduct runs through its centre.
Function
Carries the motor tracts from cortex to spinal cord, controls eye movements and pupil reflexes (oculomotor and trochlear nuclei), orients the eyes and head to sights and sounds, and contains the substantia nigra.
Size
About 2 cm long and 2.5-3 cm wide across the peduncles
Clinical note
Parkinson disease results from loss of the dopamine neurons of the substantia nigra; a swollen temporal lobe herniating through the tentorial notch compresses the midbrain.

Nearby nervous structures Arachnoid mater, Cerebral aqueduct, Dura mater (cranial), Fornix, Hypothalamus, Oculomotor nerve root (CN III), Optic tract, Optic tract root (CN II), Third ventricle, Trochlear nerve root (CN IV)