The midbrain (mesencephalon) is the shortest and highest part of the brainstem, passing through the tentorial notch. The cerebral aqueduct runs through it, dividing a dorsal roof (tectum) from a larger ventral part made of the tegmentum and cerebral peduncles. It contains the nuclei of the oculomotor and trochlear nerves and key centres for vision, hearing, movement and pain control.
| Location | Between the diencephalon above and the pons below, passing through the tentorial notch, surrounded by the interpeduncular, ambient and quadrigeminal cisterns |
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| Parts |
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| Connections |
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| Blood Supply | Posterior cerebral artery; Superior cerebellar artery; Basilar artery tip (paramedian perforators); Posterior choroidal arteries; Collicular (quadrigeminal) artery |
Controls eye movements and pupil size, relays visual and auditory reflexes, supports movement via the substantia nigra and red nucleus, and modulates pain and arousal.
Midbrain syndromes include Weber syndrome (oculomotor palsy with contralateral hemiplegia) and Benedikt syndrome (oculomotor palsy with contralateral tremor). Downward herniation through the tentorial notch compresses the midbrain and is life-threatening. In progressive supranuclear palsy, midbrain atrophy produces the hummingbird sign on sagittal MRI.
A ventral midbrain stroke damaging the oculomotor nerve fascicles and cerebral peduncle, causing a same-side third nerve palsy and opposite-side hemiplegia.
A neurodegenerative disease with midbrain atrophy causing vertical gaze palsy, early falls and stiffness.
Bleeding in the midbrain and pons caused by stretching of perforating arteries during downward transtentorial herniation.
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