The shortest section of the brainstem, whose Greek name means 'middle brain'
Midbrain — also the mesencephalon.
Anatomy · Brain & nerves
Midbrain
Also called the mesencephalon
Brain · brainstem (top part)
The uppermost and smallest part of the brainstem, about 2 cm long, connecting the pons and cerebellum below to the forebrain above. It contains the four colliculi that handle visual and auditory reflexes, the substantia nigra whose dopamine-producing cells are lost in Parkinson's disease, and the nuclei of the oculomotor and trochlear nerves that move the eyes.
How to spot them
- ▸Short segment above the pons, with two thick 'legs' (the cerebral peduncles) diving up into the forebrain
- ▸Four small bumps on the back: two superior and two inferior colliculi
- ▸A dark band of pigmented cells (substantia nigra) and a reddish nucleus in cross-section
- ▸A narrow channel, the cerebral aqueduct, runs through it; on an axial MRI it resembles a Mickey Mouse face
What to know
Four bumps on its back, the upper pair handling visual reflexes and the lower pair hearing
Midbrain — the superior and inferior colliculi (corpora quadrigemina).
A dark band of pigmented dopamine-producing cells whose loss causes tremor and rigidity
Midbrain — the substantia nigra and Parkinson's disease.
A narrow channel running through it links the third and fourth ventricles; blockage causes hydrocephalus
Midbrain — the cerebral aqueduct (aqueduct of Sylvius).
The origin of the nerves that raise the eyelid and move most of the eye's muscles, and of the only cranial nerve to leave from the back of the brainstem
Midbrain — oculomotor (III) and trochlear (IV) nerves; the trochlear leaves dorsally.
Two thick columns on its front carry motor fibres from the cerebrum down towards the pons
Midbrain — the cerebral peduncles.
On an axial MRI it forms the face of a cartoon mouse, with the peduncles as ears
Midbrain — the 'Mickey Mouse' outline of an axial section; the cerebral peduncles form the ears.
Key ideas
- It is a relay for the eyes and ears: the superior colliculus turns the head towards a sudden movement; the inferior colliculus does the same for a sudden sound.
- Parkinson's disease follows loss of dopamine-producing neurons in the substantia nigra, which project to the basal ganglia.
Quick-fire facts
- •Only about 2 cm long, the shortest part of the brainstem
- •Its Greek name is mesencephalon, meaning 'middle brain'
- •The trochlear nerve (IV) is the only cranial nerve to emerge from the dorsal surface of the brainstem, and it crosses to the opposite side before doing so
- •The oculomotor nerve (III) emerges from the interpeduncular fossa at the front
- •Divided into the tectum (roof), the tegmentum and, at the front, the cerebral peduncles
- •The red nucleus and substantia nigra lie in the tegmentum
- •The cerebral aqueduct connects the third and fourth ventricles; blockage causes hydrocephalus
- •Compression of the oculomotor nerve by a herniating temporal lobe causes a fixed, dilated pupil
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Sources
- TeachMeAnatomy — Midbrain
- OpenStax — 13.2 The Central Nervous System — Anatomy and Physiology 2e