Midbrain

At a glance

English

Midbrain

Latin (TA2)

Mesencephalon

System

Brain and spinal cord

Region

Head

Side

Paired, left and right

About the midbrain

The midbrain, or mesencephalon (Greek for middle brain), is the shortest segment of the brainstem. It joins the diencephalon above to the pons and cerebellum below and passes through the gap in the tentorium cerebelli, the tentorial notch. Through its centre runs the narrow cerebral aqueduct, which links the third ventricle with the fourth.

Behind the aqueduct lies the tectum, the roof, carrying the four colliculi of the quadrigeminal plate: the superior pair, concerned with visual reflexes and directing the eyes and head, and the inferior pair, a relay for hearing. In front of the aqueduct lies the tegmentum, much the bulkier part. Around the aqueduct is the periaqueductal grey, important in suppressing pain and in defensive behaviour. In front of the grey, at the superior collicular level, lie the oculomotor nuclei with the Edinger-Westphal nucleus, which constricts the pupil and focuses the lens; at the inferior collicular level lie the trochlear nuclei. The tegmentum also holds the red nuclei, the reticular formation, the ventral tegmental area, the raphe nuclei, the decussation of the superior cerebellar peduncles and ascending sensory tracts such as the medial lemniscus and spinothalamic tract.

Furthest forward are the cerebral crura, two thick bundles of descending fibres from the cortex: corticospinal and corticobulbar fibres in their middle, corticopontine fibres at the sides. Between the crura lies the interpeduncular fossa, from which the oculomotor nerves emerge. Separating each crus from the tegmentum is the substantia nigra, a band of grey matter whose compact part is dark with neuromelanin and makes dopamine for the basal ganglia. The crus, nigra and tegmentum together form the cerebral peduncle. The trochlear nerve is unique in leaving the back of the brainstem, just below the inferior colliculus.

Clinically, loss of dopamine cells in the substantia nigra underlies Parkinson disease. The ventral tegmental area is central to reward and addiction. Narrowing of the aqueduct, present from birth or caused by a tumour of the tectum or pineal region, causes hydrocephalus. Raised pressure above the tentorium can push the uncus against the midbrain and the oculomotor nerve. Vascular lesions give named syndromes: Weber syndrome (oculomotor palsy with weakness of the opposite limbs) when the crus is involved, and Parinaud syndrome (loss of upward gaze) from pressure on the tectum.

Function and supply

Function

Relays and integrates motor and sensory pathways; holds centres for eye movement, pupil and lens control, visual and auditory reflexes, pain control, arousal, and dopamine output for movement and reward.

Blood supply

Paramedian and circumferential branches of the basilar artery, the posterior cerebral arteries, the superior cerebellar arteries and the posterior communicating arteries; the collicular and posterior choroidal arteries supply the tectum.