Anatomy Atlas

Midbrain

Mesencephalon
Also: Mesencephalon · Cerebral peduncles · Substantia nigra · Superior colliculus · Tectum · Parinaud syndrome
OrganHigh-yielddraftNeuro

Uppermost part of the brainstem, around the cerebral aqueduct. It contains the CN III and IV nuclei, the substantia nigra and the corticospinal tracts in the cerebral peduncles, and is compressed in transtentorial herniation.

Core anatomy

Location

Between the diencephalon above and the pons below, passing through the tentorial notch.

Parts
  • Tectum (dorsal, behind the aqueduct): superior colliculi (visual reflexes, vertical gaze) and inferior colliculi (auditory pathway)
  • Cerebral aqueduct (of Sylvius) connecting the third and fourth ventricles, surrounded by the periaqueductal gray (pain modulation)
  • Tegmentum: nuclei of CN III (with Edinger–Westphal) and CN IV, the red nucleus, medial lemniscus, spinothalamic tract, MLF, reticular formation
  • Substantia nigra (dopaminergic neurons of the pars compacta → striatum; see basal ganglia)
  • Cerebral peduncles (crus cerebri): descending corticospinal and corticobulbar fibers; the interpeduncular fossa between them, where oculomotor nerve (CN III) emerges
  • Pretectal area: pupillary light reflex relay (bilateral to both Edinger–Westphal nuclei → consensual response)
Relations

The uncus of the temporal lobe and CN III lie against the free tentorial edge beside the peduncle; the pineal gland sits above the superior colliculi.

Arterial supply

Branches of the posterior cerebral artery, superior cerebellar artery and top of the basilar artery (see vertebrobasilar system).

Venous drainage

Basal veins → great cerebral vein (of Galen) → straight sinus.

Notes

Contains the nuclei of CN III and IV; the trochlear nerve exits dorsally below the inferior colliculus (trochlear nerve (CN IV)).

Mentioned in

Generated automatically from links in other entries.

Where the cranial nerves leave the brainstem (ventral view)The front of the brainstem. The optic nerves meet at the chiasm above. The oculomotor nerve emerges from the interpeduncular fossa between the cerebral peduncles of the midbrain. The trochlear nerve is the only one to leave from the back, just below the inferior colliculus, and winds forward around the peduncle. The trigeminal nerve leaves the lateral pons. The abducens leaves at the pontomedullary junction near the midline; the facial and vestibulocochlear nerves leave at the cerebellopontine angle. The glossopharyngeal and vagus nerves leave the postolivary sulcus of the medulla, the hypoglossal leaves the preolivary sulcus between the pyramid and the olive, and the accessory nerve rises from the upper cervical cord.Optic (II)Oculomotor (III)Trochlear (IV) — from the dorsal sideTrigeminal (V)Abducens (VI)Facial (VII)Vestibulocochlear (VIII)Glossopharyngeal (IX)Vagus (X)Accessory (XI) — from C1–C5Hypoglossal (XII)Midbrain (cerebral peduncle)PonsPyramidOliveSpinal cordMedullaVentral view · dashed IV leaves from the back

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Where the cranial nerves leave the brainstem. Ventral view. III leaves the interpeduncular fossa of the midbrain; IV is the only nerve to leave from the back (dashed); V leaves the lateral pons; VI the pontomedullary junction near the midline; VII and VIII the cerebellopontine angle; IX and X the postolivary sulcus; XII the preolivary sulcus between pyramid and olive; and XI rises from the upper cervical cord. Nuclei follow the same levels: III–IV midbrain, V–VIII pons, IX–XII medulla.
Original schematic, created for this atlas.
Coronal section showing the thalamus, internal capsule, lentiform and caudate nuclei, red nucleus, substantia nigra, cerebral peduncle and hippocampus
Coronal section through the midbrain: red nucleus, substantia nigra and cerebral peduncle.
Henry Vandyke Carter, Gray's Anatomy of the Human Body, 20th ed. (1918), Fig. 717. Source · Public domain (Public domain in the US: published before January 1, 1931 (per Wikimedia Commons))
Dorsal view of the brainstem with the cerebellum partly removed: superior and inferior colliculi, trochlear nerve, facial colliculus, hypoglossal trigone, area postrema and the cerebellar peduncles
Dorsal midbrain with the superior and inferior colliculi (corpora quadrigemina).
Henry Vandyke Carter, Gray's Anatomy of the Human Body, 20th ed. (1918), Fig. 709. Source · Public domain (Public domain in the US: published before January 1, 1931 (per Wikimedia Commons))

Clinical correlations

Parkinson disease PANCE

Mechanism / lesion

Loss of dopaminergic neurons in the substantia nigra pars compacta (Lewy bodies, alpha-synuclein) → reduced dopamine in the striatum.

Classic presentation

Resting (pill-rolling) tremor, rigidity (cogwheel), bradykinesia, postural instability; shuffling gait, masked face, micrographia; often asymmetric onset. Clinical diagnosis; levodopa-carbidopa.

Parinaud (dorsal midbrain) syndrome PANCE

Mechanism / lesion

Compression of the dorsal midbrain/superior colliculi — pineal tumor (germinoma in young males), hydrocephalus, dorsal midbrain stroke.

Classic presentation

Upward gaze palsy, light–near dissociation (pupils react to accommodation but not light), convergence–retraction nystagmus, eyelid retraction (Collier sign); often obstructive hydrocephalus from aqueduct compression.

Weber syndrome PANCE

Mechanism / lesion

Infarct of the ventral (basal) midbrain — PCA/paramedian branches.

Classic presentation

Ipsilateral CN III palsy + contralateral hemiparesis (cerebral peduncle).

Transtentorial (uncal) herniation and aqueductal stenosis PANCE

Mechanism / lesion

A supratentorial mass pushes the uncus through the tentorial notch, compressing CN III, the peduncle and the PCA; congenital or acquired narrowing of the aqueduct blocks CSF flow.

Classic presentation

Herniation: ipsilateral fixed dilated pupil, contralateral (or ipsilateral — Kernohan notch) hemiparesis, declining consciousness. Aqueductal stenosis: noncommunicating hydrocephalus with dilated lateral and third ventricles but a normal fourth ventricle.

High-yield PANCE pearls

  • Substantia nigra degeneration → Parkinson disease (TRAP: tremor, rigidity, akinesia, postural instability).
  • Pineal tumor → Parinaud syndrome (upgaze palsy, light–near dissociation) + hydrocephalus.
  • Ipsilateral CN III + contralateral hemiparesis → Weber (midbrain).
  • Dilated lateral and third ventricles, normal fourth → aqueductal stenosis.

Imaging anatomy

CT

Noncontrast CT for herniation, hemorrhage and hydrocephalus.

MRI

MRI is best for the brainstem (CT has bone artifact in the posterior fossa); DaT-SPECT can support a diagnosis of parkinsonism in unclear cases.

Physical exam

Strength / function testing

Pupils (light and near response), vertical and horizontal eye movements, CN III/IV function; tone, tremor and bradykinesia (finger tapping), gait and pull test for parkinsonism.

Self-quiz

Q1.

A 15-year-old boy has headaches, cannot look upward, and his pupils constrict when he looks at a near object but not to light. MRI shows a mass above the superior colliculi with dilated lateral and third ventricles. What is the diagnosis?

Show answer

B

A pineal region tumor compressing the dorsal midbrain causes Parinaud syndrome (upgaze palsy, light–near dissociation, convergence–retraction nystagmus) and obstructive hydrocephalus at the aqueduct.

Q2.

A 66-year-old has a resting tremor of the right hand, cogwheel rigidity and slow finger tapping. Degeneration of which structure explains this?

Show answer

B

Parkinson disease results from loss of dopaminergic neurons in the substantia nigra pars compacta. Caudate atrophy is Huntington disease; a subthalamic lesion causes hemiballismus.

Q3.

Weber syndrome (ipsilateral CN III palsy with contralateral hemiparesis) results from a lesion where?

Show answer

B

Weber syndrome is a ventral (basal) midbrain infarct involving the CN III fascicles and the cerebral peduncle (corticospinal fibers). Dorsal midbrain lesions cause Parinaud syndrome.

Q4.

On MRI, which pattern of ventricular enlargement indicates aqueductal stenosis?

Show answer

D

Obstruction at the cerebral aqueduct dilates everything upstream (lateral and third ventricles) while the fourth ventricle stays normal, a noncommunicating hydrocephalus.

Q5.

Which midbrain area relays the pupillary light reflex to both Edinger–Westphal nuclei, producing the consensual response?

Show answer

A

The pretectal nuclei receive optic tract input and project to both Edinger–Westphal nuclei, so light in one eye constricts both pupils (direct and consensual responses).

Flashcards

See also