The midbrain is the smallest division of the brainstem — about two centimeters long — but it is densely packed with structures that have outsized clinical importance. The substantia nigra (degeneration of which produces Parkinson disease) lies here. The red nucleus relays cerebellar information to the cortex. The third and fourth cranial nerves emerge here. The cerebral peduncles carry corticospinal fibers on their way to the cord. The dorsal midbrain houses the centers for vertical gaze, the pupillary reflex, and the auditory and visual reflexes. Lesions of the midbrain produce some of the most recognizable syndromes in neurology — Weber, Benedikt, Claude, Parinaud — each a small territory damaged with predictable consequences.

This page covers midbrain anatomy in cross section, the nuclei and tracts at each level, and the clinical syndromes produced by midbrain lesions. The point is to give the trainee a working map of a small but high-yield region.

Position and General Architecture

The midbrain (mesencephalon) lies between the diencephalon (above) and the pons (below). It transmits the cerebral aqueduct through its core, with structures arranged around the aqueduct in a recognizable plan:

Tectum (roof): dorsal to the aqueduct. Contains the superior and inferior colliculi.
Tegmentum (floor): between the aqueduct and the cerebral peduncles. Contains the cranial nerve nuclei, the red nucleus, the substantia nigra, the reticular formation, and the long ascending and descending tracts.
Cerebral peduncles (basis pedunculi): the ventral massive bundles carrying corticospinal, corticobulbar, and corticopontine fibers.

The midbrain is divided into two anatomical levels by the colliculi:

Superior collicular level: superior part of the midbrain, with CN III nucleus.
Inferior collicular level: inferior part, with CN IV nucleus.

The Tectum (Roof)

Superior Colliculi

Paired rounded prominences on the dorsal surface of the midbrain. They are visual reflex centers — they receive input from the retina (via the optic tract bypass to the colliculus), from the visual cortex, and from auditory and somatosensory systems. Their output drives:

Saccadic eye movements to visual targets (especially novel or surprising stimuli).
Head and neck movements via tectospinal projections.
Visual orienting responses.

The superior colliculi are also important for the pupillary light reflex (with relay through the pretectal area, adjacent to the colliculus) and for the accommodation reflex.

Inferior Colliculi

Paired prominences below the superior colliculi. They are the principal relay station for the auditory pathway — virtually all ascending auditory information converges here on its way to the medial geniculate nucleus of the thalamus. They participate in auditory localization, processing of complex sounds, and acoustic reflexes.

Quadrigeminal Plate and Pineal Gland

The four colliculi together form the quadrigeminal plate (corpora quadrigemina). The pineal gland sits in the midline just above the superior colliculi and is involved in melatonin secretion and circadian regulation. Pineal tumors (pinealomas, germinomas) commonly produce Parinaud syndrome by compressing the dorsal midbrain.

The Tegmentum at the Superior Collicular Level

At the level of the superior colliculi:

CN III (Oculomotor) Nucleus and Fascicles

The oculomotor nucleus sits in the periaqueductal gray, just ventral to the aqueduct. It contains:

Somatic motor subnuclei for the extraocular muscles (medial rectus, superior rectus, inferior rectus, inferior oblique) and levator palpebrae superioris.
Edinger-Westphal nucleus: the parasympathetic subnucleus, providing preganglionic fibers to the ciliary ganglion for pupillary constriction and accommodation.

The fascicles of CN III pass ventrally through the red nucleus and the medial portion of the cerebral peduncle to emerge from the interpeduncular fossa.

Red Nucleus

A large, round, pink nucleus (named for its iron-rich pink color in fresh tissue) in the central tegmentum. It receives input from the cerebellum (via the superior cerebellar peduncle, which decussates immediately below the red nucleus) and from the cerebral cortex. Its output goes via the rubrospinal tract (small in humans) and to the inferior olive. The red nucleus is part of the cerebellar outflow circuit. Lesions involving the red nucleus produce Holmes (rubral) tremor — a coarse, low-frequency tremor with rest, postural, and intention components.

Substantia Nigra

A dark-pigmented nucleus (named for its melanin-containing dopaminergic neurons) in the ventral tegmentum, just dorsal to the cerebral peduncle. Divided into:

Pars compacta (SNc): dopaminergic neurons projecting to the striatum (the nigrostriatal pathway). Loss of these neurons defines Parkinson disease.
Pars reticulata (SNr): GABAergic neurons that are part of the basal ganglia output, functionally equivalent to the globus pallidus internus.

Ventral Tegmental Area (VTA)

Medial to the substantia nigra. Contains dopaminergic neurons projecting to limbic structures (mesolimbic pathway) and to prefrontal cortex (mesocortical pathway). Critical for reward, motivation, and addiction.

Medial Longitudinal Fasciculus (MLF)

Runs through the dorsal midbrain near the midline. Connects the abducens nucleus (in the pons) with the contralateral CN III medial rectus subnucleus, coordinating horizontal gaze. MLF lesions produce internuclear ophthalmoplegia (INO): ipsilesional eye fails to adduct on attempted contralateral gaze, with abducting nystagmus of the contralateral eye, and preserved convergence.

Other Tracts in the Midbrain Tegmentum

Medial lemniscus: carries dorsal column information to the thalamus.
Spinothalamic tract: carries pain and temperature.
Reticular formation: continues from the medulla and pons.
Posterior longitudinal fasciculus: descending fibers from the hypothalamus.

The Tegmentum at the Inferior Collicular Level

At the level of the inferior colliculi:

CN IV (Trochlear) Nucleus and Fascicles

The trochlear nucleus sits in the periaqueductal gray, dorsal to the medial longitudinal fasciculus. Its fascicles take an unusual course: they curve dorsally around the central gray, decussate in the anterior medullary velum, and emerge from the dorsal surface of the brainstem — the only cranial nerve to exit dorsally and the only one to decussate before exit. The right superior oblique is therefore innervated by the left trochlear nucleus.

Decussation of the Superior Cerebellar Peduncle

The cerebellar outflow fibers (dentatorubrothalamic tract) decussate at this level, on their way from the cerebellum to the contralateral red nucleus and thalamus. Lesions of the superior cerebellar peduncle below the decussation produce ipsilateral cerebellar signs; lesions above the decussation produce contralateral cerebellar signs.

Trigeminal Mesencephalic Nucleus

A unique nucleus extending up from the principal sensory nucleus, containing the cell bodies of primary sensory neurons (the only such cells with somas within the CNS rather than in peripheral ganglia). Mediates jaw proprioception and the jaw jerk reflex.

The Cerebral Peduncles (Basis Pedunculi)

The ventral surface of the midbrain bulges outward as the cerebral peduncles, which carry the major descending fibers from cortex to lower CNS structures:

Corticobulbar fibers: most medially. Project to brainstem cranial nerve motor nuclei.
Corticospinal fibers: in the middle three-fifths of the peduncle. Project to the spinal cord. Somatotopically arranged with arm fibers medial, leg fibers lateral.
Corticopontine fibers: laterally and medially. Frontopontine fibers medially, temporopontine, parietopontine, and occipitopontine fibers laterally. Project to the pontine nuclei (relay for the cerebellar input from cortex).

The somatotopic arrangement matters clinically: a lesion of the medial cerebral peduncle can selectively affect face (corticobulbar) more than limb fibers; a lesion of the lateral peduncle can affect leg more than arm.

The Cerebral Aqueduct

The cerebral aqueduct (of Sylvius) connects the third ventricle (above) with the fourth ventricle (below). It is surrounded by the periaqueductal gray, a region with diverse functions including descending pain modulation (involved in opioid analgesia), defensive behaviors, and vocalization. The aqueduct is the narrowest portion of the ventricular system and a common site of obstructive hydrocephalus (aqueductal stenosis).

Vascular Supply

The midbrain receives blood from:

Paramedian (thalamoperforating) arteries: from the P1 segment of the PCA, supplying the medial midbrain including the CN III nucleus and fascicles, red nucleus, and medial cerebral peduncle.
Circumferential arteries: short and long, from the PCA, supplying the lateral midbrain.
Superior cerebellar artery branches: supplying parts of the dorsal midbrain.
Quadrigeminal arteries: from the PCA, supplying the colliculi and adjacent regions.

Bilateral paramedian midbrain infarction (often together with bilateral paramedian thalamic infarction, in the artery of Percheron variant) produces a striking syndrome of altered consciousness, vertical gaze palsy, and sometimes bilateral CN III palsies.

Midbrain Syndromes

Weber Syndrome

Ventral midbrain infarction involving the CN III fascicles and the corticospinal fibers of the medial cerebral peduncle. Clinical picture:

Ipsilateral CN III palsy (down-and-out eye, pupil dilated and unresponsive, ptosis).
Contralateral hemiparesis (face spared if corticobulbar fibers are not involved).

Benedikt Syndrome

Tegmental midbrain lesion involving CN III fascicle, red nucleus, and (variably) medial lemniscus and corticospinal tract:

Ipsilateral CN III palsy.
Contralateral choreoathetosis or tremor (red nucleus involvement, Holmes tremor).
Sometimes contralateral hemibody sensory loss (medial lemniscus).

Claude Syndrome

A small midbrain lesion involving the CN III fascicle and the superior cerebellar peduncle (already decussated):

Ipsilateral CN III palsy.
Contralateral cerebellar ataxia.

Nothnagel Syndrome

Lesion at the level of the superior cerebellar peduncle affecting CN III fascicles and the cerebellar outflow:

Unilateral or bilateral CN III palsy.
Cerebellar ataxia.

Parinaud (Dorsal Midbrain) Syndrome

Dorsal midbrain lesion involving the rostral interstitial nucleus of the MLF (vertical gaze center), the posterior commissure, and the pretectal area:

Upgaze palsy: paralysis of upward gaze.
Light-near dissociation: pupils respond to near but not to light (pretectal involvement).
Convergence-retraction nystagmus: on attempted upgaze.
Lid retraction (Collier sign).

Classical cause: pineal region tumor compressing the dorsal midbrain. Other causes: midbrain stroke, MS plaque, hydrocephalus (causing pressure on the dorsal midbrain).

Locked-In Syndrome (Midbrain Component)

Although classically a ventral pontine lesion, the locked-in state can occur with midbrain involvement, particularly when both cerebral peduncles are damaged bilaterally. The patient is conscious but unable to move except for vertical eye movements (preserved supranuclear control of vertical gaze through the rostral interstitial nucleus of the MLF).

Internuclear Ophthalmoplegia (INO)

Lesion of the medial longitudinal fasciculus. On attempted contralateral gaze, the ipsilesional eye fails to adduct, with abducting nystagmus of the contralateral eye. Convergence is preserved. Bilateral INO in a young patient suggests multiple sclerosis; unilateral INO in an older patient suggests brainstem stroke.

🔍 Did You Know?

The cerebral peduncle was the surgical target for the original “cordotomy”-type procedure for severe contralateral pain — mesencephalic tractotomy. Lesioning the spinothalamic tract as it ascends through the lateral midbrain tegmentum produced contralateral analgesia. The procedure has largely been abandoned in favor of less invasive modern pain treatments, but the anatomical basis — the spinothalamic tract lies just lateral to the medial lemniscus in the midbrain tegmentum — illustrates an important principle: long ascending tracts can be selectively lesioned at brainstem levels with predictable functional consequences, and the patterns of brainstem syndromes from stroke reflect the same anatomical organization.

Pitfalls and Pearls

The midbrain is small but densely packed: substantia nigra, red nucleus, CN III/IV, cerebral peduncles, vertical gaze centers, and major ascending/descending tracts all fit within about 2 cm.
CN IV is unique in two ways: it decussates within the brainstem and exits dorsally. A nuclear CN IV lesion affects the contralateral eye.
The CN III fibers run through the red nucleus and cerebral peduncle on their way out. This explains the combinations in Weber, Benedikt, and Claude syndromes.
Parinaud syndrome is dorsal midbrain. Look for upgaze palsy, light-near dissociation, convergence-retraction nystagmus, and lid retraction.
Pineal region tumors classically cause Parinaud syndrome by compressing the dorsal midbrain.
The substantia nigra pars compacta loss is Parkinson disease. The dopaminergic neurons project to the striatum.
The cerebral aqueduct is the narrowest part of the ventricular system. Aqueductal stenosis causes non-communicating hydrocephalus.
Bilateral midbrain stroke from artery of Percheron occlusion can produce altered consciousness with vertical gaze palsy — easily mistaken for metabolic encephalopathy.
Holmes (rubral) tremor reflects red nucleus / cerebellar outflow injury and has all three tremor components (rest, postural, intention).
Periaqueductal gray is involved in descending pain modulation and is one of the targets of endogenous opioid analgesia.

References

Brazis PW, Masdeu JC, Biller J. Localization in Clinical Neurology. 7th ed. Wolters Kluwer; 2017.
Naidich TP, Duvernoy HM, Delman BN, et al. Duvernoy’s Atlas of the Human Brain Stem and Cerebellum. Springer; 2009.
Haines DE. Neuroanatomy in Clinical Context. 9th ed. Wolters Kluwer; 2015.
Mtui E, Gruener G, Dockery P. FitzGerald’s Clinical Neuroanatomy and Neuroscience. 8th ed. Elsevier; 2020.
Standring S, ed. Gray’s Anatomy. 42nd ed. Elsevier; 2021.
Liu GT, Crenner CW, Logigian EL, et al. Midbrain syndromes of Benedikt, Claude, and Nothnagel: setting the record straight. Neurology. 1992;42(9):1820-1822.