Midbrain

Mesencephalon

Also known as: mesencephalon

The shortest and most rostral part of the brainstem (≈2 cm), passing through the tentorial notch; it contains the nuclei of CN III and IV, the red nucleus, substantia nigra and periaqueductal grey, and the colliculi for visual and auditory reflexes.

Overview

Length≈2 cm

Cranial nerve nucleiIII, IV (and mesencephalic V)

The midbrain connects the diencephalon to the pons and passes through the tentorial notch, where it is vulnerable to transtentorial herniation. It is traversed by the narrow cerebral aqueduct, which divides it into a dorsal tectum and a ventral part made up of the tegmentum and the paired cerebral peduncles (crura cerebri).

The tectum carries four swellings, the corpora quadrigemina:
• the superior colliculi, centres for visual orienting reflexes and saccades, with the adjacent pretectal area for the pupillary light reflex;
• the inferior colliculi, auditory relays to the medial geniculate.

The tegmentum contains:
• the oculomotor nucleus and Edinger–Westphal nucleus, at the level of the superior colliculus;
• the trochlear nucleus, at the level of the inferior colliculus;
• the mesencephalic nucleus of V;
• the red nucleus, which relays cerebellar output and is the origin of the rubrospinal tract;
• the periaqueductal grey (PAG), which controls pain and defensive behaviour;
• the ventral tegmental area, source of mesolimbic and mesocortical dopamine;
• the ascending lemnisci, the MLF and the reticular formation.

The substantia nigra lies between the tegmentum and the crus. Each crus carries corticospinal fibres in its middle three-fifths, flanked by corticopontine fibres. CN III emerges into the interpeduncular fossa between the crura, and CN IV emerges dorsally just below the inferior colliculi.

Location

Upper end of the brainstem, in the tentorial notch, between the diencephalon above and the pons below.

Parts & subdivisions

Tectum (corpora quadrigemina)

Superior colliculi (visual orienting, saccades) and inferior colliculi (auditory relay); pretectal nuclei (pupillary light reflex).

Tegmentum

CN III and IV nuclei, Edinger–Westphal nucleus, red nucleus, periaqueductal grey, VTA, medial lemniscus, MLF, reticular formation, decussation of the superior cerebellar peduncles.

Cerebral peduncles (crura cerebri)

Descending corticospinal, corticobulbar and corticopontine fibres.

Substantia nigra

Pigmented dopaminergic nucleus between the crus and tegmentum.

Red nucleus

Receives the contralateral dentate nucleus and the motor cortex; gives rise to the rubrospinal tract (flexor tone in the upper limb) and the rubro-olivary loop.

Periaqueductal grey

Descending pain inhibition (PAG → nucleus raphe magnus → dorsal horn; opioid-sensitive); defensive behaviour; vocalisation; micturition coordination.

Cerebral aqueduct

CSF channel from the third to the fourth ventricle.

Function

Eye movements (CN III, IV; vertical gaze centre — rostral interstitial nucleus of the MLF)

Pupillary light reflex and accommodation (pretectum, Edinger–Westphal)

Visual and auditory orienting reflexes (colliculi)

Motor control via the substantia nigra and red nucleus

Pain modulation and defensive responses (PAG)

Reward and motivation (VTA dopamine)

Arousal (upper reticular activating system)

Clinical significance

Weber and Benedikt syndromes

• Weber (ventral midbrain): ipsilateral oculomotor palsy (ptosis, "down and out" eye, dilated pupil) + contralateral hemiplegia.
• Benedikt (tegmentum, red nucleus): ipsilateral CN III palsy + contralateral tremor and ataxia.

Uncal herniation and the midbrain

A supratentorial mass pushes the uncus through the tentorial notch. It compresses the ipsilateral CN III first (dilated pupil), then the cerebral peduncle (hemiparesis), and finally the midbrain reticular formation (coma). Haemorrhages from stretched paramedian perforators (Duret) are often fatal.