5.3 - The midbrain

Midbrain location

Superior brain stem segment between pons (inferior) and diencephalon (superior)

Anterior midbrain structures

Crus cerebri (cerebral peduncles) - descending motor tracts

Interpeduncular fossa - CN 3 exit point

Substantia nigra

Posterior midbrain structures (forms the quadrigemina)

Superior colliculi (visual reflex)

Inferior colliculi (auditory reflex)

Trochlear nerve (CN 4) exits BELOW inferior colliculi

Cranial nerves emerging from the midbrain

Oculomotor (CN 3) - ONLY motor neve exiting ventrally

Trochlear (CN 4) - ONLY nerve exiting dorsally

3 main divisions of midbrain cross-sections

Crus cerebri: descending motor tracts

Tegmentum: ascending motor tracts

Tectum: superior/inferior colliculi (visual/auditory reflex)

Gray matter structures in the midbrain

Motor nuclei: CN 3&4

Substantia nigra: dopamine production

Red nucleus: motor coordination

Periaqueductal gray: pain modulation

White matter tracts of midbrain

Crus cerebri: corticospinal

Tegmentum: medial lemniscus (touch), MLF (eye movements)

Tectum: inferior colliculus (auditory)

Midbrain syndrome (clinical conditions)

Weber syndrome: CN 3 palsy + contralateral hemiparesis

Parinaud syndrome: vertical gaze palsy (superior colliculus/tectal lesion)

Parkinson’s: Substantia nigra degeneration

Arteries that supply midbrain

Posterior cerebral artery (main)

Superior cerebellar artery

Thalamoperforating branches

Midbrain development

Develops from mesencephalon

Alar plate > Tectum (colliculi)

Basal plate > Tegmentum (motor nuclei)