Midbrain Anatomy

Midbrain

• Sectional and regional anatomy, modulatory nuclei of brainstem, midbrain perfusion, vascular syndromes, and herniation are all key topics
• The midbrain is rostral to parts of the brainstem
• Each of these brain regions (diencephalon and midbrain) contain many distinct nuclei
• Diencephalon is the thalamus and is not considered part of the brainstem
• The midbrain tectum contains the corpora quadrigemina (superior and inferior colliculi)
• The midbrain tegmentum has modulatory and cranial nerve nuclei, as well as fiber tracts
• The basilar midbrain is made up of cerebral peduncles, also known as crus cerebri
• Inferior colliculus and lateral lemniscus are located in the caudal midbrain
• Anterolateral and medial lemniscus also reside in the caudal midbrain
• Other key structures in the caudal midbrain are the substantia nigra, reticular nuclei, and raphe nuclei
• The mesencephalic nucleus (V) is located there
• Superior cerebellar peduncle decussation, rubrospinal tract, corticospinal tract, and corticobulbar/nuclear fibers are caudal midbrain features
• Additionally, the trochlear nucleus (IV), cerebral aqueduct, periaqueductal gray, and MLF are key
• Rostral midbrain contains the superior colliculus, anterolateral system, and medial lemniscus
• The region contains the substantia nigra, reticular nuclei, and raphe nuclei
• Similar to the caudal midbrain, the mesencephalic nucleus and superior cerebellar peduncle are significant
• The red nucleus, corticospinal tract, oculomotor nucleus, cerebral aqueduct, periaqueductal gray, and MLF are observed

Modulatory Nuclei of the Brainstem

• The dopamine nuclei include the substantia nigra pars compacta and ventral tegmental nucleus, located in the midbrain
• Dopamine is involved with movement, as well as working memory, mood, and anxiety
• The serotonin nuclei are the raphe nuclei, associated with regulating sleep-wake cycles, mood, and appetite
• The reticular system has ascending (arousal) and descending (motor) components
• Norepinephrine arises from the locus coeruleus, situated in the lateral tegmentum of the pons
• Norepinephrine is involved with arousal and attention, and pain modulation

Midbrain Perfusion

• The superior cerebellar artery supplies the superior colliculus and oculomotor nucleus
• The quadrigeminal artery supplies the periaqueductal gray and anterolateral system
• The medial lemniscus and red nucleus are supplied
• The substantia nigra, crus cerebri/cerebral peduncle and decussation of the superior cerebellar peduncle are supplied

Midbrain Lesions

• Weber syndrome is characterized by oculomotor palsy, ptosis, mydriasis (dilated pupil), and contralateral hemiplegia (corticospinal)
• Tongue protrusion is deviated (corticobulbar)
• Claude syndrome results in oculomotor palsy, ptosis, mydriasis - dilated pupil and contralateral tremor that affects the red nucleus
• There is cerebellar ataxia affecting the cerebellar peduncle

Herniation

• Central herniation occurs when the thalamic tissue is pushed downward through the tentorial notch
• Upward cerebellar herniation involves cerebellar tissue being pushed upward through the tentorial notch
• Tonsillar herniation is when cerebellar tissue is pushed through the foramen magnum and results in medullary compression
• Complications include lethargy, vomiting, headache, blackouts, cardiac dysfunction, respiratory dysfunction, tachypnea, and apnea
• Uncal herniation happens when the medial temporal lobe (uncus) is pushed over the tentorium cerebelli, which causes midbrain compression
• Symptoms include eye-motor palsy, pupillary dysfunction, visual deficits, hemi-paresis or hemi-plegia

Diencephalon

• The epithalamus includes the habenula and pineal gland and regulates circadian rhythm.
• The dorsal thalamus relays sensory information as a gateway to the cortex
• The ventral thalamus includes the subthalamic nucleus and pre-rubral area and controls motor function
• The hypothalamus regulates autonomic NS, regulates anterior pituitary and releases oxytocin and ADH
• The hypothalamus responds to chemicals and O2 levels in blood
• Dorsal thalamic sensory nuclei are VPL (ALS, DCML), VPM (trigeminal system), as well as the lateral and medial geniculate
• These are fiber tracts in the Internal Capsule

Thalamic Perfusion

• The thalamo-geniculate, thalamo-perforating, posterior choroidal, anterior choroidal, and medial choroidal arteries supply blood to the thalamus
• Key components of the internal capsule are the anterior limb, posterior limb, and genu
• The lenticulostriate arteries (off M₁ MCA), medial striate, and anterior choroidal arteries help perfuse