midbrain

**Embryology:** Derived from the mesencephalon (primary vesicle), which does not subdivide into secondary vesicles. Its cavity becomes the cerebral aqueduct (of Sylvius), connecting the 3rd and 4th ventricles.

**Gross divisions (dorsal to ventral):** 1. **Tectum**: Superior colliculi (visual reflexes, saccades/conjugate gaze; the afferent limb of the pupillary light reflex goes via the pretectal nucleus) and inferior colliculi (auditory relay to the MGN of the thalamus). 2. **Tegmentum**: CN III (oculomotor) nucleus at the level of the superior colliculus; CN IV (trochlear) nucleus at the level of the inferior colliculus; Edinger-Westphal nucleus (parasympathetic: pupillary constriction, accommodation); red nucleus (motor coordination, rubrospinal tract); periaqueductal gray (PAG; pain modulation, descending analgesia); reticular formation (arousal); medial lemniscus, spinothalamic tract, and the MLF. 3. **Substantia nigra**: Pars compacta (dopaminergic, degenerates in Parkinson disease; part of the nigrostriatal pathway) and pars reticulata (GABAergic output). The ventral tegmental area (VTA) provides the mesolimbic/mesocortical dopamine pathways (reward, psychosis). 4. **Cerebral peduncles (crus cerebri)**: Contain corticospinal, corticopontine, and corticobulbar fibers.

**Blood supply:** Posterior cerebral artery (PCA) branches, superior cerebellar artery (SCA), and basilar artery perforators (paramedian branches). The interpeduncular fossa is pierced by perforators.

**Cranial nerves:** CN III exits ventrally via the interpeduncular fossa. CN IV is the only cranial nerve to exit dorsally and it decussates, so each CN IV nucleus innervates the contralateral superior oblique.

**Clinical correlations:** - **Weber syndrome** (ventral midbrain, PCA branch infarct): Ipsilateral CN III palsy ("down and out" eye, ptosis, mydriasis) + contralateral hemiparesis (cerebral peduncle, corticospinal tract involvement before decussation). - **Benedikt syndrome**: CN III palsy + contralateral tremor/ataxia (red nucleus involvement). - **Claude syndrome**: CN III palsy + contralateral ataxia (red nucleus without peduncle involvement). - **Parinaud (dorsal midbrain) syndrome**: Vertical gaze palsy (especially upgaze), light-near dissociation, convergence-retraction nystagmus, eyelid retraction (Collier sign). Causes: pineal tumors (germinoma), hydrocephalus (aqueductal stenosis), stroke. - **Parkinson disease**: Loss of dopaminergic neurons in the substantia nigra pars compacta, with Lewy bodies (α-synuclein). Features are resting tremor, rigidity, bradykinesia, and postural instability. - **Transtentorial (uncal) herniation**: Compresses CN III (blown pupil, "down and out" eye) and the midbrain. Duret hemorrhages (paramedian midbrain/pontine) are a late sign. Kernohan notch phenomenon gives ipsilateral hemiparesis (false localizing sign) from compression of the contralateral cerebral peduncle. - **Aqueductal stenosis**: Obstructive (non-communicating) hydrocephalus with dilated lateral and 3rd ventricles and a normal 4th ventricle. - **Decorticate vs decerebrate posturing**: Decerebrate (extensor) posturing implies a lesion at or below the red nucleus (midbrain/upper pons). Decorticate (flexor) posturing implies a lesion above the red nucleus. - **Opioid/analgesia**: The PAG is a key site of opioid action in descending pain modulation.

**High-yield mnemonic:** "Rule of 4s" for the brainstem: 4 CNs in the medulla (IX–XII), 4 in the pons (V–VIII), 2 in the midbrain (III, IV), and 2 above the brainstem (I, II). The midbrain is at the level of CN III and IV.