Three Brainstem Levels
Midbrain, pons, and medulla — distinct functions at each level
The brainstem connects the cerebral hemispheres to the spinal cord. It consists of three parts rostral to caudal: midbrain (mesencephalon), pons, and medulla oblongata. The reticular formation runs throughout, regulating arousal and vital autonomic functions.
The brainstem houses cranial nerve nuclei III through XII. Brainstem lesions produce crossed syndromes: ipsilateral cranial nerve deficit (from the nucleus at that level) + contralateral motor/sensory deficit (from tracts that cross above or below).
💡 Brainstem Syndromes — Crossed Deficits
Brainstem lesions produce crossed syndromes: ipsilateral CN deficit + contralateral motor/sensory deficits.
Wallenberg syndrome (lateral medullary): PICA occlusion. Ipsilateral: facial numbness (CN V), Horner syndrome, hoarseness/dysphagia (CN IX/X), limb ataxia. Contralateral: body pain/temp loss (spinothalamic tract). Classic crossed sensory loss.
Weber syndrome (midbrain): CN III palsy ipsilateral (blown pupil, eye down-out) + contralateral hemiplegia (cerebral peduncle).
Locked-in syndrome (bilateral ventral pons): Bilateral corticospinal/corticobulbar destruction → quadriplegia and anarthria. Reticular activating system intact → fully conscious. Patient communicates only by blinking.
Mid
Midbrain — eye movement and dopamine
Key midbrain structures:
CN III nucleus (oculomotor): Controls most extraocular muscles and pupillary constrictor (parasympathetic). CN III palsy: eye deviated down and out (unopposed CN IV/VI), ptosis, mydriasis (blown pupil). A blown pupil with CN III palsy = posterior communicating artery aneurysm compressing CN III — neurosurgical emergency.
CN IV (trochlear): Controls superior oblique — depresses and intorts the eye. Only CN exiting dorsally. Palsy: vertical diplopia worse going downstairs, patient tilts head away from affected side.
Substantia nigra: Dopaminergic nigrostriatal neurons. Degeneration = Parkinson disease.
Superior/inferior colliculi: Visual and auditory reflexes.
Memory trick: Midbrain = CN III + IV + substantia nigra (Parkinson) + colliculi. CN III palsy = eye down-out + ptosis + blown pupil. Blown pupil = posterior comm artery aneurysm until proven otherwise.
Pons
Pons — facial expression, sensation, and hearing
Key pontine structures:
CN V (trigeminal): Face sensation (V1/V2/V3) and motor to muscles of mastication (V3). Trigeminal neuralgia = severe lancinating V2/V3 pain triggered by light touch; treatment: carbamazepine.
CN VI (abducens): Lateral rectus — abducts eye. Longest intracranial course → most susceptible to raised ICP (false localizing sign). Palsy: eye deviated medially.
CN VII (facial): Facial expression, taste anterior 2/3 tongue, lacrimal/salivary glands. Bell palsy: peripheral CN VII palsy → complete ipsilateral facial paralysis including forehead. Central CN VII lesion → forehead spared (bilateral cortical innervation of upper face).
CN VIII (vestibulocochlear): Hearing and balance. Acoustic neuroma: CN VIII tumor → unilateral sensorineural hearing loss, tinnitus, balance problems.
Memory trick: Pons = CN V, VI, VII, VIII. Bell palsy = peripheral VII = forehead drops too. Central VII = forehead spared. CN VI = lateral gaze; raised ICP = false localizing CN VI palsy.
Med
Medulla — vital autonomic centers
The medulla contains the most vital structures in the brain:
Cardiorespiratory centers: Dorsal respiratory group generates breathing rhythm. Cardiac centers regulate heart rate via vagus nerve. Medullary damage = apnea, cardiac arrest, death.
CN IX (glossopharyngeal): Posterior tongue taste, pharyngeal sensation (gag reflex afferent), parasympathetic to parotid gland.
CN X (vagus): Parasympathetic to heart (bradycardia), lungs, and GI tract. Motor to pharynx and larynx (voice). Uvula deviates AWAY from CN X lesion.
CN XI (accessory): Motor to SCM and trapezius. Palsy: cannot turn head away from lesion, shoulder drop.
CN XII (hypoglossal): Motor to tongue. LMN palsy: tongue deviates TOWARD the lesion on protrusion.
Pyramidal decussation: Corticospinal tracts cross here — explaining contralateral motor deficits from cortical lesions.
Memory trick: Medulla = IX, X, XI, XII + cardiorespiratory centers = potentially fatal to damage. CN X = vagus = voice + viscera. Uvula AWAY from lesion. CN XII = tongue TOWARD lesion (LMN). Pyramids cross here.
🔬 Clinical Scenario — Brainstem Lesions
Brainstem lesions produce precise, localizable syndromes:
A
Uncal herniation. Expanding supratentorial mass → temporal lobe uncus compresses CN III → ipsilateral blown pupil (first sign) → then midbrain compression → coma → death. Blown pupil in a deteriorating patient = neurosurgical emergency requiring immediate decompression.
B
Basilar artery occlusion. Supplies pons and midbrain. Occlusion → bilateral pontine ischemia → locked-in syndrome or coma → nearly universally fatal without treatment. Can be treated with tPA or thrombectomy beyond the usual 4.5-hour window given the catastrophic natural history.
C
Bell palsy. Most common acute facial paralysis — usually HSV-1 reactivation. Complete ipsilateral facial paralysis including forehead (distinguishes from central/UMN lesion). Treatment: oral prednisone + valacyclovir within 72 hours. Eye protection critical — corneal exposure risk.
D
Posterior fossa tumors in children. Children: posterior fossa (brainstem + cerebellum) is the most common tumor location (unlike adults — supratentorial). Medulloblastoma (midline, malignant) and pilocytic astrocytoma (cerebellar, cystic, low-grade) are most common. Morning headache/vomiting from CSF obstruction at 4th ventricle.
📌 Exam Application
1. Midbrain: CN III (eye + pupil — blown pupil = PComm aneurysm), CN IV (superior oblique), substantia nigra (Parkinson).
2. Pons: CN V (face sensation + mastication), CN VI (lateral gaze), CN VII (facial expression — Bell palsy), CN VIII (hearing/balance).
3. Medulla: CN IX–XII, cardiorespiratory centers, pyramidal decussation. Wallenberg = PICA occlusion.
4. Crossed syndromes: Ipsilateral CN + contralateral motor/sensory = brainstem lesion.
5. Bell palsy: Peripheral CN VII = forehead involved. Central CN VII lesion = forehead spared.
⚠️ Most Common Brainstem Mistakes
Bell palsy affects entire face INCLUDING forehead. Upper face receives bilateral cortical innervation — so a central lesion (stroke) spares the forehead (the intact opposite hemisphere still drives the upper face). A peripheral CN VII lesion (at the nucleus or nerve itself) produces complete ipsilateral facial paralysis including forehead drooping. This is the most important facial nerve distinction.
CN XII tongue deviates TOWARD the LMN lesion. The intact genioglossus on the opposite side pushes the tongue toward the weak side. Uvula (CN X) deviates AWAY from the lesion.
Reticular activating system is in the BRAINSTEM pons/midbrain. Locked-in syndrome: bilateral ventral pons destroyed (motor output gone) but RAS intact (patient fully conscious). This demonstrates that consciousness requires the brainstem RAS to be intact.
✓ Quick Self-Test
1. Three brainstem levels and their main cranial nerves?
2. What is a crossed brainstem syndrome?
3. What is Wallenberg syndrome and what causes it?
4. How do you distinguish Bell palsy from a central facial nerve lesion?
5. What is locked-in syndrome?
Answers:
1. Midbrain: CN III, IV. Pons: CN V, VI, VII, VIII. Medulla: CN IX, X, XI, XII.
2. Ipsilateral cranial nerve deficit (from the nucleus at the level of the lesion) combined with contralateral motor or sensory deficit (from descending/ascending tracts that cross above or below the lesion level). This pattern localizes the lesion to the brainstem.
3. Wallenberg syndrome (lateral medullary syndrome) from PICA (posterior inferior cerebellar artery) occlusion: ipsilateral facial numbness (CN V), Horner syndrome, hoarseness/dysphagia (CN IX/X), and limb ataxia; contralateral body pain/temperature loss (spinothalamic tract). Classic crossed sensory pattern.
4. Bell palsy (peripheral CN VII): complete ipsilateral facial paralysis including forehead — because the lesion is at the nucleus or nerve before bilateral cortical input becomes relevant. Central facial palsy (UMN lesion/stroke): contralateral lower face only — forehead spared because the intact ipsilateral hemisphere still drives the upper face via bilateral cortical innervation.
5. Locked-in syndrome from bilateral ventral pontine infarction (usually basilar artery occlusion): bilateral corticospinal and corticobulbar tract destruction → quadriplegia and inability to speak. The reticular activating system (dorsal pons/midbrain) is intact → patient is fully conscious and aware. Can only communicate via vertical eye movements and blinking.