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Basic Science
Anatomy
13 notes available
1 Last Minute Review 2 Embryology & Developmental Anatomy 3 Vascular Anatomy 4 Cerebral Cortex 5 Subcortical Nuclei (BG, Thalamus, Hypothalamus) 6 Limbic System 7 Cerebellum 8 Brainstem 9 Cranial Nerves 10 Spinal Cord 11 Motor System 12 Sensory System 13 Peripheral Nerves and Muscles
Basic Science › Anatomy

Brainstem

Brainstem Anatomy & Syndromes

What You'll Learn

  • Internal organization of each brainstem level (midbrain, pons, medulla) — ventral-to-dorsal layers and key structures at each cross-section
  • Rule of 4s — the single most efficient framework for brainstem localization on boards
  • Major tract locations at each level — corticospinal, medial lemniscus, spinothalamic, MLF, sympathetic pathways
  • Classic named brainstem stroke syndromes — Weber, Benedikt, Claude, Millard-Gubler, Foville, Wallenberg, medial medullary, locked-in → know artery, structures, and clinical features for each
  • "Crossed" findings — ipsilateral CN deficit + contralateral long-tract signs = brainstem
  • Brainstem reflexes — tested in coma and brain death examinations
  • Key neuro-ophthalmology localizations — INO, one-and-a-half syndrome, Parinaud syndrome, ocular bobbing
HighYield Pearls
  • Rule of 4s — Medial 4 M’s: Motor (CST) + Medial lemniscus + MLF + Motor CN nuclei (III, IV, VI, XII) → supplied by paramedian perforators; lesion = contralateral hemiparesis + contralateral lemniscal loss + INO + ipsilateral CN palsy
  • Rule of 4s — Lateral 4 S’s: Spinothalamic + Spinal trigeminal + Sympathetic + Spinocerebellar → supplied by circumferential arteries (PICA, AICA, SCA); lesion = contralateral body pain/temp + ipsilateral face pain/temp + ipsilateral Horner + ipsilateral ataxia
  • Crossed findings = brainstem: ipsilateral CN deficit + contralateral long-tract sign localizes to brainstem at the level of the CN nucleus
  • CN IV is the odd one: only CN that exits DORSALLY and supplies the CONTRALATERAL superior oblique — trochlear nerve palsy → vertical diplopia worse on downgaze, head tilt away from lesion
  • Wallenberg (lateral medullary, PICA/vertebral): ipsi face pain/temp loss + ipsi Horner + ipsi ataxia + hoarseness/dysphagia (nucleus ambiguus) + vertigo/nystagmus + CONTRA body pain/temp; NO weakness, NO tongue involvement
  • Medial medullary (Dejerine, anterior spinal): contra hemiparesis + contra lemniscal loss + IPSI tongue deviation (CN XII)
  • Weber vs Claude vs Benedikt: all have ipsi CN III; Weber adds contra hemiparesis (crus), Claude adds contra ataxia (red nucleus), Benedikt = ipsi CN III + contra hemiparesis + contra ataxia (combined)
  • Millard-Gubler vs Foville: both ventral pons with ipsi CN VI + CN VII LMN + contra hemiparesis; Foville adds ipsi horizontal gaze palsy (PPRF)
  • Locked-in syndrome: ventral pons (basilar) → quadriplegia + anarthria with PRESERVED consciousness; only vertical eye movements + blinking spared
  • INO (MLF lesion): impaired ADDuction of ipsilateral eye + abducting nystagmus of contralateral eye + PRESERVED convergence; bilateral → MS in young, unilateral → vascular in elderly
  • One-and-a-half syndrome: paramedian pontine lesion of PPRF/CN VI nucleus + MLF → ipsi horizontal gaze palsy + ipsi INO; only contralateral abduction preserved
  • Parinaud (dorsal midbrain) syndrome: upgaze palsy + convergence-retraction nystagmus + light-near dissociation + Collier sign (lid retraction); think pineal germinoma in young, aqueductal stenosis, midbrain stroke
  • Gaze deviation rule: cortical (frontal eye field) lesion → eyes deviate TOWARD lesion; pontine destructive lesion → eyes deviate AWAY from lesion; seizure → eyes deviate AWAY from epileptic focus
  • DCML vs spinothalamic crossing: medial lemniscus crosses in CAUDAL MEDULLA (internal arcuate fibers); spinothalamic crosses at SPINAL CORD level near entry; corticospinal crosses at PYRAMIDAL DECUSSATION (cervicomedullary junction, 85-90%)
  • Top-of-basilar syndrome: embolic occlusion → bilateral PCA + thalami + midbrain → visual, behavioral, oculomotor findings; classic combo of cortical blindness + memory loss + vertical gaze palsy
  • AICA vs PICA vs SCA: AICA = lateral pons + ipsi CN VII LMN + ipsi hearing loss/vertigo (labyrinthine artery); PICA = Wallenberg + cerebellar dysmetria; SCA = superior cerebellum + ipsi ataxia + ipsi Horner
🔍 Quick ReferenceAnatomy / CN nuclei · Tract / function · Stroke syndromes
Anatomy / CN nuclei
  • Edinger-Westphal nucleus → CN III parasympathetic → pupillary constriction + accommodation (midbrain, near aqueduct)
  • Trochlear nucleus (CN IV) → only CN exiting DORSALLY + crossing to supply CONTRALATERAL superior oblique (caudal midbrain at inferior colliculus)
  • Substantia nigra pars compacta → dopaminergic nigrostriatal pathway → Parkinson disease when depleted (midbrain)
  • Red nucleus → rubrospinal tract + cerebellar relay → Claude/Benedikt syndromes when infarcted (midbrain tegmentum)
  • Periaqueductal gray (PAG) → endogenous opioid analgesia + autonomic + vocalization (surrounds cerebral aqueduct)
  • Superior colliculus → visual reflexes + saccade generation (rostral midbrain tectum)
  • Inferior colliculus → auditory relay → MGN → primary auditory cortex (caudal midbrain tectum)
  • Pretectum + Edinger-Westphal → pupillary light reflex afferent + efferent loop (midbrain)
  • Rostral interstitial nucleus of MLF (riMLF) + interstitial nucleus of Cajal → vertical gaze center (rostral midbrain)
  • Abducens nucleus (CN VI) + genu of facial nerve → facial colliculus on floor of 4th ventricle (caudal pons)
  • PPRF (paramedian pontine reticular formation) → horizontal gaze center → ipsilateral conjugate gaze (pons, near CN VI nucleus)
  • Locus coeruleus → norepinephrine source for cortex + arousal (dorsal rostral pons)
  • Raphe nuclei → serotonin source → mood + sleep + analgesia (midline brainstem)
  • Nucleus tractus solitarius (NTS) → visceral afferents + taste from CN VII/IX/X (anterior 2/3 via geniculate, posterior 1/3 via CN IX) (medulla)
  • Nucleus ambiguus → motor to palate/pharynx/larynx via CN IX, X, XI → hoarseness, dysphagia in Wallenberg (lateral medulla)
  • Dorsal motor nucleus of vagus → parasympathetic to thoracoabdominal viscera (medulla)
  • Hypoglossal nucleus (CN XII) → tongue motor → deviates TOWARD side of lesion (medial medulla, near midline)
  • Inferior olivary nucleus → climbing fibers to contralateral cerebellum → hypertrophic olivary degeneration + palatal myoclonus (medulla)
Tract / function
  • Corticospinal tract (pyramids) → descending voluntary motor → crosses at pyramidal decussation (cervicomedullary junction)
  • Medial lemniscus (DCML) → fine touch + vibration + proprioception → crosses in CAUDAL MEDULLA as internal arcuate fibers
  • Spinothalamic tract → pain + temperature + crude touch → crosses at SPINAL CORD level near entry (anterior white commissure)
  • Spinal trigeminal nucleus + tract → IPSILATERAL face pain/temperature → extends from pons through medulla to upper cervical cord
  • Medial longitudinal fasciculus (MLF) → connects CN VI nucleus to contralateral CN III → conjugate horizontal gaze; lesion = INO
  • Rubrospinal tract → red nucleus → flexor tone in upper extremities (decorticate posturing if lesion rostral to red nucleus)
  • Sympathetic descending fibers → ipsilateral Horner syndrome (miosis, ptosis, anhidrosis) when interrupted in lateral brainstem
  • Middle cerebellar peduncle → pontine nuclei → contralateral cerebellum (cortico-ponto-cerebellar circuit)
  • Olivocerebellar tract → inferior olive → contralateral cerebellum via inferior cerebellar peduncle (climbing fibers)
Brainstem stroke syndromes
  • Wallenberg (lateral medullary) → PICA/vertebral → ipsi face pain/temp + ipsi Horner + ipsi ataxia + hoarseness/dysphagia + vertigo/nystagmus + CONTRA body pain/temp; no weakness
  • Dejerine (medial medullary) → anterior spinal artery → contra hemiparesis + contra lemniscal loss + IPSI tongue deviation (CN XII)
  • Weber syndrome → medial midbrain (PCA paramedian) → ipsi CN III + contra hemiparesis (crus cerebri)
  • Claude syndrome → midbrain tegmentum → ipsi CN III + contra ataxia (red nucleus/SCP)
  • Benedikt syndrome → midbrain → ipsi CN III + contra hemiparesis + contra ataxia + involuntary movements
  • Millard-Gubler syndrome → ventral pons → ipsi CN VI + ipsi CN VII LMN + contra hemiparesis
  • Foville syndrome → caudal pons → ipsi CN VI + ipsi CN VII LMN + ipsi horizontal gaze palsy (PPRF) + contra hemiparesis
  • Locked-in syndrome → ventral pons (basilar) → quadriplegia + anarthria + PRESERVED consciousness; only vertical eye/blink spared
  • Top-of-basilar syndrome → embolic → bilateral PCA + thalami + rostral midbrain → cortical blindness + memory loss + vertical gaze palsy + behavioral changes
  • AICA syndrome → lateral pons + labyrinthine → ipsi CN VII LMN + ipsi hearing loss/vertigo + ipsi face sensory loss + contra body pain/temp
  • SCA syndrome → superior cerebellum + rostral lateral pons → ipsi ataxia + ipsi Horner ± contra body pain/temp
  • One-and-a-half syndrome → paramedian pontine PPRF + MLF lesion → ipsi horizontal gaze palsy + ipsi INO; only contralateral eye abduction preserved
  • Parinaud (dorsal midbrain) syndrome → pineal mass/germinoma in young, aqueductal stenosis, midbrain stroke → upgaze palsy + convergence-retraction nystagmus + light-near dissociation + Collier sign
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