Location: Between diencephalon (above) and spinal cord (below), anterior to cerebellum
Components (rostral → caudal):
| Region | Contents | Function |
|---|---|---|
| Basis (Ventral) | Descending motor tracts (corticospinal, corticobulbar, corticopontine) | Motor output |
| Tegmentum (Middle) | CN nuclei, ascending tracts, reticular formation | Sensory, autonomic, CN functions |
| Tectum (Dorsal) | Colliculi (midbrain only); roof of 4th ventricle (pons/medulla) | Visual/auditory reflexes |
General rule (medial → lateral):
Motor nuclei are MEDIAL, Sensory nuclei are LATERAL. Think: “M&M” = Motor is Medial. This follows from embryological development (alar and basal plates).
| Region | Blood Supply |
|---|---|
| Midbrain | Basilar artery (top), PCA, SCA |
| Pons | Basilar artery (paramedian and circumferential branches), AICA |
| Medulla | Vertebral artery, PICA, anterior spinal artery |
Level: Between pons and diencephalon
Key landmarks: Superior and inferior colliculi, cerebral peduncles, red nucleus, substantia nigra
| Structure | Location | Notes |
|---|---|---|
| Superior Colliculus | Dorsal (tectum) | Visual reflexes, saccades; CN III level |
| Inferior Colliculus | Dorsal (tectum) | Auditory relay; CN IV level |
| Cerebral Peduncles | Ventral | Contain corticospinal, corticobulbar, corticopontine tracts |
| Interpeduncular Fossa | Between peduncles | CN III exits here |
| Structure | Function | Clinical Correlation |
|---|---|---|
| Red Nucleus | Motor coordination; receives cerebellar input (dentatorubral) | Benedikt syndrome (tremor/ataxia) |
| Substantia Nigra | Dopaminergic neurons (pars compacta) → striatum | Parkinson’s disease (loss of dopamine) |
| Periaqueductal Gray (PAG) | Pain modulation, autonomic function | Target for deep brain stimulation |
| Cerebral Aqueduct | CSF pathway (3rd → 4th ventricle) | Aqueductal stenosis → hydrocephalus |
| MLF (Medial Longitudinal Fasciculus) | Conjugate eye movements | INO (internuclear ophthalmoplegia) |
| CN | Nucleus Location | Exit | Function |
|---|---|---|---|
| CN III (Oculomotor) | Superior colliculus level; includes Edinger-Westphal (parasympathetic) | Interpeduncular fossa (ventral) | Eye movement (SR, IR, MR, IO), levator, pupil constriction |
| CN IV (Trochlear) | Inferior colliculus level | Dorsal (ONLY CN to exit dorsally); decussates | Superior oblique (depression, intorsion) |
CN IV is unique: Only CN that exits DORSALLY and DECUSSATES. Has longest intracranial course → vulnerable to trauma. Nucleus at inferior colliculus level.
Key structures (ventral → dorsal):
Tracts present:
Key structures:
Level: Between midbrain and medulla
Key landmarks: Basilar pons (ventral bulge), middle cerebellar peduncles, 4th ventricle
| Structure | Location | Notes |
|---|---|---|
| Basilar Pons | Ventral | Contains pontine nuclei, corticospinal fibers |
| Middle Cerebellar Peduncle (MCP) | Lateral | Pontocerebellar fibers (largest peduncle, AFFERENT only) |
| 4th Ventricle | Dorsal | Floor formed by pons and medulla |
| Cerebellopontine Angle (CPA) | Lateral junction | CN VII, VIII exit here; acoustic neuroma site |
| Structure | Function | Clinical Correlation |
|---|---|---|
| Locus Coeruleus | Norepinephrine production; arousal, attention | Implicated in anxiety, PTSD, depression |
| Raphe Nuclei | Serotonin production; mood, sleep | Target of SSRIs |
| PPRF (Paramedian Pontine Reticular Formation) | Horizontal gaze center | Lesion → ipsilateral gaze palsy |
| MLF | Connects CN VI to contralateral CN III for conjugate gaze | INO (MS, stroke) |
| Superior Olivary Nucleus | Sound localization (auditory pathway) | Part of ascending auditory pathway |
| CN | Nucleus Location | Exit | Function |
|---|---|---|---|
| CN V (Trigeminal) |
Motor nucleus (mid-pons) Chief sensory nucleus (mid-pons) Spinal nucleus (extends to medulla) Mesencephalic nucleus (midbrain) |
Lateral mid-pons | Facial sensation, mastication |
| CN VI (Abducens) | Floor of 4th ventricle (facial colliculus) | Pontomedullary junction | Lateral rectus (abduction) |
| CN VII (Facial) |
Motor nucleus (lower pons) Superior salivatory nucleus (parasympathetic) |
Cerebellopontine angle | Facial expression, taste (ant 2/3), lacrimation, salivation |
| CN VIII (Vestibulocochlear) |
Cochlear nuclei (pontomedullary) Vestibular nuclei (pontomedullary) |
Cerebellopontine angle | Hearing, balance |
Facial colliculus: Bump on floor of 4th ventricle formed by CN VII fibers looping around CN VI nucleus. Lesion here causes ipsilateral CN VI and VII palsy together.
Key structures (ventral → dorsal):
Key structures:
Level: Between pons and spinal cord (foramen magnum)
Key landmarks: Pyramids, olives, gracile and cuneate tubercles
| Structure | Location | Notes |
|---|---|---|
| Pyramids | Ventral midline | Corticospinal tracts; decussation at caudal medulla |
| Olives (Inferior Olivary Nucleus) | Lateral to pyramids | Climbing fibers to cerebellum; motor learning |
| Gracile Tubercle | Dorsal (medial) | Nucleus gracilis (lower body proprioception) |
| Cuneate Tubercle | Dorsal (lateral) | Nucleus cuneatus (upper body proprioception) |
| Inferior Cerebellar Peduncle (ICP) | Posterolateral | Connects medulla to cerebellum |
| Structure | Function | Clinical Correlation |
|---|---|---|
| Inferior Olivary Nucleus | Climbing fibers to cerebellum; motor learning | Hypertrophic olivary degeneration (palatal tremor) |
| Nucleus Gracilis/Cuneatus | Relay for dorsal column sensation | Proprioception, vibration, fine touch loss |
| Nucleus Ambiguus | Motor to pharynx, larynx (CN IX, X, XI) | Dysphagia, dysarthria, hoarseness |
| Nucleus Solitarius | Taste (VII, IX, X), visceral sensation | Taste loss, autonomic dysfunction |
| Dorsal Motor Nucleus of Vagus | Parasympathetic to thoracoabdominal viscera | Autonomic dysfunction |
| Area Postrema | Chemoreceptor trigger zone (outside BBB) | Nausea/vomiting |
| Respiratory Centers | Control breathing rhythm | Respiratory failure with bilateral lesions |
| CN | Nucleus Location | Exit | Function |
|---|---|---|---|
| CN IX (Glossopharyngeal) |
Nucleus ambiguus (motor) Inferior salivatory (parasympathetic) Nucleus solitarius (taste, visceral) |
Postolivary sulcus | Stylopharyngeus, taste post 1/3, parotid |
| CN X (Vagus) |
Nucleus ambiguus (motor) Dorsal motor nucleus (parasympathetic) Nucleus solitarius (visceral sensory) |
Postolivary sulcus | Pharynx, larynx, parasympathetic to viscera |
| CN XI (Spinal Accessory) | Spinal accessory nucleus (C1-C5/6) | Enters foramen magnum, exits jugular foramen | SCM, trapezius |
| CN XII (Hypoglossal) | Hypoglossal nucleus (floor of 4th ventricle) | Preolivary sulcus (between pyramid and olive) | Tongue movement |
Nucleus ambiguus = motor for swallowing and speech (CN IX, X, XI). Located in lateral medulla. Damaged in Wallenberg syndrome → dysphagia, dysarthria, hoarseness.
Key structures (ventral → dorsal):
Key structures:
Important decussations:
| Tract | Function | Decussation | Brainstem Location |
|---|---|---|---|
| Medial Lemniscus | Proprioception, vibration, fine touch | Caudal medulla (internal arcuate fibers) |
Medulla: paramedian, vertical Pons: ventral tegmentum, horizontal Midbrain: lateral to red nucleus |
| Spinothalamic Tract | Pain, temperature, crude touch | Spinal cord (anterior white commissure) | Lateral tegmentum throughout |
| Trigeminothalamic Tract | Facial sensation | Pons (after synapse in trigeminal nuclei) | Adjacent to medial lemniscus |
| Lateral Lemniscus | Auditory pathway | Superior olive (bilateral) | Lateral pons → inferior colliculus |
| Tract | Function | Decussation | Brainstem Location |
|---|---|---|---|
| Corticospinal Tract | Voluntary movement (limbs) | Pyramidal decussation (caudal medulla) |
Midbrain: cerebral peduncle (middle 3/5) Pons: scattered in basilar pons Medulla: pyramids |
| Corticobulbar Tract | Voluntary movement (face, tongue) | Variable (bilateral to most CN nuclei) | With corticospinal in basis |
| Rubrospinal Tract | Flexor tone (upper limb) | Ventral tegmental decussation (midbrain) | Lateral tegmentum |
Medial lemniscus orientation changes: Vertical in medulla (beside pyramid) → horizontal in pons → lateral in midbrain. Remember: “Medial lemniscus Moves around.”
| Tract | Function | Clinical Significance |
|---|---|---|
| MLF (Medial Longitudinal Fasciculus) | Conjugate eye movements; connects CN VI → contralateral CN III | INO: impaired adduction on lateral gaze, nystagmus of abducting eye |
| Central Tegmental Tract | Connects red nucleus → inferior olive | Lesion → hypertrophic olivary degeneration, palatal tremor |
| Spinal Trigeminal Tract | Pain/temperature from face → spinal trigeminal nucleus | Lateral medullary lesion → ipsilateral facial pain/temp loss |
| Region | Medial | Lateral |
|---|---|---|
| Midbrain | Basilar bifurcation, PCA (paramedian branches) | SCA, PCA |
| Pons | Basilar artery (paramedian branches) | AICA, SCA (circumferential branches) |
| Medulla | Vertebral artery, anterior spinal artery | PICA, vertebral artery |
Structures affected in MEDIAL brainstem stroke:
Blood supply: Paramedian branches (basilar, vertebral, anterior spinal)
Structures affected in LATERAL brainstem stroke:
Also affected:
Blood supply: Circumferential branches (PICA, AICA, SCA)
Medial = Motor (4 M’s). Lateral = Sensory + Spinocerebellar (4 S’s). This helps predict deficits based on vascular territory: paramedian branches → medial; circumferential branches → lateral.
| Syndrome | Location | Structures Involved | Clinical Features |
|---|---|---|---|
| Weber Syndrome | Ventral midbrain | CN III fascicle + cerebral peduncle |
Ipsilateral: CN III palsy (ptosis, “down and out,” dilated pupil) Contralateral: Hemiparesis (face, arm, leg) |
| Benedikt Syndrome | Tegmentum (midbrain) | CN III + red nucleus + cerebral peduncle |
Ipsilateral: CN III palsy Contralateral: Tremor/ataxia (red nucleus) + hemiparesis |
| Claude Syndrome | Tegmentum (midbrain) | CN III + red nucleus (spares peduncle) |
Ipsilateral: CN III palsy Contralateral: Ataxia (NO hemiparesis) |
| Parinaud Syndrome | Dorsal midbrain (tectum) | Pretectal area, superior colliculus | Upgaze palsy, light-near dissociation, convergence-retraction nystagmus, eyelid retraction (Collier’s sign) |
Weber = ventral (motor), Benedikt = tegmentum (motor + cerebellar), Claude = tegmentum (cerebellar only). All have ipsilateral CN III palsy. Parinaud = dorsal midbrain compression (pineal tumor, hydrocephalus).
| Syndrome | Location | Structures Involved | Clinical Features |
|---|---|---|---|
| Medial Inferior Pontine (Foville) | Medial lower pons | CN VI, VII + corticospinal + PPRF |
Ipsilateral: CN VI palsy, CN VII palsy, lateral gaze palsy (PPRF) Contralateral: Hemiparesis |
| Lateral Inferior Pontine (AICA) | Lateral lower pons | CN VII, VIII + spinothalamic + MCP |
Ipsilateral: CN VII palsy, hearing loss, vertigo, ataxia, Horner’s, facial sensory loss Contralateral: Body pain/temp loss |
| Medial Superior Pontine | Medial upper pons | Corticospinal + medial lemniscus + MLF |
Ipsilateral: INO, ataxia Contralateral: Hemiparesis, proprioception loss |
| Lateral Superior Pontine (SCA) | Lateral upper pons | SCP + spinothalamic + spinal trigeminal |
Ipsilateral: Ataxia (severe), Horner’s, facial sensory loss Contralateral: Body pain/temp loss |
| Locked-in Syndrome | Bilateral ventral pons | Bilateral corticospinal + corticobulbar (spares tegmentum) | Quadriplegia, anarthria, preserved consciousness and vertical eye movement (only way to communicate) |
Locked-in syndrome: Patient is awake but cannot move or speak. Only vertical eye movements preserved (spares CN III nucleus in midbrain). Usually basilar artery thrombosis. Must distinguish from coma!
| Syndrome | Location | Structures Involved | Clinical Features |
|---|---|---|---|
| Lateral Medullary (Wallenberg) | Lateral medulla (PICA) |
Vestibular nuclei Nucleus ambiguus (IX, X) Spinal trigeminal Spinothalamic Sympathetics ICP |
Ipsilateral: • Vertigo, nystagmus, nausea • Dysphagia, dysarthria, hoarseness • Facial pain/temp loss • Horner’s syndrome • Ataxia Contralateral: • Body pain/temp loss NO motor weakness! |
| Medial Medullary (Dejerine) | Medial medulla (ASA, vertebral) |
Pyramid Medial lemniscus CN XII |
Ipsilateral: CN XII palsy (tongue deviates toward lesion) Contralateral: • Hemiparesis (arm/leg, spares face) • Proprioception/vibration loss |
Wallenberg (lateral medullary) = MOST COMMON brainstem stroke syndrome. Key features: Crossed sensory loss (ipsi face, contra body) + NO weakness. Often misdiagnosed as peripheral vertigo. Remember: “5 D’s” – Dysphagia, Dysarthria, Diplopia, Dizziness, Dysmetria.
| Level | Medial Syndrome | Lateral Syndrome |
|---|---|---|
| Midbrain | Weber (CN III + hemiparesis) | Benedikt/Claude (CN III + ataxia) |
| Pons | Foville (CN VI, VII + hemiparesis) | AICA syndrome (CN VII, VIII + ataxia) |
| Medulla | Dejerine (CN XII + hemiparesis) | Wallenberg (CN IX, X + crossed sensory) |
| Level | Cranial Nerves | Mnemonic |
|---|---|---|
| Midbrain | CN III (superior colliculus), CN IV (inferior colliculus) | 3, 4 at the door (midbrain) |
| Pons | CN V, VI, VII, VIII | 5, 6, 7, 8 at the gate (pons) |
| Medulla | CN IX, X, XII | 9, 10, 12 keep the medulla fine |
| Spinal Cord | CN XI (C1-C5/6) | 11 is in the spine |
| Concept | Key Point |
|---|---|
| Eyes deviate toward lesion | Cortical lesion (frontal eye field). Eyes deviate AWAY from lesion in pontine (PPRF) lesion. |
| INO | MLF lesion. Impaired ADduction on lateral gaze + nystagmus of ABducting eye. MS if bilateral, stroke if unilateral. |
| One-and-a-half syndrome | PPRF + MLF lesion. Ipsilateral gaze palsy + INO. Only ABduction of contralateral eye works. |
| Parinaud syndrome causes | Pineal tumor, hydrocephalus, MS, stroke. Upgaze palsy + light-near dissociation. |
| Wallenberg (lateral medullary) | Most common brainstem stroke. Crossed sensory loss. NO weakness. |
| CN VI false localizing | Longest subarachnoid course – vulnerable to increased ICP. Doesn’t mean pontine lesion. |
