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AN58.1-4 | Medulla Oblongata — Self-Directed Learning

Part 1: External Features (AN58.1)

Anterior Surface of the Medulla:
• Pyramid — rounded ridge on each side of the anterior median fissure; contains the corticospinal fibres
• Pyramidal decussation — at the caudal end; 85% of corticospinal fibres cross here → visible as an "X" pattern obliterating the anterior median fissure
• Olive — oval eminence lateral to the pyramid; overlies the inferior olivary nucleus
• Rootlets of CN XII (hypoglossal) — emerge between the pyramid and olive (preolivary sulcus)

Posterior Surface of the Medulla (lower closed part):
• Posterior median sulcus — continuation of spinal cord
• Gracile tubercle (clava) — overlies nucleus gracilis; medial
• Cuneate tubercle — overlies nucleus cuneatus; lateral
• Obex — small projection at the lower end of the 4th ventricle floor (marking the opening of the central canal)

Posterior Surface (upper open part — floor of 4th ventricle):
• Hypoglossal triangle — overlies CN XII nucleus
• Vagal triangle — overlies dorsal nucleus of vagus
• Vestibular area — lateral to the sulcus limitans (CN VIII vestibular nuclei)
• Stria medullaris — fibres crossing to the cerebellum (arcuate fibres)
• Rootlets of CN IX, X, XI — emerge lateral to the olive (postolivary sulcus)

Part 2: Transverse Sections of the Medulla (AN58.2)

Section 1: Level of Pyramidal Decussation (most caudal)

This level is CAUDAL to the obex; resembles a spinal cord cross-section.

• Anteriorly: The pyramids (corticospinal fibres) are crossing from right to left and left to right — the "decussation" is visible as fibres interdigitating across the midline
• Posteriorly: Posterior column nuclei are beginning to appear:
• Nucleus gracilis (medial) — nucleus for fasciculus gracilis
• Nucleus cuneatus (lateral) — nucleus for fasciculus cuneatus
• Lateral: The spinal tract of CN V (carrying pain/temperature from the face) and its nucleus
• The central canal is still closed at this level

Section 2: Level of Sensory Decussation (internal arcuate fibres)

Just above the pyramidal decussation.

• Nucleus gracilis and nucleus cuneatus are now prominent (in the posterior part)
• Internal arcuate fibres — axons of 2nd order neurons from nucleus gracilis and cuneatus curve anteromedially and CROSS the midline to form the medial lemniscus on the opposite side
• This is the sensory (internal arcuate) decussation — point where posterior column information crosses
• After crossing, the medial lemniscus lies as a vertical slab in the midline posterior to the pyramid
• Lateral: Spinal nucleus of CN V + spinal tract of CN V
• Central canal — still closed

Section 3: Level of the Inferior Olivary Nucleus (rostral medulla)

The 4th ventricle has opened at this level.

Ventral (anterior) structures:
• Pyramid — contains corticospinal (15–20% uncrossed, some crossed)
• Medial lemniscus — ribbon-shaped, now lying vertically just dorsal to the pyramid
• Inferior olivary nucleus — wrinkled, bag-shaped (on cross-section = crenated outline); a relay for the cerebellum; forms the olive surface feature

Dorsal (posterior) structures (floor of 4th ventricle):
• Hypoglossal nucleus (CN XII) — most medial, near midline
• Dorsal nucleus of vagus (CN X) — lateral to CN XII nucleus; parasympathetic
• Nucleus of tractus solitarius — lateral; receives visceral sensation (taste for VII, IX, X; visceral sensation for X)
• Vestibular nuclei — in the lateral floor of the 4th ventricle (medial and inferior vestibular nuclei here)

Lateral structures:
• Nucleus ambiguus — in the lateral reticular formation; motor for CN IX, X, XI (branchiomotor); important for swallowing and phonation
• Spinal nucleus of CN V — continues from cord; pain/temperature from face
• Inferior cerebellar peduncle — connects medulla to cerebellum; contains spinocerebellar, olivocerebellar, vestibulocerebellar fibres

Part 3: Cranial Nerve Nuclei in the Medulla (AN58.3)

CN IX — Glossopharyngeal

Component	Nucleus	Function
GSA (somatic sensory)	Spinal nucleus of CN V	Skin of ear
GVA (visceral sensory)	Nucleus tractus solitarius (NTS)	Carotid sinus/body sensation
SVA (special visceral — taste)	NTS (gustatory nucleus, upper)	Taste from posterior 1/3 tongue
SVE (branchiomotor)	Nucleus ambiguus	Stylopharyngeus muscle
GVE (parasympathetic)	Inferior salivatory nucleus	Parotid gland

CN X — Vagus

Component	Nucleus	Function
GSA	Spinal nucleus of CN V	Skin of ear (Arnold's nerve)
GVA	Nucleus tractus solitarius	Thoracic and abdominal viscera
SVA	NTS (gustatory nucleus)	Taste from epiglottis
SVE	Nucleus ambiguus	Pharyngeal and laryngeal muscles
GVE	Dorsal nucleus of vagus	Thoracoabdominal parasympathetics

CN XI — Accessory

Component	Nucleus	Function
Cranial root	Nucleus ambiguus	Pharyngeal/laryngeal (joins vagus)
Spinal root	C1–C5 anterior horn	Sternocleidomastoid + trapezius

CN XII — Hypoglossal

Component	Nucleus	Function
GSE (somatic motor)	Hypoglossal nucleus	All intrinsic + most extrinsic tongue muscles

Part 4: Medullary Syndromes (AN58.4)

Lateral Medullary Syndrome (Wallenberg Syndrome)
• Cause: Occlusion of posterior inferior cerebellar artery (PICA) or vertebral artery → infarct of lateral medulla
• Structures affected:
- Nucleus ambiguus (CN IX, X) → ipsilateral dysphagia, hoarseness (LMN palsy of pharynx/larynx)
- Vestibular nuclei → ipsilateral vertigo, nausea, vomiting, nystagmus
- Spinal nucleus and tract of CN V → ipsilateral facial pain/temperature loss
- Spinothalamic tract → contralateral body pain/temperature loss
- Sympathetic fibres (descending) → ipsilateral Horner's syndrome (ptosis, miosis, anhidrosis)
- Inferior cerebellar peduncle / spinocerebellar tracts → ipsilateral limb ataxia
• Summary: "DOVES" mnemonic — Dysphagia, vertigo, ipsilateral (Ocular Horner, vestibular), contralateral spinothalamic Sensation loss

Sign	Side	Structure
Dysphagia, hoarseness	Ipsilateral	Nucleus ambiguus
Horner's syndrome	Ipsilateral	Descending sympathetics
Facial pain/temp loss	Ipsilateral	Spinal nucleus CN V
Body pain/temp loss	Contralateral	Spinothalamic tract
Ataxia	Ipsilateral	Inf. cerebellar peduncle
Vertigo, nystagmus	Ipsilateral	Vestibular nuclei

Patient A in the hook = RIGHT lateral medullary infarct

Medial Medullary Syndrome (Dejerine Syndrome)
• Cause: Occlusion of anterior spinal artery (anterior medullary artery) → infarct of medial medulla
• Structures affected:
- Pyramid (corticospinal) → contralateral hemiplegia (arm + leg, face spared)
- Medial lemniscus → contralateral loss of fine touch, vibration, proprioception
- CN XII rootlets or nucleus → ipsilateral tongue paralysis (LMN) — tongue deviates TOWARD the side of lesion
• The "crossed hemiplegia" (contralateral body + ipsilateral CN XII) is classic

Sign	Side	Structure
Hemiplegia (arm + leg)	Contralateral	Pyramid
Loss of fine touch/vibrn/prop	Contralateral	Medial lemniscus
Tongue deviation (LMN)	Ipsilateral	CN XII nucleus/rootlets

Patient B in the hook = LEFT medial medullary infarct → right hemiplegia + right tongue deviation (LMN)

Crossed Diplegia (Diplegia of the Cross)
• Bilateral medullary involvement
• Quadriplegia + bilateral LMN cranial nerve palsies
• Rare; caused by bilateral medullary infarcts or basilar artery occlusion