Anatomy Atlas

Pons

Pons
Also: Locked-in syndrome · Osmotic demyelination syndrome · Central pontine myelinolysis · PPRF
OrganHigh-yielddraftNeuro

Middle part of the brainstem that links the cerebrum to the cerebellum. It holds the nuclei of CN V–VIII and horizontal gaze centers; basilar artery occlusion causes locked-in syndrome.

Core anatomy

Location

Between the midbrain and the medulla oblongata, in front of the cerebellum and the fourth ventricle, resting on the clivus.

Parts
  • Basilar (ventral) pons: descending corticospinal and corticobulbar fibers, pontine nuclei, and transverse pontocerebellar fibers forming the middle cerebellar peduncle
  • Tegmentum (dorsal): nuclei of CN V (motor and principal sensory), VI, VII and VIII (vestibular and cochlear); the PPRF (paramedian pontine reticular formation — horizontal gaze center); MLF; medial lemniscus and spinothalamic tract; locus coeruleus (norepinephrine)
  • Floor of the fourth ventricle with the facial colliculus (CN VII fibers looping around the CN VI nucleus)
  • Respiratory centers (pneumotaxic and apneustic) that modulate medullary rhythm
Relations

The basilar artery runs in the basilar groove on its anterior surface.

Arterial supply

Basilar artery — paramedian and short circumferential pontine branches; AICA (lateral lower pons) and SCA (upper pons) — see vertebrobasilar system.

Venous drainage

Pontine veins → superior petrosal sinus and basal veins.

Notes

Emergence of CN V (lateral pons), CN VI (pontomedullary junction, medially), CN VII and VIII (cerebellopontine angle) — see trigeminal nerve (CN V), abducens nerve (CN VI), facial nerve (CN VII), vestibulocochlear nerve (CN VIII).

Mentioned in

Generated automatically from links in other entries.

Where the cranial nerves leave the brainstem (ventral view)The front of the brainstem. The optic nerves meet at the chiasm above. The oculomotor nerve emerges from the interpeduncular fossa between the cerebral peduncles of the midbrain. The trochlear nerve is the only one to leave from the back, just below the inferior colliculus, and winds forward around the peduncle. The trigeminal nerve leaves the lateral pons. The abducens leaves at the pontomedullary junction near the midline; the facial and vestibulocochlear nerves leave at the cerebellopontine angle. The glossopharyngeal and vagus nerves leave the postolivary sulcus of the medulla, the hypoglossal leaves the preolivary sulcus between the pyramid and the olive, and the accessory nerve rises from the upper cervical cord.Optic (II)Oculomotor (III)Trochlear (IV) — from the dorsal sideTrigeminal (V)Abducens (VI)Facial (VII)Vestibulocochlear (VIII)Glossopharyngeal (IX)Vagus (X)Accessory (XI) — from C1–C5Hypoglossal (XII)Midbrain (cerebral peduncle)PonsPyramidOliveSpinal cordMedullaVentral view · dashed IV leaves from the back

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Where the cranial nerves leave the brainstem. Ventral view. III leaves the interpeduncular fossa of the midbrain; IV is the only nerve to leave from the back (dashed); V leaves the lateral pons; VI the pontomedullary junction near the midline; VII and VIII the cerebellopontine angle; IX and X the postolivary sulcus; XII the preolivary sulcus between pyramid and olive; and XI rises from the upper cervical cord. Nuclei follow the same levels: III–IV midbrain, V–VIII pons, IX–XII medulla.
Original schematic, created for this atlas.
Anterior view of the pons and medulla showing the pyramids, the olives, the decussation of the pyramids and the middle cerebellar peduncle (brachium pontis)
Anterior surface of the pons and medulla.
Henry Vandyke Carter, Gray's Anatomy of the Human Body, 20th ed. (1918), Fig. 679. Source · Public domain (Public domain in the US: published before January 1, 1931 (per Wikimedia Commons))
Dorsal view of the brainstem with the cerebellum partly removed: superior and inferior colliculi, trochlear nerve, facial colliculus, hypoglossal trigone, area postrema and the cerebellar peduncles
The floor of the fourth ventricle with the facial colliculus.
Henry Vandyke Carter, Gray's Anatomy of the Human Body, 20th ed. (1918), Fig. 709. Source · Public domain (Public domain in the US: published before January 1, 1931 (per Wikimedia Commons))

Clinical correlations

Locked-in syndrome PANCE

Mechanism / lesion

Basilar artery occlusion (or hemorrhage) destroying the ventral pons — corticospinal and corticobulbar tracts — while sparing the tegmentum (reticular activating system) above.

Classic presentation

Quadriplegia and anarthria with preserved consciousness; vertical eye movements and blinking are preserved (midbrain) and used to communicate. Sudden basilar occlusion can present with coma, and needs emergency thrombectomy.

Osmotic demyelination syndrome (central pontine myelinolysis) PANCE

Mechanism / lesion

Overly rapid correction of chronic hyponatremia (risk: alcohol use disorder, malnutrition, liver disease, hypokalemia).

Classic presentation

Days after correction: dysarthria, dysphagia, quadriparesis, locked-in state. Prevent by correcting sodium no faster than about 8 mEq/L per 24 hours in high-risk patients (10–12 limit otherwise).

Pontine hemorrhage and gaze palsies PANCE

Mechanism / lesion

Hypertensive hemorrhage from paramedian perforators; lesions of the PPRF, abducens nucleus or MLF.

Classic presentation

Pontine hemorrhage: coma, quadriplegia, pinpoint reactive pupils, hyperthermia. PPRF/abducens nucleus lesion → ipsilateral horizontal gaze palsy. MLF → internuclear ophthalmoplegia (MS). "One-and-a-half syndrome" → both.

High-yield PANCE pearls

  • Awake, quadriplegic, communicates by vertical eye movements/blinking → locked-in syndrome (ventral pons, basilar).
  • Rapid correction of hyponatremia → osmotic demyelination → ≤ 8 mEq/L/24 h in high-risk patients.
  • Coma + pinpoint pupils → pontine hemorrhage (or opioids).
  • Pons houses nuclei of CN V, VI, VII, VIII.

Imaging anatomy

CT

CT shows pontine hemorrhage; small infarcts are often missed in the posterior fossa.

MRI

MRI with DWI for pontine infarcts and demyelination (a "trident" or "bat-wing" T2 lesion in osmotic demyelination, which may appear only after 1–2 weeks); MRA/CTA for basilar occlusion.

Physical exam

Strength / function testing

Corneal reflex, facial strength, horizontal gaze (PPRF, CN VI), hearing and vestibular function; limb strength and reflexes for corticospinal involvement; level of consciousness and pupil size.

Self-quiz

Q1.

A 60-year-old man is found unable to move his limbs or speak. He is alert and answers yes/no questions by looking up and down, but cannot move his eyes horizontally. Which artery is most likely occluded?

Show answer

B

Locked-in syndrome from basilar artery occlusion infarcts the ventral pons (motor tracts) and the horizontal gaze centers, sparing consciousness and midbrain-controlled vertical gaze and blinking.

Q2.

A woman with alcohol use disorder has a sodium of 108 mEq/L, which rises to 128 mEq/L in 24 hours. Four days later she develops dysarthria and weakness of all limbs. What is the diagnosis?

Show answer

B

Correcting chronic hyponatremia by 20 mEq/L in 24 hours is far too fast and causes osmotic demyelination (central pontine myelinolysis) — dysarthria, dysphagia and quadriparesis days later.

Q3.

A comatose patient has pinpoint but reactive pupils and quadriplegia after sudden collapse. Where is the likely lesion (if opioids are excluded)?

Show answer

B

Pontine hemorrhage causes coma, quadriplegia and pinpoint reactive pupils (interrupted descending sympathetic fibers). Opioid overdose is the main mimic.

Q4.

The locus coeruleus in the pons is the main source of which neurotransmitter?

Show answer

D

The locus coeruleus is the brain's main source of norepinephrine. The substantia nigra supplies dopamine and the raphe nuclei serotonin.

Q5.

What is the function of the paramedian pontine reticular formation (PPRF)?

Show answer

A

The PPRF is the horizontal gaze center, driving the ipsilateral abducens nucleus. Vertical gaze is controlled in the midbrain.

Flashcards

See also