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AN59.1-4 | Pons — Practice Quiz
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The transverse fibres of the pons cross the midline and form which structure connecting the pons to the cerebellum?
Correct! Pontocerebellar fibres (from pontine nuclei, after crossing) form the MIDDLE cerebellar peduncle (brachium pontis) — the largest of the three cerebellar peduncles. Carries cerebrocerebellar pathway fibres.
Three cerebellar peduncles: Superior (brachium conjunctivum) = output of cerebellum to midbrain (dentatorubrothalamic). Middle (brachium pontis) = input from cerebral cortex via pons (corticopontocerebellar). Inferior (restiform body + juxtarestiform) = input from spinal cord + vestibular nuclei + inferior olive.
Incorrect. Middle cerebellar peduncle = largest, from pons. Superior = from midbrain (cerebellum→thalamus, dentatorubrothalamic). Inferior = from medulla (spinocerebellar, vestibulocerebellar, olivocerebellar).
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The facial colliculus on the floor of the 4th ventricle is a surface landmark produced by:
Correct! CN VII motor nerve fibres take an unusual course: they initially emerge from the facial nucleus, loop posteriorly around the abducens (CN VI) nucleus, then turn laterally to exit. This internal genu of CN VII creates the facial colliculus over the CN VI nucleus on the 4th ventricle floor.
The genu of CN VII around CN VI nucleus explains the frequent co-involvement of CN VI and CN VII in lower pontine lesions (Millard-Gubler: ipsilateral CN VI palsy + CN VII LMN palsy + contralateral hemiplegia). Clinical test: asking patient to close both eyes tightly — if upper AND lower face weakness = LMN (pontine); if only lower face = UMN (cortical).
Incorrect. Facial colliculus = CN VII fibres looping OVER CN VI nucleus. This is why lower pons lesions often affect BOTH CN VI and CN VII (they are anatomically very close — Millard-Gubler syndrome).
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The medial longitudinal fasciculus (MLF) in the pons primarily coordinates:
Correct! MLF = supranuclear control of horizontal conjugate gaze. It connects the CN VI nucleus (lateral rectus — abduction) on one side to the CN III nucleus (medial rectus — adduction) on the other side, so both eyes move together horizontally.
INO (internuclear ophthalmoplegia): MLF lesion → on horizontal gaze TOWARD the lesion side, the ipsilateral eye ADDUCTS sluggishly (or fails completely). The contralateral eye abducts but shows nystagmus. Bilateral INO = virtually diagnostic of MS. In the elderly, unilateral INO after stroke.
Incorrect. MLF = conjugate horizontal gaze coordination (CN VI ↔ contralateral CN III). Damage = INO. Pain/temp from face = spinothalamic + spinal nucleus CN V. Facial muscles = CN VII.
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The superior salivatory nucleus in the pons provides parasympathetic fibres (via CN VII) to:
Correct! Superior salivatory nucleus (CN VII): submandibular + sublingual (via chorda tympani → lingual nerve → submandibular ganglion) + lacrimal gland (via greater petrosal nerve → pterygopalatine ganglion). Parotid = inferior salivatory nucleus (CN IX).
Bell's palsy (LMN CN VII palsy): ALL branches of CN VII affected. Loss of: forehead wrinkling, eye closure (risk of exposure keratitis), nasolabial fold, smile. If the lesion is proximal to the geniculate ganglion: also loss of taste (anterior 2/3 tongue via chorda tympani) + dry eye (greater petrosal) + hyperacusis (nerve to stapedius). Important distinction from cortical (UMN) facial palsy where forehead is spared.
Incorrect. CN VII (superior salivatory nucleus) = submandibular + sublingual + lacrimal. Parotid = CN IX (inferior salivatory nucleus → lesser petrosal → otic ganglion). Mnemonic: "VII = sublingual + submandibular + lacrimal; IX = parotid".
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Which nucleus of the trigeminal nerve (CN V) is responsible for proprioception from the muscles of mastication?
Correct! Mesencephalic nucleus of CN V = proprioception from jaw muscles (masseters, pterygoids, temporalis). Unique: the cell bodies are 1st order neurons located WITHIN the CNS (in the midbrain) rather than in a peripheral ganglion (like the trigeminal ganglion).
CN V nuclei summary: Motor (SVE) = mastication. Principal sensory (GSA) = fine touch face. Mesencephalic (GSA) = proprioception jaw (CNS neuron). Spinal (GSA) = pain/temperature face (extends from pons to C2). Jaw jerk reflex: afferent = mesencephalic nucleus; efferent = motor nucleus. Brisk jaw jerk = UMN lesion ABOVE the pons (bilateral corticobulbar tract lesion = pseudobulbar palsy).
Incorrect. Mesencephalic nucleus = proprioception from jaw (unique CNS location). Principal sensory = fine touch from face. Spinal nucleus = pain/temperature from face. Motor nucleus = muscles of mastication.
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In locked-in syndrome from pontine haemorrhage, the patient is quadriplegic and cannot speak, but remains fully conscious and can blink and move eyes vertically. Consciousness is preserved because:
Correct! Locked-in syndrome = bilateral VENTRAL (basis) pontis destruction. The ascending reticular activating system (ARAS) is located in the DORSAL tegmentum of the brainstem → spared → consciousness maintained. Vertical gaze is controlled by the midbrain (not pons) → also spared.
Key anatomical distinction in locked-in syndrome: VENTRAL pons (basis pontis) = motor outputs (corticospinal, corticobulbar, PPRF, CN VI/VII) → destroyed. DORSAL pons (tegmentum) = sensory tracts, ARAS, cranial nerve nuclei → relatively spared. Vertical gaze = midbrain (rostral interstitial MLF) → intact. Communication via vertical eye movement or blinking.
Incorrect. Consciousness = ARAS in dorsal brainstem tegmentum. Locked-in = BASIS pontis destroyed (ventral), tegmentum (dorsal) relatively intact → ARAS and vertical gaze centres preserved.
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Millard-Gubler syndrome involves a lesion in the lower pons. Which combination of deficits is characteristic?
Correct! Millard-Gubler syndrome: lower pontine infarct affecting CN VI fascicles (lateral rectus palsy → convergent squint) + CN VII fascicles (LMN facial palsy) + corticospinal tract (contralateral hemiplegia). Ipsilateral CN VI + CN VII + contralateral hemiplegia.
Lower pontine syndromes: Raymond = CN VI only + hemiplegia. Millard-Gubler = CN VI + CN VII + hemiplegia. Foville = GAZE palsy (nucleus/PPRF, both eyes) + CN VII + hemiplegia. CN VI palsy in Raymond/MG is a fascicular (LMN) palsy = ipsilateral eye cannot abduct. GAZE palsy in Foville = both eyes cannot look to ipsilateral side (nucleus/PPRF level).
Incorrect. Millard-Gubler = CN VI (abducens, lower pons) + CN VII + contralateral hemiplegia. CN III + contralateral hemiplegia = Weber syndrome (midbrain). CN XII + contralateral = medial medullary (Dejerine). Foville = gaze palsy (nucleus level) + CN VII + hemiplegia.
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Pin-point (1 mm) but reactive pupils are seen in pontine haemorrhage. The mechanism is:
Correct! Descending sympathetic fibres (from hypothalamus through lateral tegmentum of pons to ciliospinal centre T1) are bilaterally destroyed → bilateral pupil constriction (sympathetics for dilation are lost, parasympathetics from CN III remain intact → bilateral miosis). Still reactive to light because CN III is intact.
Pupil signs in brainstem lesions: (1) Pontine haemorrhage = bilateral pin-point, reactive (sympathetics destroyed, parasympathetics intact). (2) CN III compression = ipsilateral dilated, fixed (parasympathetics on outside of CN III compressed). (3) Horner's syndrome = ipsilateral miosis + ptosis + anhidrosis (sympathetics). (4) Midbrain tectal lesions = mid-position, fixed (both sympathetic and parasympathetic inputs disrupted).
Incorrect. Pin-point pupils in pons bleed = bilateral sympathetic destruction (not CN III compression). CN III compression causes DILATED, fixed pupil (parasympathetics destroyed). The distinction is critical: dilated + fixed = CN III compression (herniation); pin-point + reactive = pontine bleed.
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The fibres of the cochlear (auditory) pathway from the cochlear nuclei cross the midline at the level of the pons as the:
Correct! Cochlear fibres from the dorsal and ventral cochlear nuclei cross the midline in the TRAPEZOID BODY (transverse fibres in the lower pontine tegmentum) to reach the contralateral lateral lemniscus and medial geniculate body.
Auditory pathway: Cochlea → CN VIII → Cochlear nuclei (pontomedullary junction) → most fibres cross as trapezoid body → lateral lemniscus → inferior colliculus (midbrain) → medial geniculate body (thalamus) → Heschl's gyrus (primary auditory cortex, superior temporal gyrus). Bilateral cortical representation means unilateral cortical lesion does NOT cause complete deafness.
Incorrect. Auditory fibres cross as the TRAPEZOID BODY in the pons. Medial lemniscus = posterior column pathway crossing in medulla. Internal arcuate = same as medial lemniscus. Transverse pontine fibres = corticopontocerebellar (not auditory).
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The ventral (basis) pons contains all of the following EXCEPT:
Correct! The CN VI nucleus and facial colliculus are in the DORSAL pontine TEGMENTUM (floor of 4th ventricle), NOT in the ventral basis pontis. The basis pontis contains longitudinal corticospinal fibres, transverse pontocerebellar fibres, and scattered pontine nuclei.
Pons = two parts: (1) Ventral BASIS PONTIS = longitudinal motor fibres (corticospinal/bulbar) + pontine nuclei + transverse fibres. (2) Dorsal TEGMENTUM = cranial nerve nuclei (V, VI, VII, VIII), sensory tracts (medial lemniscus, spinothalamic), MLF, ARAS. This distinction explains locked-in syndrome (basis destroyed = motor lost; tegmentum spared = consciousness preserved).
Incorrect. CN VI nucleus and facial colliculus are TEGMENTUM (dorsal pons), not basis pontis. Basis pontis = corticospinal (longitudinal, scattered) + pontine nuclei (grey) + transverse fibres (crossing to middle cerebellar peduncle).
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