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CN III = oculomotor, Distinguishing a 3rd nerve palsy due to damage of the…
CN III = oculomotor
Anatomy
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Visceral motor component = innervates the intrinsic ocular muscles:
- Constrictor pupillae
- Ciliary muscle
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Oculomotor complex consists:
- Somatic motor subnuclei
- Edinger-westphal nucleus = visceral motor neurons
- Located at the superior colliculus
1) Axons of both nuclei course ventrally in the midbrain -> oculomotor nerve
2) Emerges from the interpeduncular fossa in the midbrain
3) Passing between the posterior cerebral and the superior cerebellar arteries
4) Pierces the dura and enters the cavernous sinus (sup to the trochlear nerve)
5) Split into:
- Smaller superior division
- Larger inferior division
and then passes through the superior orbital fissure
Divisions
Inferior division:
- Medial rectus
- Inferior rectus
- Inferior oblique muscles
Superior division:
- Superior rectus
- Levator palpebrae superioris muscles
Axons
Visceral motor axons
- Run with the nerve to the inferior oblique muscle for a short distance, then leave it to terminate in the ciliary ganglion
Postganglionic axons
- Leave the ciliary ganglion as 6 to 10 short ciliary nerves to enter the eye at the posterior aspect near the exit of the optic nerve
Parasympathetic motor component
- Pupillary Light reflex
- Accommodation Reflex
Edinger-Westphal (visceral motor) nucleus
- Located in midbrain
- Preganglionic visceral motor axons leave the nucleus and course ventrally
- The parasympathetic and somatic axons together constitute CN III
- The parasympathetic axons are located on the superomedial surface of the nerve
- The parasympathetic axons branch from the nerve to the inferior oblique muscle and terminate in the ciliary ganglion near the apex of the orbit
- Postganglionic axons leave the ciliary ganglion as 6 to 10 short ciliary nerves --> terminate in the constrictor pupillae muscle and the ciliary muscle
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Pupillary light reflex
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The pupillary light reflex involves two cranial nerves:
- The optic nerve (CN II): forms the sensory limb by carrying the sensory signal to the brainstem
- The oculomotor nerve (CN III): forms the motor limb by carrying motor signals to the pupillary constrictor muscle
- Light entering the eyes causes signals to be sent to both the ipsilateral and the contralateral pretectal olivary nuclei
- The pretectal olivary nucleus cells send signals bilaterally to the Edinger- Westphal nuclei
- Visceral motor signals are sent to the ciliary ganglia
- Postganglionic axons terminate in the constrictor pupillae muscles of the iris
In the normal reflex, light shone in either eye causes:
- Constriction of the pupil in the same eye (the direct light reflex)
- Constriction in the other eye (the consensual light reflex)
Somatic motor component
Subnuclei within the oculomotor nucleus supply individual muscles:
- Lateral subnuclei
- Medial subnucleus
- Central subnucleus
Medial subnucleus:
- contralateral superior rectus muscle
Central subnucleus:
- Levators palpebrae superioris bilaterally
Lateral subnuclei:
- Ipsilateral inferior rectus
- Inferior oblique
- Medial rectus muscles
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Eyelids
upper eyelid = palpebra superioris
- Elevated by elevator palpebra superioris
- Levator and the superior rectus muscles are innervated by the superior division of the oculomotor nerve
lower eyelid = palpebra inferioris
- Does not have its own muscle, instead = tendon of the inferior rectus muscle -> Inferior tarsus
- Both eyelids also moved by smooth muscle called "Muller's muscles" or sup and inf tarsal muscles
- The smooth muscle is innervated by sympathetic nerves
Oculomotor nerve damage
Peripheral axons
Damage to axons in the subarachnoid space may be due to infection, tumor infiltration, or infarction
- (loss of blood supply, usually caused by diabetes or hypertension)
Compression of axons may be due to aneurysms, most typically in the posterior communicating artery and sometimes in the basilar artery
Compression of axons may be caused by the uncus of the temporal lobe during cerebral herniation if there is raised intracranial pressure
Compression of axons in the cavernous sinus may be due to tumors, inflammation, infection, or thrombosis
Damage may be caused by trauma to the area where axons pass through the superior orbital fissure to enter the orbit
Nucleus
Trauma, ischemia, or demyelination within the midbrain
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Distinguishing a 3rd nerve palsy due to damage of the oculomotor nucleus from damage to the nerve itself
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Nuclear lesion
- No ptosis
- No weakness in upward gaze in the ipsilateral eye, but weakness of upgaze in the contralateral eye
- Ipsilateral weakness of downward gaze
- Ipsilateral weakness of adduction
- Dilated unresponsive pupil
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Disorders
Peripheral lesions
GSE component lesions will produce lower motor neuron paralysis of the extraocular eye muscles supplied by this nerve
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Central lesions
Lesion located near the ventromedial aspect of the midbrain
- Such a lesion invariably affects both fibers of the third nerve and corticospinal fibers
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