CN II Flashcards
Function of CN III
- GSE = SKELETAL MOTOR to upper eyelid and extraocular muscles
–> levator palpebrae superioris (LPS), Super, medial and inferior rectus, and inferior oblique
- GVE = PARASYMPATHETIC visceral motor to internal muscles of eye
–> sphincter pupillae muscle and ciliary muscle
describe oculomotor nucleus (GSE)
- Located ventral to periaqueductal gray in ROSTRAL MIDBRAIN
- contains motor neurons whose axons gather to form the oculomotor nerve
describe Edinger-Westphal nucleus (EW-N)
- Contains cell bodies of PREGANGLIONIC PARASYMPATHETICS
- axons join the main oculomotor nerve to course to the orbit
- fibers terminate in the CILIARY GANGLION with post ganglionic parasympathetic neurons
- Postganglionic fibers terminate in eye bulb via short ciliary nerves
**INNERVATE ciliary muscles (LENS ACCOMMODATION for near vision) and SPHINCTER PUPILLAE MUSCLES (for pupillary constriction)**
oculomotor and trochlear nerve passes between what two structures
- posterior cerebral artery and superior cerebellar artery
What are the muscular deficits resulting from damage to the oculomotor nerve?
- IPSILATERAL LPS IS PARALYZED = Ptosis
- Superior and inferior rectus paralyzed = inability to move eye vertically
- Medial rectus paralyzed = inability to move eye medially
- Inferior oblique paralyzed = Antagonist pulls eye DOWN AND OUT
**Since LR6SO4 are intact, the affected eye deviates DOWNWARD AND LATERALLY**
–> patient presetns with LATERAL STRABISMUS (lateral deviation) and the resulting MISALIGNMENT causes DIPLOPIA (double vision)
describe the parasympathetic deficits resulting from damage to oculomotor nerve.
- Sphincter pupillae = nonfunctional - pupil on the affected side remains dilated (MYDRIASIS)
–> Pupil does NOT constrict in response to light
- CILIARY MUSCLE = Nonfunction - cannot accommodate lens (cannot focus on near objects)
–> Lens is flat
what is the result on intracranial pressure on GVE fibers (which run with the GSE fibers)
- Results in DILATED PUPIL unresponsive to light
- May be confused with HORNER’s SYNDROME
compare and contrast Pressure/damage to CN III and Horner’s syndrome
CN III
- Ptosis (due to paralysis of LPS)
- DEFECTIVE EYE MOVEMENTS (eye is deviated DOWN AND OUT)
- DILATED, LARGE pupil (Mydriasis) due to nonfunctional sphincter pupillae
HORNOR’S SYNDROME
- Ptosis (due to loss of sympathetic innervation to Muller’s muscle)
- CONSTRICTED PUPIL (MIOSIS) due to interruption of sympathetic innervation to the DILATOR PUPILLAE
- ANHIDROSIS
describe the trochlear nucleus
- contains GSE nerve cell bodies (SKELETAL MOTOR to superior oblique)
- located at the level of the inferior colliculus in the CAUDAL MIDBRAIN
- fibers leave nucleus, DECUSSATE POSTERIORLY and exit the brainstem just inferior to the inferior colliculus
describe tochlear nerve
- Arises from the CONTRALATERAL tochlear nucleus
- emerges from the POSTERIOR surface of the brainstem
- Has longest intracranial course
- is the thinnest
- Function = intorsion, depression, abduction (DOWNWARD AND OUTWARD)
What is the result of Damage to trochlear nucleus
- Paralysis or paresis of the contralateral SO = eye drift’s upward caused by unopposed action of the antagonist caused in ELEVATION known as HYPERTROPIA
Result of damage to trochlear nerve (after it exits brainstem)
- Paralysis of paresis of the IPSILATERAL SO = eye drifts upward causing elevation (HYPERTROPIA)
When does HYPERTROPIA become WORSE
- downward gase (when looking down) = becomes partially depressed by IR, but not completely (vertical diplopia)
- Medial downward gaze
- When tilting the head toward the affected side
Why does the eye Extort with a lesion to the trochlear nerve
- caused by action of unopposed inferior oblique muscle (which extorts, elevates and abducts) and results in EXTERNAL STRABISMUS
- eyes become misaligned, the patient experiences VERTICAL DIPLOPIA (double vision)
What makes symptoms of a trochlear nerve lesion better
- Diplopia is minimized when:
–> tilting the head TOWARD the NORMAL side = causes normal eye to rotate inward (intort) and become aligned with affected eye which is rotated OUTWARD (extorted)
–> Pointing chin Downward (chin tuck) which rolls normal eye upward = compensates for the hypertropia of the affected eye