CN's 3,4,6 Flashcards
What type of nerves are CN’s 3,4 and 6
3: Somatic motor and Visceral motor
4: Somatic motor
6: Somatic motor
CN 3 (Occulomotor):
- Axon type(s)
- CNS origin/termination
- Peripheral origin/termination
- Somatic motor
- Occulomotor nucleus (Rostral midbrain)
- Superior, inferior, medial recti, inferior oblique, and levator palpebrae superioris
- Autonomic efferent
- Edinger-Westphal nucleus
- Pupillary sphincter, ciliary muscle (after synapse in ciliary ganglia)
CN 4 (Trochlear):
- Axon type(s)
- CNS origin/termination
- Peripheral origin/termination
- Somatic motor
- Trochlear nucleus (caudal midbrain)
- Superior oblique
CN 6 (Abducens):
- Axon type(s)
- CNS origin/termination
- Peripheral origin/termination
- Somatic motor
- Abducens nucleus
- Lateral rectus
Where do the extrinsic ocular muscles insert?
-Sclera
Movements of superior and inferior oblique muscles?
- Superior: Internal rotation (intorsion)
- Inferior: External rotation (extorsion)
People with weakness in superior oblique contain of what?
-Double vision
What structures are in the cavernous sinus?
- CN’s 3,4,6
- Vi and Vii
- ICA
How is levitator palpebrae superioris innervated?
-Bilaterally
How is the Superior rectus innervated?
-Contralaterally
How is medial rectus, inferior oblique and inferior rectus innervated?
-Ipsilateral
Results of CN III injury?
- Ipsilateral eye deviates laterally
- Vertical movement impaired
- Diplopia: double vision
- Ptosis: ipsilateral levator palpebrae superioris is weak
- Mydriasis: ipsilateral pupil is dilated
- Pupil does not constrict in response to light
- Lens cannot be focused for near vision
What does CN IV (Trochlear) innervate?
-Contralateral Superior Oblique
Why is CN IV unique?
-Only one to originate entirely from a contralateral nucleus and on dorsal brainstem surface
What are the results of CN IV injury?
-Patients complain of diplopia when going downstairs or reading
Select the true statement:
A) Trochlear deficits lead to a relatively mild deficit
B) Oculomotor nerve injury is associated with a dilated pupil
C) Oculomotor nerve injury is associated with lateral strabismus
D) All the above are true
D
Results of CN VI (Abducens) injury (on left side)
- Medial strabismus
* Can look right, but not left. Strabismus also present when looking straight ahead
Result of CN VI Nuclear damage?
- Medial strabismus and lateral gaze paralysis
- Ipsilateral eye will not abduct past mid position and contralateral eye will not adduct past mid position.
- Nucleus contains motor neurons and internuclear neurons that ascend in MLF (MLF crosses in midline)
What is the function of the Medial Longitudinal Fasciculus?
-It interconnects 3,4, and 6 nuclei to allow coordination of head and eye movement
Results of an injury to Medial Longitudinal Fasciculus?
- Internuclear opthalmoplegia
- removes excitatory input to ipsilateral 3 nucleus
- eye ipsilateral to lesion fails to move past mid position during horizontal gaze
- Both CN 6 nuclei are intact lateral movements of both eyes are intact
What are the 4 sets of neurons involved in the pupillary light reflex?
- Afferent limb: (CN II) Retinal ganglion cells to contralateral pretectal nuclei in midbrain
- Each pretectal nucleus us linked to both EWN, fibers reach the contralateral EWN via posterior commisure
- Preganglionic parasympathetic fibers enter CN III nerve, synapse in ciliary ganglion
- Postganglionic fibers in short ciliary nerves enter iris, supply pupillary sphincter
What should happen when light is shined into an eye?
-Both eyes should contract. If not, could be a lesion in CN III of midbrain
Describe Accommodation.
- To view an object up close, the ciliary muscle must contract, which relaxes lens suspensory ligaments and the lens bulges.
- sphincter pupillae contracts, decreasing light through periphery of lens and forces light thru thick part of lens (center)
- Convergence: both eyes converge due to increased tone in medial rectus muscles
Select the true response:
A) The afferent limb of the pupillary reflex is III
B) The efferent limb of the pupillary reflex is IV
C) Accommodation consists of contraction of pupillary sphincter and the ciliary body
D) All the above are true
C
Describe the sympathetic pathway to the eye
- Central fibers from hypothalamus cross midline in midbrain to join ipsilateral descending fibers in pons/medulla
- Preganglionic fibers emerge from ventral root, travel up sympathetic chain to SCG
- Postganglionic fibers run with ICA, leave to join Vi in cavernous sinus, then to ciliary nerves
Describe the scanning movement.
-Eyes move from one visual target to another very fast, known as saccades
Describe the tracking movement.
-AKA smooth pursuit. Eyes follow area of interest across a visual field
Describe the compensation movement.
Gaze held on object of interest during head movements, known as vestibuloocular reflex
In scanning, where are the gaze centers located?
-Brainstem reticular formation (4)
Where is the gaze center for horizontal saccades? Upward? Downward?
- Horizontal: Paramedian pontine reticular formation (PPRF) each pulls eye to own side
- Upward: Midbrain, rostral interstitial nuclei of MLF (riMLF)
- Downward: in midbrain next to upward center
What happens during a voluntary saccade to the left?
- Right frontal eye field activates left PPRF
- Some PPRF neurons activate adjacent VI neurons (left lateral rectus)
- Other PPRF neurons send heavily myelinated internuclear fibers in MLF to activate III (right medial rectus)
Select the true response:
A) Trochlear nerve: Exits brainstem posteriorly
B) Oculomotor nerve: Superior oblique muscle
C) Abducens nerve: Inferior oblique
D) Lateral rectus: Eye adduction
E) Inferior rectus: Eye elevation
A
Select the correct location: A) Edinger- Westphal nucleus: Caudal midbrain B) Oculomotor nucleus: Caudal medulla C) Trochlear nucleus: Rostral midbrain D) Abducens nucleus: Caudal pons E) None of the above are correct
D
Select correct statement:
A) Oculomotor nerve injury results in a mild deficit of eye movement
B) Trochlear nerve injury results in a large deficit of eye movement
C) Abducens injury results in medial strabismus
D) Oculomotor nerve injury results in a constricted pupil due to interruption of sympathetic fibers
C
Select correct statement about pupillary reflex:
A) Efferent limb is the oculomotor nerve
B) Afferent limb is retina ganglion cell fibers in cranial nerve II
C) Input from cranial nerve II is sent to the Edinger Westphal nucleus bilaterally
D) Postganglionic fibers from the ciliary ganglion innervate the iris and supply the sphincter muscle of the pupil
E) All the above are true
E
Accommodation includes:
A) Contraction of the ciliary muscle to allow the lens to thicken centrally
B) Convergence of eyes by slight increase in medial rectus muscle tone bilaterally
C) Contraction of sphincter pupillae to constrict pupils
D) All the above are part of accommodation
D
