Cranial nerves III, IV, VI Flashcards
What structure separates the motor and sensory nuclei in the spinal cord and in the brainstem?
The sulcus limitans
What is the action of the superior rectus muscle?
It elevates the eye- make you look up
What is the action of the inferior rectus muscle?
It depresses the eye- makes you look down
What is the action of the lateral rectus muscle?
Adbducts the eyeball
What is the action of the medial rectus muscle?
It adducts the eyeball
What is the action of the superior oblique muscle?
Internal rotation- helps you go cross-eyed- up and in
What is the action of the inferior oblique muscle?
External rotation- down and out
Where are CN III, IV, and VI located on the brainstem?
III and IV are located in the midbrain and VI is located in the pons
Which muscles does the oculomotor nucleus innervate?
Superior, inferior, and medial recti
Inferior oblique
Levator palpebrae superioris
Which muscles does the Edigner-Westphal nucleus innervate?
The pupillary sphincter as well as (after a synapse in the parasympathetic ganglion) the ciliary muscle
Which muscle does the trochlear and adbucens nuclei innervate respectively?
Trochlear- superior oblique
Lateral rectus- abducens
What is the only CN to exit the posterior side of the brainstem?
The trochlear nerve (CN IV)
The Edinger-Westphal nucleus is the autonomic component.
Truth.
Which CN is the red nucleus closest to?
CN III- oculomotor
Let’s talk about the results of a CN III lesion.
Holy crap- there’s a lot (ocho of them)
- Ipsilateral eye deviates laterally because the medial rectus would be useless and abducens is unopposed.
- Patient can’t move eye medially.
- Vertical movement is impaired due to weak superior and inferior recti and inferior oblique
- Double vision
- Levator palpebrae superioris is weak so you get a droopy eyelid
- Mydriasis: the pupil on the affected side is dilated because the pupillary sphincter and ciliary muscle are dysfunctional
- Pupil doesn’t constrict in response to light
- Lens cannot be focused for near vision.
CN ___ is innervated contralaterally.
CN IV
T/F all eyeball related CNs have ipsilateral innervation, except for CN IV?
Trooth. (That’s some appalachan spelling fer ya)
T/F- it is a big deal if you’ve got a CN IV lesion?
Not really- the SO muscle moves the eye downwards and laterally, so patients report diplopia (double vision) when going downstairs or reading
Because of its long intracranial course, CN ___ is susceptible to increased intracranial pressure.
CN VI
Where in the brainstem is CN VI located?
In the caudal pons, the floor of the 4th ventricle
CN VI nucleus and the internal genu (of CN VII) together are called:
the facial colliculus.
Where do the fibers of CN VI wrap around and turn to exit?
The internal genu of CN VII
What is a medial strabismus and what causes it?
It is where the affected eye deviates medially and is caused by a lesion in CN VI
Describe the eye movements of someone with a medial strabismus.
Ipsilateral eye will not abduct past the midposition and the contralateral eye will not adduct past the midposition.
The medial longitudinal fasciculus does what?
- Allows the coordination of head and eye movement.
2. Interconnects CN nuclei III, IV and VI
Looking to one side requires what to happen?
The lateral rectus and the contralateral medial rectus to contract simultaneously. MLF allows for this to occur
Let’s talk casualties. Medial longitudinal fasciculus injuries- what happens? (3 things)
- Eyeball paralysis due to damage between the nuclei- they can no longer coordinate with each other
- MLF damage removes the excitatory input to ipsilateral III nucleus (I assume this is the sympathetic functions?)
- Ipsilateral to the lesion, the eye can’t move past the mid point during a moving horizontal gaze
If an MLF injury exists, what happens to lateral eye movements (CN VI)?
Nothing- they still work because a median longitudinal fasciculus lesion doesn’t damage the CN VI nuclei.
What is the pupillary light reflex?
It is the constriction of pupils in response to light.
How does the pupillary light reflex work?
4 stages/4 neurons
- (afferent limb) The retina receives light and sends the signal to the pretectal nuclei.
- Each pretectal nucleus is linked to both EWN (Edinger-Westphal Nucleus)- one ipsilaterally and the other one via the posterior commissure.
- Preganglionic parasympathetic fibers from the EWN enter CN III and synapse in the ciliary ganglion.
- Postganglionic fibers in the short ciliary nerves enter the iris and supply the pupillary sphincter.
- Boom. Constriction. In both eyes.
The lens of the eye is involved in focusing. At rest the lens is flattened by tension on its capsule exerted by the suspensory ligament. How can you see up close (accommodation)?
The ciliary muscle contracts, which relaxes the suspensory ligaments- this allows the lens to bulge and focus on something close up.
What is the near response of accommodation?
It is when the sphincter pupillae contracts- light is decreased through the periphery of the lens. You want to view the object through the thicker part of the middle of the lens instead of the periphery.
T/F- there is convergence of the visual axis of both eyes due to increased tone in the lateral rectus?
False- it would be true if we were talking about the medial rectus.
What is the sympathetic pathway to the eyes?
Well, I’m not very glad you asked- here we go: (–> means “fibers go to”)
Central fibers in the hypothalamus –> 1st thoracic vertral nerve root –> Superior cervical ganglion–> CN V–> ciliary nerve.
Eyes moving as a pair is known as what type of movements?
Conjugate movements
What are the three types of conjugate movements?
- Scanning- eyes move from one visual target to another in high-speed movement known as saccades
- Tracking- aka smooth pursuit- eyes follow the area of interest across the visual field
- Compensation- gaze is held on an object of interest during head movement
In the conjugate movements of the eyes, what is compensation also known as?
The vestibulo-ocular reflex
How many gaze centers do we have?
4- up, down, left, right
Where are the gaze centers located?
In the brainstem reticular formation
Specifically, where are the horizontal saccades located?
PPRF (paramedian pontine reticular formation)
Specifically, where are the upward saccades located?
In the midbrain- riMLF (rostral interstitial nuclei of the MLF)
Specifically, where are the downward saccades located?
In the midbrain next to the upward center
Concerning voluntary saccade to the left: Which frontal eye field activates the left PPRF?
The right frontal eye field
Concerning voluntary saccade to the left: T/F some PPRF neurons activate adjacent VI neurons
Truth
