Cranial Nerves III, IV, VI Flashcards
The sulcus limitans separates motor & sensory nuclei in ____ & ______
the spinal cord and brainstem
What are the extrinsic ocular muscles and their movements?
1) superior oblique - internal rotation (intorsion)
2) Superior rectus - elevation
3) medial rectus - adduction
4) lateral rectus - abduction
5) inferior rectus - depression
6) inferior oblique - external rotation (extortion)
What are the multiple columns of neurons in III?
- Levator palpebrae superioris, in midline innervates muscle bilaterally
- Superior rectus innervated contralaterally
- Column supplying medial rectus, inferior oblique, inferior rectus are project to ipsilateral muscles
- Edinger-Westphal nucleus, preganglionic parasympathetic projection to ipsilateral ciliary ganglion, innervates pupillary sphincter and ciliary muscle
True or false: The nuclei of III are very close together so a lesion often affects both.
True
True or False: The partly crossed nature of III is of major clinical significance.
False
True or false: Once III leaves the brainstem it contains fibers the supply only ipsilateral structures, so a nerve lesion affects only one eye.
True
What are the results of an injury to III?
-Eye ipsilateral to lesion deviates laterally
-Medial rectus is weak and lateral rectus is unopposed, lateral strabismus
-Patient cannot move the eye medially
-Vertical movement impaired, weak superior and inferior recti and inferior oblique ipsilaterally
-Diplopia: double vision
-Ipsilateral levator palpebrae superioris weak: ptosis
-Mydriasis: pupil on affected side is dilated
Pupillary dilator is unopposed as pupillary sphincter and ciliary muscle are dysfunctional
-Pupil does not constrict in response to light
-Lens cannot be focused for near vision
True or false: IV is unique among the cranial nerves in that it is the only one to originate entirely from a contralateral nucleus and it is the only cranial nerve on the dorsal brainstem surface.
True
What extrinsic ocular muscles does IV innervate?
the contralateral superior oblique muscles
What is the path of IV?
Fibers leave the nucleus and turn caudally in PAG, arch dorsally to decussate and leave brainstem in midbrain-pons junction
What are the results of an injury to IV?
- Much less noticeable than deficits of III
- Superior oblique moves eye downwards and laterally
- Patients report diplopia when going downstairs or reading
True or false: The long intracranial course of the abducens nerve (VI) makes it susceptible to intracranial pressure.
True
What extrinsic ocular muscles does VI innervate?
ipsilateral lateral rectus, which abducts eye
VI begins in the _____.
floor of 4th ventricle, caudal pons
Medial to VI nucleus are the _______.
MLF and motor fibers of VII
What are the results of an injury to VI?
- medial strabismus (affected eye deviated medially)
- VI nuclear damage causes medial strabismus and lateral gaze paralysis
- Ipsilateral eye will not abduct past midposition and contralateral eye will not adduct past midposition
- Nucleus contains motor neurons and internuclear neurons that ascend in MLF
MLF interconnects ____, ____ & ____ to allow coordination of head and eye movement
III, IV & VI
How does the MLF allow both eyes work together during lateral gaze?
To look to one side requires ipsilateral rectus and the contralateral medial rectus to contract simultaneously
MLF injury results in?
-Internuclear ophthalmoplegia -
“paralysis of the eye due to damage between the nuclei”
-MLF damage removes excitatory input to ipsilateral III nucleus
-Eye ipsilateral to lesion fails to move past midposition during horizontal gaze
-Both VI nuclei are intact so lateral movements of both eyes are intact
What is the pupillary light reflex?
Constriction of pupils in response to light
What four sets of neurons are involved in the pupillary light reflex?
1) Afferent limb: retinal ganglion cells to pretectal nuclei
2) Each pretectal nucleus is linked to both EWN, fibers reach the contralateral EWN via the posterior commissure
3) Pregang. Parasym. fiber enter III nerve, synapse in ciliary ganglion
4) Postgang. fibers in short ciliary nerves, enter iris, supply sphincter (constrictor) of pupillae
What is accommodation or ‘the near response’?
- At rest lens is flattened by tension on its capsule exerted by the suspensory ligament
- The lens bulges passively when ciliary muscle contracts
- To view an object close up: ciliary muscle contracts, which relaxes lens suspensory ligaments
- Other features:
- Sphincter pupillae contracts - Decreased light through periphery of lens (which is thinner)
- Convergence: visual axis of both eyes converge due to increased tone in medial rectus muscles
What is the sympathetic pathway to the eyes?
1) Central fibers from hypothalamus, cross midline in midbrain join ipsilateral descending fibers in pons and medulla
2) Preganglionic fibers emerge in first thoracic ventral nerve root and ascend sympathetic chain to SCG
3) Postganglionic fibers run along external and internal carotid arteries and their branches - Fibers leave IC, join Vi in cavernous sinus, then to ciliary nerves
What are the 3 main types of conjugate movements (when the eyes move as a pair)?
1) Scanning: eyes move from one visual target to another in high-speed movement know as saccades
2) Tracking: a.k.a, smooth pursuit, eyes follow area of interest across visual field
3) Compensation: gaze held on object of interest during head movement, known as vestibulo-ocular reflex (vestibular system also at work)
What are the gaze centers for scanning?
Gaze centers: in brainstem reticular formation; 4 (left, right, up, down)
1) Horizontal saccades: paramedian pontine reticular formation (PPRF), each pulls eye to its own side
2) Upward saccades: midbrain, rostral interstitial nuclei of the MLF (riMLF)
3) Downward saccades: in midbrain next to upward center
What are pathways involved in the voluntary saccade to the left?
1) Projection from right frontal eye field activates left paramedian pontine reticular formation (PPRF).
2) Some PPRF neurons activate adjacent abducens neurons
3) Other PPRF neurons send heavily myelinated (fast) internuclear fibers along the medial longitudinal bundle to activate oculomotor neurons serving medial rectus.
*Simultaneous contraction of the respective rectus muscles yields a saccade to the left.
What activates the left PPRF?
Right frontal eye field
True or false: PPRF neurons do not activate adjacent VI neurons
False, some do.
What do other PPRF neurons do?
They send heavily myelinated (fast) internuclear fibers in MLF to activate III serving medial rectus.
