Lec 25 Anatomy of Eye and Central Visual Pathways Flashcards
What is the path of innervation upstream to the PPRF?
- from frontal eye field [FEF] projects through anterior limb of internal capsule and decussates [crosses] to opposite side at midbrain-pontine junction
Is the PPRF contra or ipsi from the medial rectus it innervates? what about the lateral rectus?
medial = contra lateral = ipsi
Which direction saccade does the right FEF command?
right FEF = left PPRF = commands conjugate gaze to the left = left lateral rectus and right medial rectus activated
What happens if there is a lesion of the abducens nerve [ex. left]?
CN VI palsy –> impaired abduction of ipsilateral eye
ex. if the lesion is on the left, the left eye will be turned in medially and unable to abduct
what happens if there is a lesion of the abducens nucleus or PPRF [ex. left]?
ipsilateral lateral gaze palsy
ex. if lesion on left, then when you try to look left nothing happens. when you look right you get normal right gaze
[left gaze would require action of left lateral rectus and right medial rectus. since PPRF upstream is out don’t have left lateral rectus function + don’t have the ], nothing happens
PPRF is
what happens if there is a lesion of the MLF [ex. left]?
interuclear opthalmoplegia [INO]
ex. if lesion of left MLF, when you try to look left its fine. when you try to look right –> right eye is fine but left eye stays [unable to look right/adduct], have nystagmus in right eye
convergence normal [can get ipsilateral [left] adduction in convergence just not in conjugate]
INO due to right MLF lesion can be seen when gaze toward which side?
gaze toward left [problem with right eye adduction]
What disease should you think of if you see INO [particularly bilateral younger pt vs older unilateral]?
bilateral younger = MS
unilateral older = stroke
What comprises the external eye?
sclera + cornea
what comprises the intermediate eye?
iris + ciliary body + choroid
what comprises the internal eye?
retina
What are the 2 most important structures for capturing light?
cornea, lens
What are zonule fibers?
connect lens to ciliary muscle
What are the 3 fluid compartments of the eye?
anterior chamber: filled with aqueous
posterior: where aqueous made by ciliary body
vitreous body: filled with vitreous humor
What are the photoreceptors in the retina?
rods and cones
What are horizontal cells in the retina?
spread laterally between photoreceptor and bipolar layer, spread transmission laterally
what are amarcine cells in the retina?
spread laterally between bipolar layer and ganglion cells, spread transmission laterally
What is horner’s syndrome? possible dangerous cause?
sympathetic disorder –> could indicate dissection of cartoid artery
signs: ptosis, anhidrosis [decreased sweating], miosis
What is tonic pupil? cause?
dissociation of light-near reflexes cause by lesion in ciliary ganglion
signs: mydriasis [dilation], absent pupillary light reflex, preserved pupillary near reflex [but slow to return to dilate]
What is argyll-robertson pupil? cause?
dissociation of light-near reflexes caused by tertiary neurosyphilis or diabetes
- irregular pupils accomodate but don’t react = constrict in near reflex but not in light
What is marcus gunn pupil?
- due to optic neuropathy or severe retinal injury
decreased bilateral pupil constriction when light is shone in affected eye relative to unaffected –> look for paradoxical pupil dilation in bad eye after constriction
What is the order of cells in the retina from outside to in [same direction light travels]?
ganglion cells –> amacrine cells –> bipolar cells –> horizontal cells –> photoreceptors
What is contained in “nuclear” layers in retina? what about “plexiform” layers?
nuclear layers have cell bodies
plexiform layers have synapses
What layer of retina has ganglion cell axons?
nerve fiber layer
What is function of pigment epithelium layer of retina?
contains melanin, absorbs light + prevents back scatter, phagocytosis of discarded outer segments
What is function of rods [type of spatial resolution, light sensitivity, vision]?
- low spatial resolution
- high light sensitivity
- can detect single photon, night vision, peripheral vision
What is function of cones [type of spatial resolution, light sensitivity, vision]?
- high spatial resolution
- low light sensitivity
- acuity and color
- day vision
- central foveal vision
what is degree of convergence photoreceptors and their bipolar cells onto retinal ganglion for cones vs rods?
many rods onto few bipolar cells = high degree of convergence to detect single photon
less convergence for cones = for high visual acuity to retain spacial specificity
What is the fovea?
region of retina sensitive to acuity. has fewer cells per layer, cones only [ no rods]
What covers the retina?
covered by dura because it is a CNS structure
What is the macula?
area around the fovea, responsible for central vision
What parts are retina are responsible for peripheral vs central portion of right visual field?
central = left temporal retina peripheral = right nasal retina
Retro-chiasmally [before chiasm toward brain] what info is carried in the right optic tract?
all info from left visual field
What crosses in the optic chiasm?
only the nasal halves of the retina that are then responsible for ipsilateral peripheral vision [ipsilateral meaning in regard to side after chiasm and eye visual field]
Info from the inferior nasal visual field is mapped to what part of the retina?
superior temporal part of retina
What makes optic chiasm particularly vulnerable?
sits just next to anterior communicating artery –> vulnerable to aneurysms
right near pituitary –> pituitary may jam up into optic chiasm
What is the convention for how visual fields are drawn?
- visual fields drawn the way a person sees the world –> visual field of left eye shown on left and that of right eye on the right
What myelinates the retina?
oligodendrocytes [not schwann cells] because its part of CNS
What is the magnocellular pathway [type of info it carries, from which photoreceptors, size of receptive field]?
- one of two main functional channels of vision
- carries rod-generated signals of motion/direction
- insensitive to color + detail
- large receptive fields
- mostly from peripheral retina
what is the parvocellular pathway [type of info it carries, from which photoreceptors, size of receptive field]?
- one of two main functional channels of vision
- carries cone-generated signals
- small receptive fields
- acuity and color sensitive
Where do most optic tract fibers terminate?
lateral geniculate nucleus [LGN] of thalamus
Which layers do magnocellular vs parvocellular go in the LGN?
layers 1-2 = magnocellular
layers 3-6 = parvocellular
which layers of LGN from ipsilateral vs contralateral retina?
contralateral = 1, 4, 6 ipsilateral = 2, 3, 5
Where is the primary visual cortex?
in occipital lobe in calcarine fissure
Fibers from which part of visual field travel through temporal lobe radiation vs temporal lobe radiation [meyer’s loop]?
superior visual field [inferior retinal quadrant] = temporal
inferior visual field [superior retinal quadrant] = parietal
What is effect of lesion to optic tract or LGN [ex. left]?
homonymous heminaopia in contralateral visual field
if left optic tract/LGN –> right half of visual field for both eyes is impaired
What does homonymous hemianopia mean?
loss of half of visual field for each eye. ex. if its a left homonymous hemianopia –> the left half of visual field for each eye is impaired
What is common cause of optic tract homonymous hemianopia?
- occlusion of PCA or anterior choroidal artery
- causes lesion of optic tract or LGN lesion
What is effect of meyer’s loop lesion [ex. right side]?
- contralateral homonymous superior quadrant hemianopia
- meyer’s loop carries superior visual field –> get “pie in the sky” = quadrant of superior visual field impaired
ex. right side loop lesion –> loss of superior left quadrant visual field for both eyes
What is effect of lesion of parietal optic radiation [ex. right side]?
contralateral homonymous inferior quadrant hemianopia
ex. right side lesion –> loss of inferior left quadrant visual field for both eyes
Where does superior vs inferior visual field map to in calcarine fissure?
superior field –> inferior calcarine fissure
inferior field –> superior calcarine fissure
where does peripheral visual field vs macular part map to in calcarine fissure?
peripheral = more anterior central/macular = more posterior toward occipital pole
What is effect of lesion at optic chiasm?
bitemporal hemianopia –> peripheral vision lost in both sides because the nasal part of retina is what crosses = the temporal part of visual field
What is effect of lesion of primary visual cortex?
- homonymous hemianopia of contralateral side
- usually some central vision spared [macular sparing]
What causes macular sparing in primary visual cortex lesion?
- large extent of macular projection onto calcarine cortex
- dual blood supply from MCA and PCA
Which is dorsal vs ventral of what/where path? what fasciculi respectively?
what = dorsal = inferior longitudinal fascilucus, terminates in inferior temporal cortex where = ventral = superior longitudinal fasciculus, terminates in posterior parietal cortex
Where do fibers of pupillary light reflex path terminate?
midbrain pretectum –> ipsilateral and contralateral edinger-westphal nuclei [contralateral via posterior commissure
What is the pupillary near reflex?
pupils constrict when shift gaze from distant to near
What are the steps along the direct pupillary light reflex?
light to eye –> optic nerve –> chiasm –> tract –> midbrain pretectum
–> ipsilateral edinger westphal –> CN 3 to ciliary ganglion –> ipsilateral sphincter muscle contracts / pupil constricts
What are the steps along the consensual light reflex?
light to eye –> optice nerve/chiasm/tract –> midbrain pretectum
–> posterior commissure –> contralateral edinger westphal –> cn 3 to ciliary ganglion –> contralateral sphincter contracts /pupil constricts
What are ligh-sensitive biological clocks?
- photosensitive retinal ganglion cells transduce light energy and convert to electrical impulse
- non-image forming cells but tell you ambient light intesity
allow for: - synchronization circadian rhythms to light/dark cycle via hypothalamus
- help regulate pupil diameter via pretectal nucleus
- participated in light-dependent release of melatonin from pineal gland
