Visual System Flashcards
name of the principle refractive medium of the eye, which is avascular
cornea
the outermost layer of the eye, made of type I collagen
sclera- mechanical stability, stable medium for ocular muscles to attach
where is the blood supply of the eye that nourishes the photoreceptors found
choroid layer (middle layer of eyeball wall). anterior portion of choroid layer forms stroma of the iris (blood vessels and pigmentation)
innermost layer of eyeball
neural retina, contains photoreceptors and neurons
job of the ciliary epithelium in the eyeball?
produces and secretes aqueous humor, which is nutrition for avascular lens and cornea. extension of ciliary epithelium is retinal pigment epithelium (maintains photoreceptor metabolism)
what are the 2 smooth muscles that change diameter of the pupillary opening
pupillary sphincter (constrictor) and pupillary dilator
which is more anterior, aqueous humor or vitreous humor, in eye?
aqueous humor is more anterior (behind cornea)
how is aqueous humor circulated in the eye
secreted into posterior chamber, circulates through and drains at outer edge of anterior chamber via trabecular meshwork (drains into scleral vein, towards canal of Schlemm)
cause of glaucoma?
increase in interoccular pressure, causing painless compression of axons of optic nerve at peripheral retina–> can cause loss of peripheral vision
open-angle vs closed-angle glaucoma
open-angle: drainage problem of trabecular meshwork in anterior chamber when aqueous humor is being drained to canal of Schlemm
closed-angle: adhesion of peripheral iris to cornea that prevents aqueous humor from reaching trabecular meshwork
what happens when the suspensory ligaments of the eye relax
ciliary muscle contracts–> suspensory ligaments relax–> lens becomes rounder for near response
near response under parasympathetic control and skeletal motor fibers of CN III (oculomotor)
presbyopia
age-related, lens becomes less elastic, inability to focus on close objects (hinders near response reflex)
T/F: lens replacement (in the eye) is an effective way to restore visual clarity and accommodation
FALSE: lens replacement can restore clarity (lens becomes less flexible with age- presbyopia- so near response is hindered) but NOT accommodation
the sphincter (constrictor) pupillae is innervated by a combination of these 2 neurons (in eye)
preganglionic parasympathetic fibers of CN III (indirect)
postganglionic parasympathetic axons of ciliary ganglion in orbit (direct)
dilator pupillae muscle of eye is innervated by these two neurons (one direct, one indirect)
direct- postganglionic sympathetic axons from superior cervical ganglion in upper portion of neck
indirect- preganglionic sympathetic axons from T1 segment of spinal cord
reminder- Horner’s syndrome results from loss of sympathetic innervation to face, so patients will not be able to dilate pupil (meiosis- pupil is constricted, and ptosis- drooping eyelid)
what are the 4 neurons of the pupillary light reflex?
- optic nerve- bilateral projections, crosses at optic chiasm
- pretectal area in midbrain (rostral to superior colliculi)- also cross and project bilaterally
- preganglionic parasympathetic neurons of Edinger-Westphal in midbrain- ocular motor nerves project to orbit
- neuron in ciliary ganglion (postganglionic parasympathetic)- send axons to eyeball
contraction of the ciliary muscle (which causes suspensory ligaments to relax and produce near response) is under parasympathetic control of ___ and ____
nuclei of Edinger-Westphal and ciliary ganglia
which response, light reflex or near response, requires neurons in the pretectal nuclei?
light reflex ONLY. pretectal nuclei in midbrain needed to generate bilateral pupillary constriction in response to light
Marcus Gunn pupil is typically seen in patients that develop optic neuritis, secondary to multiple sclerosis. what kind of lesion is it, and what kind of affect is it?
isolate lesion of one optic nerve, relative afferent pupillary defect- occurs when light is presented to abnormal eye only (the eye is not “seeing” the light)
both pupils constrict when light is shown on normal eye
expected result of isolated lesion of either optic tract on light reflex?
limited effect on light reflex, because of redundancy of both optic nervous crossing to give bilateral innervation at pretectal neurons (which then cross again to bilaterally innervate Edinger-Westphal, increasing redundancy)
will a lesion of the lateral geniculate body (or any point past the optic tract) affect the pupillary light reflex?
no. light reflex axons do not project directly to visual cortex, so do not need neurons of the optic nerve, optic chiasm, or optic tract
how might an efferent pupillary defect occur?
lesion of oculomotor nerve (CN III), past the lateral geniculate body
will not get constriction in either eye in response to light to abnormal eye. light on normal eye will only cause constriction in the normal eye (not the affected eye)
will get fixed and dilated pupil that won’t respond to light or near response
what is Argyll Robertson pupil? May be seen in patients with Tabes dorsalia (caused by neurosyphilis) which is selective demyelination in dorsal columns of spinal cord. May also be seen in patients with diabetes mellitus or those with pineal tumor
simultaneous bilateral lesion or compression of pretectal area in midbrain
neither pupil will constrict in response to light, but they will constrict for near response (because pretectal area is not involved in near response)
the two projection neurons of the eye are ____; the two interactive neurons are the ____
projection neurons: bipolar and ganglion
interactive neurons: amacrine and horizontal
in what layer of the eye are the photoreceptors?
retina, in pigment epithelium
what are the neurons that photoreceptors interact with in the retina and what are their positions?
photoreceptors are on the BACK of the retina. they synapse with bipolar cells, which synapse with ganglion cells which are most ANTERIOR out of the three. the ganglion cells turn around and head back to the posterior of the eye to become the axons of the optic nerve. basically light has to go THROUGH the ganglion and bipolar cells (the projection neurons) to reach the photoreceptors
what is the limitation of rods ability to sense light
low spatial resolution- limited ability to tell us what an object is or what color it is
great for scotopic (night) vision
what is the shallow depression positioned just on the lateral (or temporal) side of each optic nerve?
fovea- where the cones are. light is focused here. NO bipolar or ganglion cells here- light hits cones directly (cones are less sensitive to light than rods)
the fovea, on the temporal side of the optic disk (where the optic nerve exits retina) is surrounded by ____, a yellow pigmented rim
macula lutea (macular processing = foveal vision = cones)
what can occur from a detached retina in the eye?
detached retina- separation of pigment epithelium from outer segment of photoreceptors. may result in degeneration of photoreceptors (pigment epithelium supplies nutrients, vit A)
age-related macular degeneration (AMD)
progressive retinal detachment- separates fovea from pigment epithelium. causes central visual field loss- decreased ability to recognize object and its color. results in central scotoma (central visual loss affecting only foveal vision)
central scotoma
central visual loss affecting only foveal vision (cones)
what are the 3 neurons of the visual system
- bipolar (in retina)
- retinal ganglion (in retina). optic nerve (axons of retinal ganglion)- only some cross on their way to thalamus
- thalamic nucleus- lateral geniculate body
describe the difference in synapses between rods/cones and bipolar cells
many rods synapse with bipolar cell- convergence makes rods a good detector of light. numerous rod bipolar cells synapse with single retinal ganglion
in fovea, only single cone synapses with bipolar cell, which synapses with one retinal ganglion cell- maximizes visual acuity
function of the interactive neurons of the eye, horizontal cells and amacrine cells?
horizontal cells- enhance contrast by inhibiting adjacent rods/cones in response to stimulus
amacrine cells- detect speed/direction of moving stimulus, elicit and respond to changes in intensity of light stimulus. synapse with retinal ganglion
T/F: optic nerve (second neuron of visual processing pathway) do not cross on their way to the thalamus
trick question! (ha ha). some do, some do not
images seen in the nasal hemifield stimulate the ___ half of the retina
nasal hemifield images stimulate temporal half of retina
temporal hemifield images stimulate nasal half of retina
T/F: most of the optic nerve axons cross
true- about 60%, at the optic chiasm. the optic nerves coming out of the nasal half of each retina are the ones that cross
conceptually, why do only some optic nerves cross?
optic nerves coming from nasal half of retina (receiving temporal light) cross, while temporal optic nerves (receiving nasal light) do not cross.
the net effect is that each hemisphere receives a complete image of everything seen on the opposite side.
for example: your left visual processing center receives info from left temporal optic nerves (which see NASAL light) and info from right nasal optic nerves (which see TEMPORAL light), allowing you to piece together the whole right side of your vision
the right side of the brain processes what is seen in the ____ and _____ hemifields
right side of brain processes what is seen in LEFT nasal and temporal hemifields (left eye for nasal, right eye for temporal- ipsilateral)
each optic tract projects to a third neuron in the lateral geniculate nucleus, which is made of 6 layers (visual radiations) that run ventral (1) to dorsal (6). which layers contain the terminating ganglion cell axons from the ipsilateral (noncrossing) retina?
2, 3, 5
ganglion cell axons that arise from contralateral retina cross in optic chiasm and terminate in layers 1, 4, 6
which layers (visual radiations) of the lateral geniculate body are the parvocellular layers?
3-6 (just not 1 and 2). receive cone directed input for visual acuity
T/F: most of the layers (of the 6) of the lateral geniculate body are driven by rod input
FALSE: parvocellular (cone) input for layers 3-6
only layer 1 and 2 receive magnocellular (rod) input that detects moving objects in sapce
parvocellular vs magnocellular input in the eye
parvocellular = cone input for visual acuity and color magnocellular = rod input for detecting movement
besides projecting to the lateral geniculate nucleus in the thalamus, the optic tracts also project to all of the following: superchiasmatic nucleus pretectal nucleus superior colliculus ...what do these do?
project to:
superchiasmatic nucleus- in hypothalamus for circadian rhythm
pretectal nucleus- in midbrain for light reflex
superior colliculus- conjugate gaze
axons leaving the lateral geniculate body in the thalamus go next to the primary visual cortex, which is located anatomically where?
medial aspect of each occipital lobe
half of the visual radiations of the lateral geniculate body course medially and dorsally through ____ to convey information from _____, while other half project more laterally and form ____ to convey information from ____
half of the visual radiations of the lateral geniculate body course medially and dorsally through PARIETAL LOBE to convey information from LOWER QUADRANTS of visual hemifields,
while other half project more laterally and form MEYER’S LOOP to convey information from UPPER QUADRANTS of visual hemifields
the visual radiations of the lateral geniculate body that form Meyer’s loop course ventrally and rostrally into and through a ____, making a ____
temporal lobe, make a hairpin turn. then course posteriorly to reach primary visual cortex
the primary visual cortex is at medial aspect of each occipital lobe and is subdivided by a horizontal fissure or sulcus known as ___
calcarine sulcus
the calcarine sulcus divides the primary visual cortex into a ___ and ___ gyrus
calcarine sulcus subdivides primary visual cortex into superior cuneus gyrus and inferior lingual gyrus
the cuneus gyrus of the primary visual cortex receives input from where?
the inferior parts of the hemifields project light to the superior part of each retinal quadrant. the superior retinal quadrants relay info to lateral geniculate nucleus, which follows parietal lobe to synapse on cuneus gyrus
so cuneus gyrus overall gets info from inferior hemifields of vision
the lingual gyrus of the primary visual cortex receives input from where?
superior quadrants of hemifields project to inferior retina, which sends axons to lateral geniculate nucleus and these follow Meyer’s loop (hairpin turn) that go through temporal lobe and synapse on lingual gyrus
so lingual gyrus overall gets info from superior hemifields of vision
visual radiations of lateral geniculate body that go through Meyer’s loop synapse where in primary visual cortex?
making hairpin turn, go through temporal lobe (instead of parietal as others do) and synapse on cuneus gyrus
which, foveal or macular vision, is represented to a greater degree in primary visual cortex?
foveal (cones)
paravoceullar vs magnocellular visual projections
both projections from lateral geniculate body
paravocellular- color and form, project to inferior aspects of each temporal lobe
magnocellular- movement and location, project to each parietal lobe
any lesion occurring in front of the optic chiasm will result in
monocular and ipsilateral visual deficits
a compression of the optic chiasm results in what kind of visual deficit
bitemporal (because nasal optic nerves that see temporal light cross) and heteronymous (meaning opposite side of each eye is affected)- loss in temporal hemifields on both sides
any lesion past the optic chiasm (optic tract part or any of visual radiations of lateral geniculate nucleus or primary visual cortex) will result in what kind of deficit
contralateral and homonymous (same side of each eye affected)
how might monocular anopsia occur (what kind of lesion)
anopsia = complete visual loss
lesion in optic nerve, before it gets to optic chiasm- monocular and ipsilateral
papilledema (from increased intracranial CSF pressure or blockage of central artery of retina) or optic neuritis (as an effect of MS) may result in what kind of visual deficit
affects optic nerve before optic chiasm–> monocular visual field loss. if there is a complete lesion or blockage of optic nerve, results in anopsia of the affected eye
an aneurysm of the internal carotid artery may cause what kind of visual deficit
aneurism may expand medially and compress axons on the outside of the optic nerve, causing ipsilateral nasal hemianopsia (located before optic nerve reaches optic chiasm, and temporal fibers are on outside of nerve, which see nasal light)
result of complete lesion or compression via pituitary tumor of fibers crossing the optic chiasm
bitemporal heteronymous hemianopsia- nasal fibers (which see temporal light) cross at chiasm
what is the only place in the visual system where a single lesion can cause bitemporal hemianopsia
optic chiasm (nasal optic nerves cross here)
pituitary tumors are a common cause of compression of what part of visual system
optic chiasm (–> bitemporal hemianopsia)
if a large vascular insult affected all of the visual radiations (of the lateral geniculate nucleus) going through the temporal (Meyer’s loop) and parietal lobe, what visual deficit would occur
contralateral homonymous hemianopsia (visual field opposite of affected side is lost in both eyes- either both L or both R fields in both eyes is lost)
lesions in the Meyer’s loop going through the temporal lobe are more common than those affecting the other visual radiations going through the parietal lobe. what occurs when this happens?
contralateral and homonymous superior/upper quadrantanopsia –> “contralateral pie in the sky”
(Meyer’s loop formed by lateral fibers coming from lower retinal quadrants, which see superior light)
a lesion of the visual radiations (of the lateral geniculate nucleus) that course through the PARIETAL (not temporal) lobe would cause what visual deficit
these would be the NON-Myer’s loop visual radiations
–> contralateral inferior homonymous quadrantanopsia
a lesion within the primary visual cortex (usually by vascular insult) results in what phenomenon
macular sparing deficit- macular (cone) cortex is represented on most posterior aspect of primary visual cortex, believed to have dual blood supply by middle cerebral artery
left superior homonymous quadrantanaopsia with macular sparing would be due to a vascular insult to ____
lesion of right lingual gyrus in primary visual cortex (lingual gyrus receives input from inferior optic nerves- those of Myer’s loop- that sense superior light)
macular sparing tells you it’s in primary visual cortex
