Visual Tracts (Dennis) Flashcards
1
Q
- formation of light image (photons) by the photoreceptive retina
- info transduced from rods/cones > bipolar cells > ganglion cells
- axons of ganglion cells and axons of higher order cells form this pathway
- relays info to primary visual cortex via thalamus
- has precise retinotopic arrangement of fibers that is maintained at each structure (small regions of retina are respresented in specific regions centrally)
A
visual system
2
Q
What are the 3 layers of the eye?
A
- fibrous layer (outer): external layer consisting of sclera and cornea
- vascular layer (middle): consists of choroid, ciliary body, and iris
- retinal layer (inner): sensory layer that gives rise to optic nerve
3
Q
- fibrous, external layer of the eye that protects internal structures and provides sites for muscle insertion
- composed of dense regular CT, w/ flat bundles of type 1 collagen and microvasculature near outer surface
- extraocular muscles insert anteriorly onto this structure
A
sclera
4
Q
What are the 5 layers of the cornea (transparent and avascular)?
A
- corneal epithelium: nonkeratinized stratified squamous
- anterior limiting membrane (Bowman’s membrane): basement membrane beneath corneal epithelium
- thick stroma: composed of keratocytes
- posterior limiting membrane (Descemet’s membrane): basement membrane of the endothelium
- inner endothelium
(picture below: goes from outer most structures on the left to innermost on the right)
5
Q
- transitional area where transparent cornea merges w/ opaque sclera, encircles the entirety of the cornea
- area denotes end of Bowman’s membrane and beginning of conjunctiva, covers anterior sclera and lines the eyelides
- epithelial stem cells at surface of this structure give rise to progenitor cells that move into corneal epithelium
A
limbus
(in the photo: labeled as CSJ or corneoscleral junction)
6
Q
What are the structures of the vascular layer?
A
- consists of choroid, ciliary body, and iris
- choroid (yellow arrows in image): loose, well-vascularized connective tissue that contains numerous melanocytes; black layer that prevents light from entering eye except via the pupil; bruch membrane (thin extracellular sheet that includes basal lamina of retina’s pigmented layer)
- ciliary body: includes ciliary muscle, processes, and zonula; each involved in shaping the lens
- iris: covers outer boundary of lens, creates the pupil
7
Q
What are the structures within the retina?
A
(develops from optic cup)
- outer pigmented layer: simple cuboidal epithelium
- inner neural layer: thick and stratified w/ various neurons/photoreceptors; extends anteriorly to the ora serrata (continues as cuboidal epithelium that covers ciliary body and posterior iris)
8
Q
- outer layer of the retina that consists of simple cuboidal or low columnar epithelium
- cells have basal nuclei, extensive junctions, and melanin granules
- surrounds neural retina an extends apical processes around tips of photoreceptors
- functions: absorbs scattered light, forms blood-retina barrier, isomerization of all-trans-retinal to 11-cis-retinal and transfers back to photoreceptors, phagocytosis/waste recycling, removal of free radicals/secretion of ATP
A
pigmented epithelium (PE)
9
Q
What are the functions of pigmented epithelium (PE)?
A
- absorbs scattered light: supplements choroid
- forms part of the blood-retina barrier: isolates retina photoreceptors from highly vascular choroid
- isomerization of all-trans-retinal > 11-cis-retinal > transfers back to photoreceptors
- phagocytosis and degradation of components/waste from photoreceptors
- removal of free radicals and secretes ATP, polypeptide growth factors, and immunomodulatory factors
10
Q
What are the 9 layers of the neural retina and their associated functions?
A
- inner limiting membrane (ILM): basement mem covered by processes of Müller cells (not seen on H&E)
- nerve fiber layer (NFL): contains ganglionic cell axons, converge at optic disc and form CN II
- ganglionic layer (GL): contains ganglion cell bodies, thicker near retina’s center than periphery
- inner plexiform layer (IPL): contains fibers/synapses of ganglion cells and bipolar neurons from INL
- inner nuclear layer (INL): bipolar neurons that integrate signals from rods/cones
- outer plexiform layer (OPL): contains fibers/synapses of bipolar neurons and rods/cones
- outer nuclear layer (ONL): cell bodies of rods/cones
- outer limiting layer (OLL): line of junctional complexes holding photoreceptors to Müller cells
-
rod/cone layer (RCL): contains outer segments of rods/cones
(10. pigmented layer)
*light flows from 1 to 10*
11
Q
- photoreceptor cells that are sensitive to light
- thin, elongated cells that are composed of: inner segment (glycogen, mitos, polyribosomes for cell’s biosyn activity) and outer segment (modified primary cilium, photosensitive, shaped like a short rod w/ 600-1000 flattened stacked membranous discs)
- rhodopsin located within discs, acts as GPCR for light transduction signal
- discs are replaced continually, old discs are pushed superficially, shed, phagocytosed, and digested by pigemented epithelium
A
rods
12
Q
- less numerous and less sensitive of the photoreceptor cells, produce color vision in adequate lighting
- elongated cells with: inner and outer segments (shorter, more conical, membranous discs are continuous invaginations of plasma membrane along one side)
- 3 types, each contain one type of visual pigment, iodopsin (photopsins) that have maximal sensitivity to light of different wavelengths (red, blue, green)
- discs are shed less frequently
A
cones
13
Q
- specialized area of the retina
- highest concentration of cones
- area where visual acuity is sharpest (20/20), drops precipitously in outer parts of retina (20/600)
- absence of vessels, cell bodies, and axons of ganglionic/inner nuclear layer
A
fovea (fovea centralis)
14
Q
- specialized area of the retina
- surrounds fovea and protects cones
- antioxidant properties and short wave UV filter
A
macula (macula lutea)
15
Q
- specialized area of the retina
- located at the head of the optic nerve
- ganglion axons from all of retina converge and dive here
- lacks photoreceptors, only ganglion cells axons > blind spot
A
optic disc
16
Q
- specialized area of the retina
- axons of retinal ganglion cells, become myelinated as they pass through sclera
- penetrates choroid and sclera, travels to brain
A
optic nerve
17
Q
- area that person is able to see when both eyes are fixed in one position
- binocular zone: broad, central region seen by both eyes
- monocular zone: seen only by R/L eye
A
visual fields
*visual system lesions are described in terms of their visual field deficits*
18
Q
- location on the retina that an object in visual field is projected
- retinal hemifields: nasal and temporal halves of retina
- quadrants: hemifields divided into upper/lower parts
A
retinal fields
19
Q
How is an image projected onto the retina?
A
- image formed on the retina is inverted in lateral and vertical dimensions
- left half of visual field: forms an image on nasal (right) half of left retina and temporal (right) half of right retina
- right half of visual field: forms an image on temporal (left) half of left retina and nasal (left) half of right retina
20
Q
fibers from temporal retina (ipsilateral eye) + fibers from nasal retina (contralateral eye)
A
optic tract
21
Q
Where does CN II decussate and why does this occur?
A
- optic chiasm: this is where the two optic nerves partially decussate; nasal half of each retina decussates to contralat tract; temporal half of each retina remains ipsilateral
- decussation is necessary to deliver info from contralat visual field to optic tract; brings together info from comparable areas of both retinas, necessary for stereoscopic vision (aka depth perception)
22
Q
How do optic tract fibers move into and out of the lateral geniculate nucleus (LGN)?
A
- optic tract curves posteriorly around cerebral peduncles to term in the LGN (maintain retinotopic pattern)
- within LGN, there are magnocellular and parvocellular layers: based on type of ganglion cell input received and the side of retina from which input originates
- outgoing fibers then form optic radiations
23
Q
How is info sent to primary visual cortex from retinas?
A
- neurons from LGN extend as large bundle forming optic radiations (retinotopic org maintained)
- relay info to PVC (striate cortex), located on upper and lower banks of calcarine sulcus
- lower quadrant of contralat hemifields > superior bank of sulcus, the cuneus
- upper quadrant of contralat hemifield > arch rostrally, pass through temporal lobe, do broad U-turn (Meyer’s loop) > inferior bank of sulcus, the lingual gyrus
25
Q
How are the fibers representing macula and fovea presented in the LGN and PVC?
A
- these fibers originate from central regions of the LGN
- in PVC: these fibers target the most posterior portion and are represented in disproportionately large volumes (relative to size) in the LGN and visual cortex
26
Q
How are visual projections conveyed in the PVC?
A
- visual field has four quadrants, each projecting to its own quadrant of the PVC (left/right hemi, above/below calcarine sulcus)
- lateral and vertical inversion occurs in projection of visual field upon PVC
- example: upper left quadrant (visual field) is represented in the lower right quadrant (visual cortex)
27
Q
If you review nothing else in this lecture, at least review this:
A
28
Q
- located above and below calcarine sulcus
- inferior visual fields project to area above calcarine, superior field project to area below sulcus
- macula is represented more posteriorly, and peripheral fields more anteriorly
A
primary visual cortex
29
Q
- ‘parieto-occipito-temporal area’
- areas 18, 19, and related parts of temporal and parietal lobes
- LGN also sends targets to these areas > helps interpret location, motion, form, and color
A
visual association cortex
38
Q
What are the 3 general visual field deficit concepts?
A
- damage anterior to chiasm affects only ipsilateral eye
- damage at chiasm causes heteronymous deficits
- damage behind chiasm causes homonymous deficits
