Visual Tracts (Dennis)

Question 1

• 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)

Answer 1

visual system

Question 2

What are the 3 layers of the eye?

Answer 2

1. fibrous layer (outer): external layer consisting of sclera and cornea
2. vascular layer (middle): consists of choroid, ciliary body, and iris
3. retinal layer (inner): sensory layer that gives rise to optic nerve

Question 3

• 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

Answer 3

sclera

Question 4

What are the 5 layers of the cornea (transparent and avascular)?

Answer 4

1. corneal epithelium: nonkeratinized stratified squamous
2. anterior limiting membrane (Bowman’s membrane): basement membrane beneath corneal epithelium
3. thick stroma: composed of keratocytes
4. posterior limiting membrane (Descemet’s membrane): basement membrane of the endothelium
5. inner endothelium

(picture below: goes from outer most structures on the left to innermost on the right)

Question 5

• 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

Answer 5

limbus

(in the photo: labeled as CSJ or corneoscleral junction)

Question 6

What are the structures of the vascular layer?

Answer 6

• 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

Question 7

What are the structures within the retina?

Answer 7

(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)

Question 8

• 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

Answer 8

pigmented epithelium (PE)

Question 9

What are the functions of pigmented epithelium (PE)?

Answer 9

• 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

Question 10

What are the 9 layers of the neural retina and their associated functions?

Answer 10

1. inner limiting membrane (ILM): basement mem covered by processes of Müller cells (not seen on H&E)
2. nerve fiber layer (NFL): contains ganglionic cell axons, converge at optic disc and form CN II
3. ganglionic layer (GL): contains ganglion cell bodies, thicker near retina’s center than periphery
4. inner plexiform layer (IPL): contains fibers/synapses of ganglion cells and bipolar neurons from INL
5. inner nuclear layer (INL): bipolar neurons that integrate signals from rods/cones
6. outer plexiform layer (OPL): contains fibers/synapses of bipolar neurons and rods/cones
7. outer nuclear layer (ONL): cell bodies of rods/cones
8. outer limiting layer (OLL): line of junctional complexes holding photoreceptors to Müller cells
9. rod/cone layer (RCL): contains outer segments of rods/cones
   (10. pigmented layer)

*light flows from 1 to 10*

Question 11

• 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

Answer 11

rods

Question 12

• 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

Answer 12

cones

Question 13

• 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

Answer 13

fovea (fovea centralis)

Question 14

• specialized area of the retina
• surrounds fovea and protects cones
• antioxidant properties and short wave UV filter

Answer 14

macula (macula lutea)

Question 15

• 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

Answer 15

optic disc

Question 16

• specialized area of the retina
• axons of retinal ganglion cells, become myelinated as they pass through sclera
• penetrates choroid and sclera, travels to brain

Answer 16

optic nerve

Question 17

• 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

Answer 17

visual fields

*visual system lesions are described in terms of their visual field deficits*

Question 18

• 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

Answer 18

retinal fields

Question 19

How is an image projected onto the retina?

Answer 19

• 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

Question 20

fibers from temporal retina (ipsilateral eye) + fibers from nasal retina (contralateral eye)

Answer 20

optic tract

Question 21

Where does CN II decussate and why does this occur?

Answer 21

• 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)

Question 22

How do optic tract fibers move into and out of the lateral geniculate nucleus (LGN)?

Answer 22

• 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

Question 23

How is info sent to primary visual cortex from retinas?

Answer 23

• 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

Question 24

What is the Meyer’s loop and what are clinical implications of lesions in this area?

Answer 24

• Meyer’s loop: fibers from upper quadrant of contralat hemifield that arch rostrally, pass through temporal lobe, and U-turn (toward PVC)
• lesions in this area of temporal lobe can lead to blindess

Question 25

How are the fibers representing macula and fovea presented in the LGN and PVC?

Answer 25

• 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

Question 26

How are visual projections conveyed in the PVC?

Answer 26

• 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)

Question 27

If you review nothing else in this lecture, at least review this:

Answer 27

Question 28

• 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

Answer 28

primary visual cortex

Question 29

• ‘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

Answer 29

visual association cortex

Question 30

• spatially directs head movements and visual reflexes (eye movements)
• retinal input bypass the LGN
• target the brachium of this area > terminate retinotopically
• also receives spinotectal (somatosensory) and auditory inputs

Answer 30

superior colliculus

Question 31

• nuclei near midbrain/forebrain junction
• receives inputs from optic tract, LGN, and suprachiasmatic nucleus
• respond to varying intensities of illuminance and non-conscious behavioral responses
• important in pupillary light reflex

Answer 31

pretectal/pretectum area

Question 32

blindess (-anopia or -anopsia) in 1/2 of the visual field

Answer 32

hemianopia

Question 33

blindness of a quadrant of the visual field

Answer 33

quadrantanopia

Question 34

conditions in visual field losses are similar in both eyes

Answer 34

homonymous visual fields

Question 35

conditions in which two eyes have non-overlapping field losses

Answer 35

heteronymous visual fields

Question 36

visual field loss that preserves vision in the center of the visual field

Answer 36

macular sparing

Question 37

• visual field loss of one eye that can be superimposed on the other eye (symmetrical)
• the closer a lesion is to the visual cortex, the more likely the deficit will be displayed in this manner
• the more anterior a lesion is, the more likely it will be incongruous

Answer 37

congruous

Question 38

What are the 3 general visual field deficit concepts?

Answer 38

• damage anterior to chiasm affects only ipsilateral eye
• damage at chiasm causes heteronymous deficits
• damage behind chiasm causes homonymous deficits

Question 39

• infarction of left occipital lobe and posterior corpus callosum (typically due to PCA damage)
• disconnects language area from visual association cortex
• patient cannot name or describe object in visual field, BUT they can recognize and demonstrate its use
• patient may also be alexic (unable to read), and writing ability may be affected (agraphia)

Answer 39

associative visual agnosia