The Visual System Part One Flashcards
Image on the retina
Inverted and reversed
Info from the upper visual space
Projected onto the lower retina
Info from the lower visual space
Projects to the upper retina
Right part of visual space
Projects to the left hemiretina in each eye and vice versa
Do the VF overlap
Yes most of it does, the minor peripheral zones do not
Outermost layer of the eye
Sclera
What is the sclera continuous with
Dura mater
How does the sclera continue posteriorly
As the sheath of the CN2
What begins at the limbus
Cornea
Vascularized middle layer
Choroid plexus (uvea)
Where is the choroid
Between the sclera and the retina
Pigment of the choroid
Densely pigmented
What is the principal route through which blood vessels and nerves travel within the wall of the eye
Choroid
What do choroid caps supply
Retinal photoreceptors
What does choroidal pigment do
Absorbs light
How does the choroid continue anteriorly
Dorms the bulk of the ciliary body
What is the choroid similar to
Arachnoid mater
What is the innermost layer of the eye
Retina
What part of the retina lies adjacent to the choroid
RPE
Where do the rods and cones point towards
The choroid, light must cross all layers of the retina to get to the photoreceptors
What are the layers of the retina from most superficial to deep (choroid to vitreous)
- Epithelial layer
- Photoreceptor cell outer and inner segments
- Outer limiting membrane
- Outer nuclear layer
- Outer plexiform layer
- The inner nuclear layer
- Inner plexiform layer
- Ganglion cell layer
- Nerve fiber layer
- Inner limiting membrane
Where are the nuclei of the photoreceptor cells
Outer nuclear layer
Where are the synaptic connections of photoreceptors with other retinal cells
Outer plexiform layer
Contains the somata of second order and some third order retinal cells
Inner nuclear layer
Another area of synaptic contact in the retina other than the outer plexiform layer
Inner plexiform layer
What part of retina contains the cell bodies of the ganglion cells
Ganglion cell layer
What layer of the retina is composed of the axons of the ganglion cells that converge at the optic disc to form the optic nerve
Nerve fiber layer
This layer of the retina consists of glial cell processes joined by tight junctions
Inner limiting membrane
This retinal layer is located between the nerve fiber layer and the vitreous
Inner limiting membrane
This layer is a single layer of polygonal, pigmented cells
Retinal pigment epithelium
How do the RPE cells support the photoreceptors
Metabolically
What are the two main roles of the RPE
Support the photoreceptors metabolically and they play a role in absorbing light
What direction do the rods and cones point
Towards the epithelial layer (and choroid)
Relative to the direction of light, how are the rods and cones positioned
Backwards
5mm diamter region in the center of the retina
Macula lutea
What kind of photoreceptors are in the macula
Both, but predominantly cones
What kind of photoreceptors are in the fovea
Cones ONLY
What is the central fovea specialized for
Vision of the highest acuity
What is different about the deep retinal layers in the foveal area?
They are pushed aside so that the cones are exposed. Light gets right to the cones
What receives inputs from individual foveal cones
Midget bipolar cells
What do the midget bipolar cells contact after receiving inputs from individual foveal cones
Individual midget ganglion cells, so that an anatomical basis for highly detailed foveal vision is maintained
1:1!!
smaller receptive field
Better VAs
The larger the receptive field
The harder it is to discern between 2 points, worse VA
What kind of photo receptors are in the optic disc
None. This is where the central axons of ganglion cells leave the eye to form the optic nerve
Where do the central axons of hte ganglion cells originate
At the border with the vitreous
What must the central axons of ganglion cells do before passing through the sclera
Traverse the retina
Where is the blind spot
Where the optic disc is
Why is there a blind spot where the optic disc is
Because there are no photoreceptors there
Are we aware of the blind spot at the optic nerve?
No, the nervous system simply fills it in. The other eye compensates in binocular vision
Where does the density of the cones decrease sharply
Outside the fovea
Where does the density of rods increase sharply
Outside the fovea, reaching a maximum use outside the macula
What portion of the rods and cones are responsible for photoreception
The outer portions
The process by which photons are detected and the information is transducer into an electrical signal
Photoreception
What connects the outer segment to the inner segment of the photoreceptor and what does it contain
The ciliary. It contains mitochondria
Where are the nucleus of the photoreceptor cells found
Outer nuclear layer
Where do the photoreceptor cells terminate?
Outer plexiform layer in an expansion that makes synaptic contacts with neurons
What is the synaptic expansion on the rod cells called
Spherule
What is the synaptic expansion on the cone cells called
Pedicle
What do the surface membranes of rods and cones outer segments contain
cGMP gated Na+ channels
When are cGMP concentrations high
In the dark
When are the Na channels open, allowing a current of Na+ ions to flow freely into the outer segment?
In the dark
When are rods and cones depoalrized?
In the dark
What do rods and cones release at a steady rate in the dark
Glutamate
What does light induce in photoreceptors
Hydrolysis of gCMP, causing cation channels to close, the membrane hyperpolarizes, and transmitter release declines
What are the G protein-coupled receptors of the rods and cones
Opsins (photopigments)
What is the ligand of the receptor opsin
11-cis retinal (vit A derivative)
What is the effect of light on the opsin and 11-cis retinal
It isomerizes 11-CIA retinal to all-trans retinal, which dissociates from opsin
What does isomerized retinal do
Causes a conformational change in the opsin-opsin (trans) activates transducin (G protein)-transducin activates phosphodiesterase E (enzyme that hydrolyzes cGMP)-cGMP concentration decreases-Na channels close-secretion of glutamate stops
When is 11-cis-retinal bound to opsins (rhodopsin)
Dark
When does 11 trans retinal release from opsin and react with transducin?
In light
How many types of opsins in cones
3
How many types of opsins in rods
1
What are the different cone opsins
- L (red)
- M (green)
- S (blue)
What is the absorption peak of each class of cones determined by
The kind of opsin a particular cone makes
What kind of retinal do each of the cone opsins bind
They all bind 11-cis-retinal
What is the basis of trichromatic color in humans
The 3 different cone opsins
Which is more sensitive to light, rods or cones?
Rods
What kind of light do rods respond to
Only up to about moonlight levels of light intensity
What is the reaction time of rods
Slow
What kind of specificity do the rods have?
Not very specific. Could take up to thousands of rods to generate signal
Cone sensitivity
They have a smaller outer segment and less visual pigment. This makes them less sensitive
What do cones require to function effectively
Greater level of illumination
There is considerably less ______ in cone pathways than in rod pathways
Convergence
What makes possible the huge resolution of fine spatial detail
There is less convergence in cone pathways than in rod pathways
Where is acuity highest
Where midget ganglion cells have receptive fields with centers the size of a single cone (1:1!!)
What kind of synapses can photoreceptor cells form with bipolar cells
Either excitatory or inhibitory
Is glutamate release all or nothing?
No, it is gradual and depends on the amount of light
Where do rod bipolar cells terminate
On processes of special amacrine cells, depolarizing them, and they in turn depolarize ganglion cells
Portion of the VF where light causes excitation or inhibition of one retinal ganglion cell
Receptive field
How are receptive fields determined
- Photoreceptors respond to light in their respective fields and form excitatory or inhibitory synapses onto bipolar cells
- bipolar cells, in turn, synapse onto ganglion cells, whihc send sons into the optic nerve
- convey info RBC releasing NT in a graded fashion
- ganglion cells fire APs as they convey info into optic nerve
Are receptive fields yes or no
Yes
Center surround receptive fields
- composed of roughly circular zones
- illumination of the central area causes an increase in firing rate, whereas illumination of the peripheral area has the opposite affect
- this is an On-center OFF-surround receptive field
- Off-center ON surround receptive fields also exist
What has to happen for he ganglion cells in a receptive field to be activate
Has to have at least part of the off or part of the ON deactivated. If they are all illuminated or all under dim conditions they will cancel each other out
Simultaneous illumination of both enter and surround causes what
Relatively little change in firing rate because the antagonistic effects of the two areas roughly cancel each other
Contrast between the two different areas of the receptive fields
At the level of the ganglion cells, the contrast between the two is of paramount importance
What is scotoma most often lay produced by
Lesions of the retina
What does a scotoma represent
A visual field defect, single or multiple
Likely causes of scotoma
Embolus, hemorrhage
Amaurosis fugax and scotoma
Treat by rubbing the eyebulb to dislodge the embolus or at least move it forward to decrease the lesion size
An inherited, degenerative disease that causes severe vision impairment due to the progressive degeneration of the rod photoreceptor cells in the retina
Retinitis pigmentosa
What is responsible for the majority of autosomal-dominant inherited retinitis pigmentosa cases
Mutations in the rhodopsin gene disrupts the rod-opsin protein
What is the temporal profile of retinitis pigmentosa
Insidious, progresses slow and steady
What is the give away for retinitis pigmentosa
Tunnel vision
Where are the genes for the red and green cone pigments
X chromosomes
What can cause a red green color blindness
Unequal crossing over during meiosis can cause one X chromosome to wind up with a missing or defective red or green gene
Lack of red pigment
Protanopia
Lack of green pigment
Deuteranopia
What percent of population is RGcolorblind
2% male population
Lack of blue cone pigment
Rare because located on chromosome 7, equally uncommon in males and females
Absence of red makes it difficult to see what colors
Red, yellow, and green objects
