Visual System 1 Flashcards
The image on the retina is
Inverted and reversed
Right part of space projects to
Left hemiretina
Left part of space projects to
Right hemiretina
Most of the visual fields of the eyes
Overlap - there is only a minor peripheral zone that doesn’t overlap
Outermost layer of the eye
Sclera
Sclera is continuous with
Dura mater
What continues posteriorly as a sheath of the optic nerve
Sclera
Beginning at the circular transition zone called the limbus, the transparent ______ lets light into the eye
Cornea
Vascularized middle layer of the eye
Choroid (plexus?) aka uvea
Choroid is between
Sclera and retina
What is the principal route of travel for blood vessels and nerves in the eye
Choroid
Choroidal capillaries supply
Retinal photoreceptors
Choroidal pigment
Absorbs light
The urea continues anteriorly to form the bulk of the
Ciliary body and the stroma of the iris
Innermost layer of eye
Retina
What lies adjacent to the choroid?
RPE
Retinal detachment occurs if
Connection between choroid and RPE is not strong
Retinal cells are metabolically dependent on what
Pigmented epithelial cells and adjacent choroidal vasculature
Retinal layers (sup to deep)
- epithelial
- photoreceptor outer and inner
- Outer limiting membrane
- Outer nuclear layer
- Outer plexiform layer
- Inner nuclear layer
- Inner plexiform layer
- Ganglion cell layer
- Nerve fiber layer
- Inner limiting membrane
Outer nuclear layer
Consisting of the nuclei of photoreceptor cells
Outer plexiform layer
Consisting of the synaptic connections of photoreceptors with other retinal cells
Inner nuclear layer
Containing somata of 2nd and some 3rd order retinal cells
Inner plexiform layer
Layer of synaptic contact
Ganglion cell layer
Containing the cell bodies of ganglion cells
Nerve fiber layer
Composed of the axons of the ganglion cells. These axons converge at the optic disc to form the optic nerve
Inner limiting membrane
Located between the nerve fiber layer and the vitreous. The limiting membranes consist of glial cell processes joined by tight junctions
Cell type of RPE
Single layer of polygonal pigmented cells
One side of each RPE cell joins the
choroid - to supply the avascular first 2 layers of the retina
Pigment epithelial cells of the RPE support photoreceptors
Metabolically
Pigment epithelial cells of the RPE play a role in
Absorbing excess light that has passed through the retina
Rods and cones point
TOWARDS the epithelial layer -> BACKWARDS to the direction of light
Yellow structure located in the central retina, about 5mm diameter, with predominantly cones
Macula
Structure located in the center of the macula that is about 1.5mm in diameter that consists of ONLY cones
Fovea
Which structure has only cones?
Fovea
What part of the fovea is specialized for vision of the highest acuity?
Central
Specialized interneurons called ______ receive their input from individual foveal cones
Midget bipolar cells
Midget bipolar cells contact _____ so that an anatomical basis for highly detailed foveal vision is maintained
Midget ganglion cells
No photoreceptors are present where
Optic disc
Location where central axons of ganglion cells leave the eye to form the optic nerve
Optic disc
Central axons of ganglion cells originate where
At the border with the vitreous, so they must traverse the retina before passing through the sclera
Blind spot
Exists because there are no photoreceptors at the optic disc, so we are blind to any object whose image falls on this part of the retina
Why are we unaware of our blind spot?
The brain fills it in for us and the other eye compensates in binocular vision
Density of cones decreases sharply where?
Outside the fovea
Density of rods increases and reaches a maximum where?
Just outside the macula
Which part of the eye is used for high-acuity color vision in reasonably bright light?
Fovea
What part of rods and cones are responsible for photoreception?
Outer portions
Photoreception
The process by which photons are detected and the information is transduced into an electrochemical signal
The ______ connects the outer segment to the inner segment
Cilium
Which part of a photoreceptor contains mitochondria?
Inner segment
Spherule
Part of a rod that terminates in the outer plexiform layer in an expansion that makes synaptic contacts with neurons
Pedicle
Part of a cone that terminates in the outer plexiform layer in an expansion that makes synaptic contacts with neurons
Surface of rods and cones contain
cGMP gated Na+ channels
In the dark
cGMP concentration is high and channels are open, Na current flows freely into the outer segment
DEPOLARIZED
Rods and cones are depolarized when/where?
In the dark
Glutamate is released at a steady rate when
The rods and cones are depolarized (in the dark)
In the light
cGMP hydrolysis causes channels to close and transmitter release declines
HYPERPOLARIZED
Glutamate is not released when/where?
In the light
Opsins (photopigments)
G protein coupled receptors of the rods and cones
Ligand of opsin
11-cis retinal (vitamin A derivative)
What does light do to 11-cis retinal?
Isomerize it to all-trans retinal, which DISSOCIATES from opsin
Isomerize retinal causes a conformational change in the opsin ->
opsin (trans) activates transducin, a G protein -> transducer activates phosphodiesterase E, an enzyme that hydrolyzes cyclic guanosine monophosphate (cGMP) -> cGMP concentration decreases -> Na+ channels close -> secretion of glutamate stops
Types of cones
Long (L), middle (m), and short (S)
Long-wavelength cones
Red
Middle-wavelength cones
Green
Short-wavelength cones
Blue
Absorption peak of a cone is determined by
The kind of opsin a particular cone makes
______ respond only up to about moonlight levels of light intensity
Rods
______ responses are slow
Rod
The responses of multiple ______ must be pooled to produce firing of ganglion cells in response to dim light
Rods
In the peripheral retina
Outputs of thousands of rods converge on hundreds of bipolar cells before ultimately reaching single ganglion cells, hence spatial resolution in dim light is relatively poor
Can rods differentiate color? Why?
No - they all contain the same rhodopsin
Rods rhodopsin absorb what wavelength of lifer most effectively
500nm
Cones have ______ outer segments and ______ visual pigment
Smaller
Less
Which is more sensitive, rods or cones?
Cones
There is considerably less convergence in ______ pathways than ______ pathways
Cone
Rod
Acuity is highest
Where the midget ganglion cells have receptive fields with centers the size of a single cone
In the absence of light, rods and cones
RELEASE glutamate
Glutamate release is
all or nothing, gradual, and depends on the amount of light
Photoreceptors form either excitatory or inhibitory synapses with ______ cells
Bipolar
Rod bipolar cells terminate
On processes of special amacrine cells, DEPOLARIZING them
Amacrine cells depolarize
Ganglion cells (1st order neurons of the optic nerve)
Through their connections with ______ cells, photoreceptors may excite or inhibit adjacent cells
Horizontal
Receptive field
Portion of the visual field where light causes excitation or inhibition of one retinal ganglion cell
Photoreceptors and bipolar cells convey information by
Releasing neurotransmitter in a graded fashion that depends on membrane potential
Ganglion cells fire action potentials as they convey information to the
Optic nerve
Ganglion cells have ______ receptive fields
Center-surround
Ganglion cell receptive fields are composed of
Two concentric, roughly circular zones
Illumination of the central area of the ganglion cell receptive field causes
Increase in firing rate
Illumination of the peripheral (surround) area of the ganglion cell receptive field causes
Decrease in firing rate
Ganglion cell receptive field is an ___-center ___-off surround receptive field
ON-center OFF-surround
Simultaneous illumination of both the center and surround causes
Relatively little change in firing rate because the antagonistic effects of the two areas roughly cancel each other out
At the level of the ganglion cells, the contrast between the areas of the receptive field is of ______ importance
Paramount (high!)
Scotoma is most often produced by lesions of the
Retina
Scotoma represents
Visual field defects - single or multiple
Most likely dx of scotoma
Embolus arising from the internal carotid artery resulting in a branch retinal artery occlusion
Other possible causes are hemorrhage or a disorder of the optic nerve
Patient with amaurosis fugax (sudden blindness) should be treated with
Massage of the eyebulb to dislodge the embolus or at least move it forward to decrease the lesion size
Retinitis pigmentosa
Inherited disease (AD,AR, X-linked, and mitochondrially defined) degenerative eye disease that causes severe vision impairment due to progressive degeneration of the rod photoreceptor cells in the retina
Most common cause of autosomal-dominant RP cases
Mutations in the rhodopsin gene, which disrupts the rod-opsin protein
DX of RP occurs
Anywhere from early infancy to late adulthood
Early stages of RP
Compromised peripheral and dim light vision
Later stages of RP
Abnormalities in the adjacent retinal pigment epithelium and deterioration of cone photoreceptor cells
Give away phrase for RP?
Tunnel vision and eventual blindness
Genes for red and green cone pigments are located next to each other on the X chromosome, and unequal crossing over during meiosis can cause one X chromosome to wind up with a missing or defective
Red or green gene
What percent of male population is colorblind?
2%
Why aren’t females often colorblind?
They likely have an X chromosome with normal red green cone pigments since they have 2 X chromosomes
Lack of the blue cone pigment is
Rare, and equal in males and females because the gene is located on chromosome 7
Absence of the cone population that absorbs in the ______ makes it difficult to discriminate among red, yellow, and green objects
Red
Protanope
Loss of red
Deuteranope
Loss of green
Tritanope
Loss of blue
