The Eye Flashcards
Dark current channels close and less Ca is entering. Ca inhibits the synthesis of cGMP by guanylyl cyclase and thus with less unerring guanylyl cyclase is more active and cGMP synthesis inc. Dark channel currents gradually open and depolarize cones so that they have a range through white they can function in the light
Light adaptation
Neural tissue at the back of the eye
Retina
Carry red-green and blue-yellow info
P and nonm-nonp ganglion cells
Respond to the opposite way the photoreceptors communicating with them do. Metabotropic glutamate receptors (light is preferred stimulus)
On type (sign inverting) bipolar cells
Fewer disks, contain red, blue, or green pigments
Cones
Only output cells, fire APs
Ganglion cells
The cornea uses this since there is air in front and fluid behind (aqueous humor)
Refraction
Bending of light when traveling from one medium to another
Refraction
What does it mean when we say our visual system doesn’t detect differences in absolute values?
Not great in uniformly light/dark environments
SLIDE 45
SLIDE 45
More sensitive to light bc rods more sensitive to low light, more photoreceptors per ganglion cell
Peripheral retina
What does the G protein (transducin) do when rhodopsin is hit by light?
Activates phosphodiesterase and reduces the level of cGMP
All the complex data we need to construct our perception of the world is gathered by this
Retina
How does light activate rhodopsin?
Light strikes retinal and changes its shape to linear and the color of rhodopsin changes
Distance between peaks
Wavelength
Are red on center, green off surround or green on center, red off surround. Red and green are opponent colors
P type cells
Release GABA as a NT
Horizontal cells
Transfer of light energy, compounds do this with light of specific wavelengths and reflect others
Absorption
Used in daylight (photopic) to detect color. Many more photons are required to activate these photoreceptors. Not very active in the dark
Cones
SLIDE 59
SLIDE 59
What happens when Na and Ca stop flowing into rhodopsin?
The cell is hyper polarized (more negative)
These photoreceptors and 1000x more sensitive to the others and are thus used for night vision (scotopic) and shut down in bright light
Rods
90% of the ganglion cells. Contact one to a few cone bipolar. They have, small concentric receptive fields, produce sustained slowly adapting responses, with a weak response to movement. Good at analyzing SHAPES
P type
Respond to light at 560 nm (l)
Red cones
Scotopic vs photopic vision
S - night vision. rods
P - day vision. cones
Light being bounced off of a surface
Reflection
SLIDE 35
SLIDE 35
Theory that brain assigns colors based on comparison fo cone readouts, more to it than this though including the color opponent process
Young-Helmholtz trichromacy theory
Light sensitive part of retina
Layer of photoreceptor outer segments
Xanthophylls here protect eye from near UV light and act as antioxidants
Macula
Ganglion cells that sustain APs for longer
P type
Overall cell response is to send information about light vs dark, red vs gree, blue vs yellow, movement, detail (like shapes)
Ganglion cells
There is usually poor contrast sensitivity at first, results from cones being hyper polarized at first due to dark current channels closing
Light adaptation
Response in center to one wavelength is cancelled by other wavelength (opponent only) in surround. This is similar to the center surround organization seen for light or dark except that specific cones are present in only the center or the surround
Color opponent cells
Respond to the same way the photoreceptors communicating with them do. Inotropic glutamate receptors (dark is preferred stimulus)
Off type (sign conserving) bipolar cells
Where the optic nerve exits the eye, blood vessels but no photoreceptors (blind spot)
Optic disc
SLIDE 46 and 47 and 48!!!
SLIDE 46 and 47 and 48!!!
This does most of the light bending for the eye
Cornea
What does light act as for a GPCR
Agonist
At back fo retina to absorb stray light, remove old discs, regenerate photopigment
Pigment epithelium
Peak minus trough
Amplitude
Respond to light at 530 nm (m)
green cones
The only light sensitive cells
Photoreceptors
The lens does this when looking at a distance and this when looking close up
Distance - flattens
Close up - more rounded
What range of intensity do photoreceptors work at? What does this mean?
100 fold range in intensity. At 1/10 of ambient light are maximally depolarized and at 10x ambient light are maximally hyperpolarized
Photoreceptors stop working and blindness in certain areas can result
Detached retina
Ganglion cells have these while photoreceptors do not. They function as an area of the visual field that feeds information a particular ganglion cells
Receptive fields
Information gathered in the retina is ultimately sent here
Primary visual cortex
5% of ganglion cells with some being color sensitive. Good with COLOR
non-m non-p type
Beyond on and off center, how are ganglion cells categorized?
P (midget) type
M (parasol) type
Non-M non-P
Said all colors created by mixing red, green, and blue light and the retina senses this. This was only a prediction at his time
Young
These photoreceptors communicate directly with bipolar cells
Center photoreceptors
Has more receptor cells per ganglion cell and more rods than cones
Peripheral retina
Transparent with no blood vessels
Cornea
Tough wall of the eyeball
Sclera
With small fields, carry info regarding fine detail
P type ganglion cells
Light is slowed by this in front of the cornea
Aqueous humor
Many disks and large amount of rhodopsin
Rods
Don’t fire APs. Respond by releasing more NT if depolarized and less NT if hyperpolarized
Bipolar cells
What are the two functions of the eye?
- camera like system to focus light on the retina
2. neural retina; generate a pattern of APs from the input of many photons striking photoreceptors
Failure of lens accommodation. Impairs close up vision needed for reading
Presbyopia
SLIDES 40 AND 41
SLIDES 40 AND 41
This is a type of electromagnetic radiation that exists on the EM spectrum that we see
Light
Involves both eyes (consensual) pupil of one does the same as the other (both dilate or constrict in light)
Pupillary light reflex
Can be thought of as all the bipolar cells communicating with them and they have receptive fields as well
Ganglion cells
Bipolar, amacrine, ganglion synaptic contacts of retina
Inner plexiform layer
Light hits behind retina, cant see close up (farsightedness)
Hyperopia
What type of receptor is rhodopsin?
GPCR (7 TM regions coupled with G protein inside cell)
Ability to distinguish between nearby points (20/20 = 0.83 degrees). Due to refractive power and spacing of photoreceptors
Visual acuity
Photoreceptor, bipolar, and horizontal cell layer contacts of retina
Outer plexiform layer
Have no opponency bc receive input from all cone types in both center and surround
m type
Membrane potential of rod outer segments in dark
-30 mV
Light hits short of the fovea (retina), cant see far away (nearsightedness)
Myopia
The complexity of this system allows for reading, art, and many of the abilities which are unique to humans
Visual
Respond to light at 430 nm (s)
blue cones
Outer nuclear layer cells
cell bodies of photoreceptors
Extraocular muscle disorders that causes input from one eye to be suppressed
esotropia - cross eyed
exotropia - wall eyed
How are cells organized in the retina?
In layers. Inner nuclear layer and outer nuclear layer (inner = closer to inside of eyeball)
Only these fire APs while the others change Vm in smaller measured ways
Retinal ganglion cells
How large is focal distance?
2.4 cm (the average diameter of an eyeball)
These communicate to bipolar cells
Photoreceptors
Analyze every point of the retinal surface
Several on type and off type ganglion cells
How much do glasses correct?
By a few diopters
Between the cornea and lens (nourishes cornea)
Aqueous humor
How must retinal change for activation and deactivation of rhodopsin?
Activate - cis to trans (straighten)
Deactivate - trans to cis (gets its bend back)
How does contrast affect perception of light and dark?
Lighter surrounds and darker surrounds can influence the perception of the center shade
Information gathered in the retina that isn’t sent to the primary visual cortex goes to areas involved with what?
Areas influencing biological rhythms, eye movement, control of the pupillary light reflex and shape of the lens
Good reference point, retina is thin here, where we have our best vision
Fovea
Loss of peripheral and night vision due to degeneration of photoreceptors (though to be genetic disorder controlled by about 100 different genes)
Retinitis pigmentosa
Muscles controlling pupil size and show the eye color
Iris
What creates he best resolution?
Low photoreceptor to ganglion cell ration (best in fovea)
With low resolution and large receptive fields, carry info regarding movement
M type ganglion cells
20/10 vision?
20/30 vision?
10 - yours is better than most
30 - yours is worse than most
These communicate to ganglion cells
Bipolar cells
Clouding of the lens usually fixed with lens removal and replacement with artificial lens
Cataract
This also bend light (12 diopters) and helps to form sharp images of close objects (within 9 meters)
Lens
Can also be designated on center and off center cells like bipolar cells
Ganglion cells
This occurs when moving form light to dark and takes about 20 minutes
Dark adaptation
4 parts of photoreceptors
- outer segment
- inner segment
- cell body
- synaptic terminal
Poor for details but can see faint stars
Peripheral retina
These types of animals can see UV while these can see infrared
UV - some fish
Infrared - snakes
Results from pupil dilation, adjustment of retinal circuitry, the synthesis of new unbleached rhodopsin (11 trans retinal converted back to 11 cis retinal)
Dark adaptation
What causes the membrane potential in rods?
Special Na channels (dark current) opened by cGMP made by guanylate cyclase, continuously made in the dark
The retina sticks to it and it helps to nourish photoreceptors
Pigment epithelium
Light that hits a surface and is transferred into it with no reflection or refraction
Absorption
Almost half of the cortex is devoted to this in some form
Vision
These photoreceptors communicate with horizontal cells
Surround photoreceptors
Intermediate light levels when both cones and rods are involved in vision
Mesotopic
More of these = work better in low light
Rods
These photoreceptors are more present in the peripheral vision
Rods
These photoreceptors are more present in the central vision
Cones
Changing lens shape, ciliary muscles contract, lens becomes rounder, more curved
Accommodation
Light reduces the level of this leading to this occurring in rods
Reduces level of cGMP, Na channels close, rods hyperpolarize in light. Light is like an agonist
Can be easily transplanted since it has no blood vessels
Cornea
Increased intraocular pressure from pushing on the optic nerve
Glaucoma
Most of the light we see is this
Reflection
Have blue on center, yellow off surround or yellow on center, blue off surround. Blue and yellow are opponent colors
non m-non p
How do the different sensitivity of cones create a picture?
Combine to produce right signal for response
Central vision, important for fine vision
Macula
What causes channels in cones to eventually open in light
Lack of Ca entering, more cGMP produced
Loss of central vision which occurs in 25% of people over age 65
Macular degeneration
Nearsightedness, correct with concave lens
Myopia
Jelly-like fluid inside the eyeball
Vitreous humor
He showed how RGCs responses affected by horizontal and bipolar cells
Dowling
What causes bad vision underwater with no goggles?
Light bending occurring at the cornea
How many photoreceptors vs bipolar cells vs ganglion cells?
100 mil to 10 mil to 1 mil (convergence of info)
Consist of a center and a surround. They have small in the center of the retina and large in the periphery
Receptive fields
Average amount of bending by the cornea (depends on curvature of cornea)
42 diopters
Membrane attaching eyelids to sclera
Conjunctiva
Area seen by eyes when looking forward
Visual field
SLIDE 34
SLIDE 34
What property of light equates to higher energy?
Higher frequency
Specialized for high resolution and color (no rods). Overlying cells grow off to the side and light has a clearer path to the photoreceptors
Fovea
Parallel streams of info leave the eye via what
Optic nerve
Constriction of pupil causes this
Increases depth of focus
More energy is required to bleach these which leads to them staying working during the day
Cones
Path of light in the retina
Passes through several layers (mostly transparent) to reach photoreceptors
Provide input to subcortical areas to synchronize response to light changes
ipRGCs
Use melanopsin, depolarize in light, large dendritic fields, dendrites sensitive to light
Intrinsically photosensitive ganglion cells (ipRGC)
Control the response of central photoreceptors
Horizontal cells
These contract to make the lens rounder, more curved, in accommodation of the lens
Ciliary muscles
5% of the ganglion cells. Synapses with many bipolar. They have large concentric receptive fields, have transient rapidly adapting responses, respond to movement across fields, and low contrast stimuli. Good at analyzing MOVEMENT
M type
Light that hits a surface and bends
Refraction
Farsightedness, correct with convex lens
Hyperopia
Enhances the ability to detect contrasts and edges moving across the boundaries of the center surround
Same stimulus in the center and surround producing opposite effects
What is the visual range of light for humans?
400-700 nm
Why is light bent when it goes to the cornea?
It is moving from one medium (air) to another (liquid)
Modify the pathway of photoreceptors to bipolar to ganglion
Horizontal and amacrine cells
The degree to which light is bent is measured in these
Diopters
How many rods and how many cones?
rods - 5 mil
cones - 92 mil
Ganglion cells that are sensitive to color
P type and some non m-non p
Eye muscles that assist with eye movement
Extraocular muscles
What must happens for rhodopsin to send another signal
Must be unbleached
Produced by retina-brain stem connections, dilate/constriction of pupils in light
Pupillary light reflex
What is retinal derived from?
Vitamin A
Work on these 4 animals helped in eye discoveries
Crabs, frogs, cats, and monkeys
What does reduces levels of cGMP cause in rhodopsin?
Dark current (cGMP channels) close and the flow of Na and Ca into the photoreceptor is reduced
Occurs after moving from dark to light. Takes 5-10 minutes
Light adaptation
Why are dashboard lights red?
To only affect cones so we don’t bleach our rods and become unable to see in the dim light
Waves per second
Frequency
3 key structures of the retina
- optic disc
- macula
- fovea
Has the effect of producing opposing responses to the same stimulus depending on whether it is striking the center or the surround
Receptive fields
Our visual system detects differences in this but not this
Differences in light intensity but not absolute values
Controls the shape of the lens
Ciliary muscle
SLIDE 50!!!
SLIDE 50!!!
Gets nourishment from the aqueous humor since it has no blood vessels
Cornea
Respond to light at 500 nm. Don’t do day vision
Rods
Total absorption of light
Black
Inner nuclear layer cells
bipolar, horizontal, amacrine
What happens when light hyperpolarizes rods?
-30 mV to -65 mV (more NT release to less NT release)
What happens when photons strike the GCPR rhodopsin?
Retinal is bleached (cis to trans)
This allows for detection of a single photon is the rhodopsin GPCR
Amplification
Dark part of the eye bc of retinal pigments
Pupil
