Phototransduction And Visual System Physiology Flashcards
What are the photoreceptors?
Rods and cones
What is rod mediated vision also called?
Scotopic vision
What happens when you lose rod vision ?
Night blindness
What stimulates rod cells?
SINGLE photon
What do rods converge onto?
What does this allow?
MANY rods to a SINGLE bipolar cell
Allows for high level of sensitivity (but low resolution)
After the rod cells converge onto the bipolar cell, what will that synapse on?
Single amacrine cell
What is cone mediated vision also called?
Photopic vision
What happens when you lose cone vision ?
Legally blind
What activates cones?
~100 photons
What do cones converge onto?
What does this allow for?
One cone cell —> one bipolar cell
Best resolution (low sensitivity)
What is mesopic vision?
When both rods and cones are activated by the light levels of environment
What is the dark current?
When it is dark,
The cells are constantly depolarized and release glutamate
What will suppresses the dark current?
What will this cause?
Light
Causes hyperpolarization of cells
Reduced release of Glu
When is the release of glutamate the hgihest?
Why? M
When it is dark
Because this will activate the off center bipolar cells (those for dark vision)
When is release of glutamate lowest?
Why?
When there is light
Low glutamate activates On center bipolar cells
What are amacrine cells needed for?
Low light pathways
When does night blindness occur?
When th rod mediated release of glutamate is disrupted
What kind of potentials do neuronal cells of the retina use?
Why?
Graded potentials
So there can be varying streams of glutamate
What kind of potentials do ganglion cells use?
Action potentials
What are the two subtypes of bipolar cells?
On center & Off center
What glutamate receptor do ON-center bipolar cells have?
What kind of receptor is this? Connected to what?
MGLuR6
metabotropic - connected to Gi
Due to the mGLuR6 receptors on ON center bipolar cells, what will occur when glutamate is decreased?
Gi is no longer active
Open cyclic CMP gated Na/Ca Channels and increases cations
What will lots of glutamate do ON center bipolar cells?
Will allow Gi to be active
Gi will close cyclic CMP gated na channel and decrease cations
What are the receptors on OFF center bipolar cells?
What type of receptor are these?
Normal glutamate receptors (non-NMDA Rs = AMPA R.)
Ionotropic
What happens when it is dark out?
Glutamate is increased
Binds to AMPA r. On OFF center bipolar cell
(Active w/ glu binidng)
—> depolarization —> off center ganglion cell —> optic nerve
What are the varieties of ganglion cells?
On and off center ganglion cell
What are the Rs. Of ganglion cells?
NMDA or non-NMDA rs.
What makes up hte fibers of the optic nerve?
Ganglion cell axons
Where will ganglion cells go to?
To the cortex where they will release glutamate
What do amacrine and other horizontal cells secrete?
Excitatory or inhibitory?
Release glycine or GABA
Inhibitory
What is the role of amacrine cells?
Allows us to see in low light conditions to enhance edges, shadows, and contrasting areas of luminance
What is the direct target of the retina?
Lateral geniculate body
How is the Lateral geniculate body organized?
Retinotopically
How are the signals from each eye composed at the lateral geniculate body?
Signals are kept separate here
What are the functions of the Lateral geniculate body?
Control motion of eyes to converge on a point of interest
Control focus of eyes based on distance
Determine relative position of objects to map them in space
Detect movement relative to an object
What does the superior colliculus create?
Map of visual space to activate appropriate motor responses required to move eyes
What is the funciton of the supeiror colliculus?
Coordinate head and eye movements to visual targets
Reflexive saccades
What does the Pretectum do?
Reflex control of pupil and lens
Where does the Pretectum send projections to?
Edinger westphal nucleus (for pupil)
and then to ciliary ganglion (for lens)
What forms the retinohypothalamic tract?
Where does this tract go?
What does it control?
Fibers branching off optic tract
To supraoptic, suprachiasmatic and paraventricular nuclei of hypothalamus
Gives visual input to hypothalamus to allow regulation of circadian rhythms etc.
What are the neurons of the medial temporal area/V5 responsive to?
Allowing us to what?
Direction of moving edge
Track motion across a scene
Does the MT/V5 area recognize color?
No - ignores it
What is cerebral akinetopsia?
What causes it?
Motion blindness - knows things are moving (V3), but movements are broken up
Lesion to MT/V5
What is the accessory optic system?
Several small nuclei involved in advanced visual process
What is the role of the accessory optic system?
Impt. Role in eye movements of Compensation and Pursuit (particularly w/ alternating saccadic type eye movements)
What is the major job of V1?
What else is this known as?
Identifying edges and contours of objects; constructs image features like size, orientation, local direction of movement and binocular disparity
Primary visual cortex or striate cortex
Where is V1?
What does it consist of?
Occipital lobe
6 cortical layers
How is V1 organized?
Retinotopically
Is V1 excitatory or inhibitory? W/ what NTRs?
Mostly excitatory w/ glutamate
Also has some GABA interneurons
What is V2 for?
Depth perception by analyzing disparities b/w eyes
What is V3a for?
Ids that motion is indeed occurring
What is V4 for?
COMPLETE processing of color
What are ocular dominance columns?
Columns running thru all 6 layers of cortex
W/ each one having a preference to input from one eye or another
How are ocular dominance columns mapped?
Into stripes
What are orientation columns?
Organizers region of neurons that are excited by VISUAL LINE STIMULI of VARYING ANGLES!
How are orientation columns oriented?
Perpendicular to the cortical surface
What are vertically responsive orientation column responsive to?
Vertical images (i.e. a tall tree)
How are orientation columns mapped?
Into swirls
What are the Blobs in V1?
Regions of neurons that are sensitive to COLOR assemble into cylindrical shapes
What is the difference in V1 and V4 processing of color?
V1 = subconscious awareness of color but cannot identify it
V4 = conscious awareness of color
What do you need for accurate color detection?
All 3 kinds of cones
What is cerebral achromatopsia?
Color loss bc damage to specific extrastriate cortical areas (V4)
Patient cannot see info supplied by retina (color blindness at level of retina)
What is color blindness?
No color Bc CONES do not funciton properly
What are melanopsin ganglion cells?
Specialized photosensitive ganglion cells that contain melanopsin
What are considered a 3rd class of photoreceptors in retina?
Melanopsin ganglion cells
Where do melanopsin ganglion cells project to?
Suprachiasmatic nucleus of hypothalamus
Light sensitive nucleus of pretectum
Limbic system
What do melanopsin ganglion cells not project to?
Making them be called what?
Do not project to visual cortex
—> non-image forming light responsive melanopsin gnaglion cells
What do melanopsin gnaglion cells do?
Helps regulate circadian rhythms, mood, and sleep
What therapy is good for those who experience shifts in light/dark cycles?
Because of what system
Light therapy
Bc of melanopsin ganglion cells
What is the ventral pathway that arises form the visual cortex?
From V1 —> V4 —> temporal lobe
What is the function of the ventral pathways?
Involved in interpreting images (recognizing or copying shapes, forms, faces)
Copying/naming objects = separate function in temporal lobe
Facial recognition = special area
What is the ventral pathway selectively activated by?
Shape, color, texture and object recognition
What is the dorsal pathway from the visual cortex?
From V1 —> V2 —> V3 —> V5/MT —> Parietal lobe
What is the function of the dorsal pathway?
Pathway allows you to complete motor actions based on visual input thru this pathway
What is the dorsal pathway selectively activated by?
Directionality and speed
What is the ventral pathway called?
The dorsal pathway?
Ventral = vhat
Dorsal - where
What does damage to the inferior temporal lobe cause?
Due to what?
Agnosia and prosopagnosia
Due to interruption of the ventral stream
What is agnosia?
Can see object but unable to copy or name it
Cannot see parts of objects contributing to it as a whole
Cannot interpret, understand or assign meaning
How is agnosia clinically seen?
Inability to construct or draw visual stimuli
Cannot recognize a picture of object
What is prosopagnosia?
Agnosia where you can identify a face as a face but NOT FACE AS BELONGING TO A PERSON
What do rod cells contain?
Contain rhodopsin
Where are rod cells concentrated?
Just outside the center of the fovea
How are rod cells positioned around cone cels?
Hexagonally packed around cones
How is the outer segment of rods constructed?
Disc membranes stacked in outer membrane w/ structure protein peripherin
What system are rod cells associated with?
Pathway?
W/ GPCR system
Rhodopsin —> transducen —> cGMP PDE
What is on the surface membrane of Rod cells?
Na/Ca exchanger
GC
cGMP gated Na channel
What are the desensitization proteins associated with rod cells?
Beta arrestin
Rhodopsin kinase
In the dark, hwo are rod cell membranes?
Partially depolarizes bc glutamate is continuously released (inhibiting optic nerve ON center bipolar cells)
CGMP na channel is open and cell is depolarized
How are rod cells’ channels in hte light?
Sodium channels = closed
Cell = hyperpolarized
Bc cGMP is hydrolyzed
What happens when the rod cell is hyperpolarized in the LIGHT?
Stops glutamate transmission
Depolarizes On center
Hyperpolarizes Off center cells
What is rhodopsin homologous to?
Beta adrenergic receptors
What is covalently bound to 11-retinal on rhodopsin?
Lysine 296 in center of 7 TM
What is retinal derived from?
Vitamin A
What is the active rhodopsin player have?
What can it absorb?
Protonated schiff base
> 440 nm
What makes up rhodopsin?
Opsin + retinal
What is the activation form of retinal?
11 trans retinal
Where are cones concentrated?
Fovea
What are the three varieties of cone receptors? At what wavelength do they absorb?
Blue = 460 nm Green = 520 nm Red = > 560 nm
What chromosomes are the photoreceptor proteins from?
Rod - chromosome 3
Blue - chromosome 7
Red - chromosome x
Green - chromosome x
What is the heritability pattern for red green color blindness?
X linked recessive trait
What modulated cGMP PDE?
Transducen
What will light activate?
Transducen (Gt), causing alpha subunit to dissociate —> activate cGMP PDE
What will cGMP PDE do?
Catalyze hydrolysis of cGMP to 5’GMP
What will low cGMP cause?
Activation of GC
Hyperpolarization of visual cells
= no visual signal tranduciton
What is the role of calcium in the visual phototransduction pathway?
Controls rate of cGMP synthesis and governs rate by which system is restored
When Ca is low, is GC active or inhibited?
Active
When it is dark, what is calcium doing?
Ca and Na enter Rod thru cGMP gated Na channels
What balances the calcium influx?
Calcium channel exchanger
One ca out, for 4 na in/1 k out
When it is dark, what is calcium doing?
Ca does not come in thru cGMP gated Na Chanel’s
But exchanger channel still working = Calcium level drops
What will the drop in calcium stimulate?
GC
What will GC do?
Increase the concentration of cGMP and open cGMP gated Na channels
What is the pathway of the visual phototransduction system when there is light?
One photon to rhodopsin
Trasnducin activated, alpha subunit dissociates
Activates cGMP PDE
Hydrolyzes cGMP to GMP
Decreases {cGMP}
CGMP gated Na channels closed
Drop in calcium
Membrane potential altered by 1 mv
Signal relayed to brain
(Drop in calcium stimulates GC, which restores {cGMP} and re-opens cGMP gated Na/Ca channel)
What are the 3 ways to terminate signals of the visual phototransduction pathway?
- Light activated rhodopsin blocked from activating transducen by Arrestin
- ATP is hydrolyzes to ADP, therefore alpha subunit cannot be bound to PDE
- Increased cGMP by GC
How is arrestin allows to bind rhodopsin?
Bc rhodopsin kinase phosphorylates C terminus of Meta-rhodopsin at Thr and Ser
What are the dietary sources of vitamin a?
Carrots Leafy greens Sweet potatoes Squash Broccoli Animal products (eggs, dairy, fish, liver)
What could deficiency of vitamin A cause?
‣ Night blindness ‣ Xerophthalmia ‣ Ckeratinization of epithelium ‣ Dry, scaly skin ‣ Failure of wounds to heal well ‣ Adv. Macular Degeneration ‣ Leading cause of preventable blindness in children ‣ Bitot’s spots
What are bitot spots?
Build up of keratin debris superficially in conjunctiva
Usually foamy in appearance
What can an excess of vitamin a cause?
Liver toxicity and joint pina
What can infantile exposure to isotretinoin in womb?
Cleft palates
Heart abnormalities
What nutritional disorder affects the cornea the most ?
Vitamin A deficiency
If you are deficient in vitamin A, how will the cornea present?
Bitot spots
Xerosis
Keratomalacia
What is xerophthalmia?
◦ Abnormal dryness of conjunctiva and cornea of eye
‣ W/ inflammation and ridge formation
What is nyctalopia?
Caused by ?
Night blindness; difficulty seeing in lowlight
Disorder of rod cells - assoc. w/ a vitamin a deficiency leading to insufficient rhodopsin
What is corneal or conjunctival xerosis?
◦ Metaplasia of conjunctival epithelium to stratified squamous type; keratinized surface
What are bitot spots ?
◦ Tangles of keratin mixed w/ saprophytic bacteria and sometimes fungi
What is macular degeneration?
Degeneration of RPE and retina
How does macular degeneration present?
Loss of central field vision
Poor night vision
Atrophy, macular hemorrhage, pigmentation, macular edema, and subretinal fluid
What is the role of macular carotenoids?
What are the macular carotenoids?
play a direct role in protection of retina against damage
- Lutein
- Zexanthin
Why is the eye susceptible to macular generation?
‣ Has high O2 flux and resp. Quotient
‣ High lipid content
‣ High glucose content
‣ UV radiation
What are the two types of AMD?
Dry type atrophy of macula
wet AMD assoc. w/ VEGF
What is the etiology of macular degeneration?
‣ Combo of envir. And genetics change REP oxidatively and thru inflammation
• Induces angiogenesis and increases inflammation and vascular permeability
‣ Genetics:
• 19 genes ID’d
• Dysregulation of alternate complement system
• Mutation in ABCA4 can cause macular degeneration
