Vision Flashcards
How much of the brain mass is devoted to visual processing?
Half
The occipital cortex is used for?
Perception of objects in space
Where are visual memories stored?
Parietal and temporal lobe
Visual reflexes are provided by what?
Brain Stem and Spinal Cord
Where do parts of the visual system that provide information requiring for setting the circadian rhythms, general metabolic rate, mood, etc. occur?
Pineal gland & diencephalon
Questions on visual memory help test?
Cognitive status
Pupil dilation can help test?
Sympathetic nerve function
What allows for fine focusing of incoming light?
Fovea
As light enters the eye what refracts it? Inverts it? Then variably refracted by?
Cornea
Pupil
Lens
What is the lens under the control of?
Suspensory ligaments
Ciliary muscles
What is the fovea the center of?
Macula
What is the sclera?
Protective layer for the retina and the choroid
What is the choroid?
Vascular bed of the outer retina
To what does the choroid provide blood flow and nutrients to?
Photoreceptors and RPE cells
What provides retinal blood flow?
Central retinal artery
Where does the central retinal artery enter?
Through the optic nerves
How much blood does the central retinal artery provide?
20%
Are there blood vessels in the fovea?
Nope
What helps maintain the visual axis of the fovea?
Extraocular muscles
Function of the retinal pigmented epithelial cells (RPE)?
Provide a barrier to the retina from the choroid
What is the retinal pathway?
Photoreceptors to bipolars to ganglion cells
Where do ganglion cell axons coalesce? What happens there?
Form optic nerve head
Blind spot
Types of interneurons?
Horizontal cells (outer plexiform) Amacrines (inner plexiform)
Other functions of the RPE?
Phagocytosis of rod outer segments
Retinal nutrition
Protection of photoreceptors from light damage
What does RPE contain? Its function
Melanin
Helps absorb some of the light that comes to the photoreceptors
What is drusen?
Autofluoresce that occurs from protein left in the RPE
What are rods names for?
Shape of the outer segment
Rods are responsible for?
Black and white (night) vision [scotopic]
Where does light transduction occur in rods?
Outer segment
In rods, what is the synaptic expansion called?
Spherule
How often do rods undergo phagocytosis? By what?
Every 10 days
RPE cell
What are cones named for?
Outer segment
What do cones process?
Color (photopic) information
Types of cones?
L-Cones
M-Cones
S-Cones
To what do L-Cones respond? M-Cones? S-Cones?
Long wavelengths -Red
Medium wavelengths -Green
Short wavelengths -Blue
Does each type of cone respond to a different color of light?
Yes
What are colors represented by?
Unique combination of L-,M-,S-cones
Which cones are carried on a X-chromosome?
M- and L- cones
What happens in color blindness?
Genetic defect causing a person to miss one type of cone
When does vision occur?
When rods and cones HYPERPOLARIZE to light
NOT DEPOLARIZE!!!!
Purpose of rods and cones?
Absorb quanta of light and convert to an electrical signal
What happens when light comes in?
Converts 11-cis-retinal to all-trans-retinal
What does all-trans-retinal activate?
Rod opsin - rhodopsin
Active rhodopsin activates? Which then activates?
GTP –> cGMP phosphodiesterase (PDE)
Active PDE in a rod does what?
Lowers cGMP lvls enough to hyperpolarize the membrane and close sodium and calcium channels
Closure of the sodium and calcium channels leads to?
Visual response
What happens when enough sodium and calcium is stuck in the membrane?
Calcium begins to be pumped out through the sodium/calcium exchanger
What happens to cGMP levels as calcium in the membrane is reduced?
Rise and repolarize the membrane to pre-stimulus levels –> reopening the channel
In rods, after the membrane is repolarized and the channels reopened what happens?
Rhodopssin is de-phosphorylated and all-trans-retinal is converted back to 11-cis-retinal
In rods, what happens after rhodopsin is de-phosphorylated?
rhodopsin kinase comes back to re-phosporylate rhodopsin and is bound to arrestin to render it inactive
Where does the signal go from the photoreceptors?
Bipolar cells
What modifies bipolar cells?
horizontal cells
Where does the signal go from bipolar cells?
Ganglion cells
What is located at the synapse between bipolar cells and ganglion cells? What do they do?
Amacrine cells –> detect major changes in activity levels
Characteristics of Alpha-Ganglion cells?
Predominate in the peripheral retina Most input from rods Extensive dendritic tree Large axons Participate little in color perception Project to Magnocellular layer of lateral geniculate nucleus
Function of alpha-ganglion cells?
Location of object in space
Characteristics of Beta-Ganglion cells?
Primarily in central retina Most inputs from cones small receptive fields Small dendritic arbors responsive to color stimuli project to parvocellular region in lateral geniculate nucleus
Function of beta-ganglion cells?
Define color and texture of object
How do cells project out of the fovea?
radially
Does convergence occur in the fovea?
NO
Ratio of photoreceptor to ganglion cell in the fovea? What does this ratio allow?
1:1
Crisp imaging
Why is the visual image reversed and inverted?
Due to pinhole effect of pupil
What happens in the optic chiasm to ganglion cell axons?
Partial decussation –> only nasal fibers cross
Where does the optic tract project to?
Lateral geniculate
What carries geniculate fibers to primary visual cortex in occipital lobe?
Optic radiations
Which cortex is the left visual field processed?
RIGHT VISUAL CORTEX
Describe the M pathway?
Originates from the magnocellular (alpha) ganglion cells –> projects to lateral geniculate layers 1 and 2 –> to layer 4C-alpha in the cortex - space info
Descibe the P pathway?
Originates from the parvocellular ganglion cells –> projects to lateral geniculate layers 3-6 –> to layer 4C-beta in the cortex - form info
Causes of diabetic retinopathy?
1) Changes in vasculature
- Angiogentic growth factors
- Loss of angiostatic growth factors
2) Changes in neurons
- Loss of photoreceptors
- Loss of ganglion cells
Characteristics of Macular degeneration?
Choroid NV
Age induced
Some genetic
Lose central vision
Characteristics of dry AMD?
Drusen
Little vision loss
often in 1 eye
Characteristics of wet AMD?
Sometimes drusen
Vision Loss
Progresses to both eyes
