Lecture 4: Vision-Retinal Processing Flashcards
What is the region of high visual acuity in the eye?
macula/fovea
What is the blindspot of the eye?
optic disk
no receptors, this is where cells/axons leave retina to become part of the optic nerve
What is the vascular supply of the eye?
branch of opthalmic artery and branch of opthalmic vein
Most of our vision is ____
binocular
this is central vision (both eyes)
Most of our peripheral vision is____
monocular
one eye
Where is light absorbed?
retina
light perception begins at retina
How can the lens accomodate when light info enters?
It can change shape so that the light is refracted at the correct angle onto the retina
lens help focus light
To see far, how does the eye accommodate?
- Ciliary muscle relaxes
- suspensory ligament taut
- lends flattens/thins
To see near, how does the eye accommodate?
- Ciliary muscle contracts
- Suspensory ligament relaxes
- lends become globular (thickens)
What is emmetropia?
Normal vision
What is myopia and where is the focal point?
near sightedness. focal point is in front of the eye
can see near, focus light in front of the retina,
What is hyperopia and where is the focal point?
far-sightedness and the focus behind the retina
can see far, light focuses behind the retina, cant see near
How do glassses/contacts help you see?
glasses/contacts changes the angle of light so it reaches the retina
Projections of the visual field/image onto the retina are ____
inverted (flipped upside down and backwards)
Visual Field is?
What you’re looking at in space
Retinal field is?
What’s projected onto the retina
All information from the RIGHT visual field will end up on the ____ portion of the brain.
LEFT
How does info from your RIGHT visual field end up on the LEFT side of your brain?
- Info projects onto temporal regions of LEFT retina and nasal region of RIGHT retina
- All info that projects to temporal stays ipsilateral (same side)
- All info projected to nasal retina will cross to contralateral side
- Then its sent up the LEFT optic tract and LEFT side of brain for processing
All information from the LEFT visual field will end up on the ____ portion of the brain.
RIGHT
How does info from your LEFT visual field end up on the RIGHT side of your brain?
- Info projects onto temporal regions of RIGHT retina and nasal region of LEFT retina
- All info that projects to temporal stays ipsilateral (same side)
- All info projected to nasal retina will cross to contralateral side
- Then its sent up the RIGHT optic tract and RIGHT side of brain for processing
Given that each eye receives information from either visual field, fibers cross at the ____ to get information from the visual field to its respective cortical region.
optic chiasm
What are the two types of photoreceptors of the retina?
Rods and cones
Where are rods located?
retina except optic disk
Where are cones located?
Macula/Fovea
What are the 5 retinal neurons?
- Photoreceptors
- Horizontal cells = release GABA (inhibitory) and form gap junctions
- Bipolar cell
- Amacrine cell = release GABA, glycine, and dopamine (inhibitory) and form gap junctions
- Ganglion cell
- Photoreceptors, bipolar cells, and horizontal cells produce graded potentials
- Amacrine cells and ganglion cells produce action potentials
Briefly explain how light reaches the retina?
- Light enters the superficial retina, passes through cell bodies, and travels towards the back in order to reach photosensitive cells (rods and cones).
- Action potential allows photoreceptor signaling to be sent back up towards the superficial retina where they can then be guided to the optic nerve and CNS.
What are rods highly sensitive to?
Low levels of light
not sensitive to color
What protein do rods have and do they have a large or small number of disks?
Opsin=rhodopsin
a large number of disks
- Rods have far larger areas of photoreceptor disks to make them highly sensitive to light.
- Opsin responds to light
What are cones sensitive to?
Cones are sensitive to colors
What protein do cones have and do they have a large or small number of disks?
Many (around 3) different opsins
smaller number of disks
different opsins are responsible for detecting different wavelenghts that give us diff colors
What is the function of pigment epithelium?
It is a “backward” organization of the retinal layer due to the need for constant recycling of photoreceptor proteins (opsins) and disks
Photoreceptors (rods and cones) are embedded in the pigment epithelium
Why do we need to recycle photoreceptor proteins and what is required to do so?
on exam
- Need to be constantly synthesizing new proteins so we stay sensitive to light, especially rods (low light conditions).
- We need VITAMIN A (retinol) which maintains rods and rhodopsin signaling
___________ of total body retinol (Vitamin A) is stored in liver
50-85%
What is scotopic vision?
Only rods are being used
like an on/off switch, no wavelength to tell you color
What is mesopic vision?
Rods and cones are being used
you can see color
What is photopic vision?
only cones are being used (rods are saturated with light)
rods are saturated with light so much that rods aren’t firing anymore, and we use cones
Which type of vision has the best acuity?
Photopic vision
bc cone and color info
Light sensitivity depends on ____ and ____.
receptive field size, intensity
Rods are usually activated at what wavelength of light?
400-600nm
What is trichromat?
This is normal vision; you contain all 3 cones subtype
have all 3 opsins
What is protanopia?
This is a type of colorblindness where you lose the cone subtype responsible for RED wavelengths
2/3 opsins, missing longest=red wavelength of light
What is deuteranopia?
This is a type of colorblindness where you lose the cone subtype responsible for GREEN wavelengths
2/3 opsins, missing middle=green wl of light
Activation of this alters a membrane current that controls photoreceptor transmitter release
Rhodopsin
In the absence of light, explain what happens to photoreceptor rhodopsin and its relation to glutamate
- Rhodopsin is INACTIVE
- Na+ channels opens and influx
- Cell is DEPOLARIZED
- HIGH rate of glutamate is released constantly
cones have the SAME signaling using different opsins.
WITH light, explain what happens to photoreceptor rhodopsin and its relation to glutamate
- Rhodopsin is ACTIVE
- Na+ channels CLOSED
- Cell is HYPERPOLARIZED
- Low rate of glutamate release
basically no glutamate released
cones have the SAME signaling using different opsins.
Explain briefly the vertical info flow
- Rods and cones release glutamate
- This is inhibitory or excitatory depending in the glut receptors expressed on the bipolar cell
- Bipolar cells release glutamate which is excitatory for ganglion cells
ganglion cells=retinol ganglion cells (RGC)
Explain vertical info flow for ON center receptor field (light comes in)
Light stimulation causes
1.HYPERPOL of photoreceptor, and no glutamate is released
2. INCR DEPOL of ON center bipolar cell, so mGLU is activated
3. INCR glutamate release from bipolar cells
4. Incr ganglion cell firing rate
Explain vertical info flow for OFF center receptor field (light comes in)
Light stimulation causes
1. HYPERPOL of photoreceptor, and no glutamate is release
2. DECR DEPOL of OFF center bipolar cell , so NMDA rec is INHIBITED (b/c hyperpol)
3. DECR glutamamte release from off bipolar cells
4. DECR ganglion cell firing rate
With light PRESENT is the following cells activated or inhibited?
A. On-center bipolar cell (mGlu)
B. Off-center bipolar cell (NMDA)
A. On-center bipolar cell (mGlu): ACTIVATED
B. Off-center bipolar cell (NMDA): INHIBITED
With NO light, the following are activated or inhibited?
A. On-center bipolar cell (mGlu)
B. Off-center bipolar cell (NMDA)
A. On-center bipolar cell (mGlu): INHIBITED
B. Off-center bipolar cell (NMDA): ACTIVATED
What variables contribute to mapping center-surrond receptive field of the retina?
- Duration of spot illumination
- Size of spot illumination
- Background light intensitiy of both center and surround area
center compared to peripheral environment
ex:a grey spot next to white looks much darker than the same grey spot next to black
What is 20/40 vision?
At 20 feet, a human is able to separate line for image recognition that a person with nominal performance can resolve at 40 feet
What is 20/20 vision?
At 20 feet, a human with normal eye performance is able to separate lines for image recognition
What are the 2 types of synaptic transmission in the retina?
chemical and electrical
need electrical to identify things moving very quickly
What are the two cells involved in lateral information flow and explain their fxn
- Horizontal cells: release GABA (inhibitory) and form gap junctions.
- Amacrine cells: release GABA, glycine, dopamine (inhibitory) and form gap junctions
What is lateral inhibition essential for?
Essential for identifying shapes (edges) and detecting motion
Horizontal cells always have a ____ output via release of ____.
inhibitory, GABA
What are horizontal cells connected to?
Connected laterally to photoreceptor cells, and they suppress vertical information flow in adjacent pathways
Explain how horizontal cells play a role in lateral inhibition in this case, for an ON-center receptive field, with NO surround light stimulation
- There is a dark surround, this inhibits center cone via GABA
- Less glutamate release and stimulation of the one-center bipolar cells,
- Incr ganglion cell firing
Dark stimulated surround ____ center response
enhances/incr
Explain how horizontal cells play a role in lateral inhibition in this case, for an ON-center receptive field, WITH surround light stimulation
- Surround photoreceptor is activated and therefore hyperpol with decr release of glut
- Reduction of glut reduces release of GABA from horizontal cells
- Reduction of gaba inhibition causes dep of photoreceptors in the center and incr release of glut
- Incr release of Glu from photoreceptor causes a decr in ON center ganglion cells firing
Light stimulated surround ____ center response
inhibits/decr
Lateral inhibition relies on what?
Gap jxns: provides cell-cell coupling
What does lateral inhibition promote and amplify?
Lateral inhibitions promotes contrast enhancement and amplifies edge detection of the retina
detect edges with shape
For information flow to the brain, the visual scene is encoded by the firing patterns of ____ ____, which is further processed in the ____ ____ by the way of the ____ ____.
retinal ganglion cells
visual cortex
optic nerve
What is the lens retinal prosthesis?
A computer chip embedded in lens that mimic retina
- An MSP3 student arrives late to lecture and decides to sit in the very back row of MDA1209. In order for the lens of his eyes to properly refract light (from the screen) onto the retina, what must occur?
A. Ciliary muscle relaxes; suspensory ligament tightens; lens flattens
B. Ciliary muscle contracts; suspensory ligament relaxes; lens becomes globular
C. Ciliary muscle relaxes; suspensory ligament tightens; lens becomes globular
D. Ciliary muscle contracts; suspensory ligament relaxes; lens flattens
A. Ciliary muscle relaxes; suspensory ligament tightens; lens flattens
- During a bright day on the beach, the rods of your retina become saturated with light and only cones are being used to perceive visual fields. Which of the following best describes this form of vision?
A. Scotopic
B. Mesopic
C. Photopic
D. Lasopic
C. Photopic
- A 24-year-old was recently diagnosed with a form of color blindness known as Deuteranopia due to a genetic malformation of the cones in his retina. Given her condition, which of the following color wavelengths is she not able to perceive?
A. Red
B. Green
C. Blue
D. Yellow
B. Green
- Neuroscientists at the Morsani College of Medicine are studying the effects of light stimulation on retinal neurons and photoreceptors. All of the following are likely to occur during their experiment EXCEPT:
A. Hyperpolarization of photoreceptors
B. Decreased glutamate release from on-center bipolar cells
C. Hyperpolarization of off-center bipolar cells
D. Decreased glutamate release from photoreceptors
B. Decreased glutamate release from on-center bipolar cells
- Horizontal cells play an important role in the modulation of lateral information flow within the retina. More specifically, they are responsible for lateral inhibition. What neurotransmitter do horizontal cells release to execute their function?
A. GABA
B. Glycine
C. Dopamine
D. All of the above
A. GABA
