Lec 05- Vision Flashcards
What is refraction?
bending of light waves at an angulated surface of a transparent material
What happens to light after striking a perpendicular surface?
continues on through WITHOUT bending
What 2 things does the degree of refraction depend on?
- Ratio of 2 refractive indices of the 2 transparent media
- Degree of angulation between the interface and the entering wave front of the light waves
What is refractive index?
ratio of the velocity of light in air/velocity of light in substance
What is the RI of air?
1
What is refractive power?
how much a lens bends light waves
diopters
How much is 1 diopter?
1 meter/focal length of lens
What is the distance from the center of the lens to the focal point?
focal length
Which lenses don’t have a focal point because they are diverging?
concave lenses
The object as projected on the retina will be _________ and _________
- reversed
- inverted
What does the iris do?
controls the amount of light entering the eye from darkness to light conditions
What does depth of focus of the lens do as the pupillary diameter decreases?
increases
What is Emmetropia?
- Normal eye (depth of focus)
- All distant objects seen clearly
- When ciliary muscle relaxed= parallel light rays from distant objects are in sharp focus on the retina
What is Hyperopia?
- Farsightedness
- Eyeball = too short
- Lens system = too weak (occasionally)
- All distant object seen clearly
What is Myopia?
- Nearsightedness
- When ciliary muscle relaxed= light rays from distant objects are focused in front of the retina
- Eyeball = too long
How is Myopia fixed?
Diverging lens
How is Hyperopia fixed?
Converging lens
Light from a distant point source will normally create a spot with a diameter of _________ and a center that is brighter than the periphery.
11 um
What is the average diameter of a cone in the retina?
1.5 um
What is the max visual acuity for two-point sources of light?
1.5 - 2mm
Which humor is formed by the ciliary processes?
aqueous humor
What is the ciliary apparatus consist of?
2 opposing layers of epithelial cells with a space between them
What ions are actively transported into the space of the ciliary apparatus?
Na+
Cl-
HCO3
What do the ions do in the ciliary apparatus?
- draw water (osmosis) into the space
- aqueous solution then passed into anterior chamber
Fluid flows from the anterior chamber into _______________ and then into aqueous veins in the sclera
Canal of Schlemm
What is normal intraocular pressure?
15 mmHg
What is glaucoma?
-
What is the photosensitive pigment in cones?
rhodopsin
What kind of proteins are photosensitive pigments?
transmembrane conjugated proteins
What are the 4 major functional segments of rods and cones?
- Outer segment
- Inner segment
- Nucleus
- Synaptic body
What major functional segment of rods and cones is the site of light-sensitive photo chemicals?
outer segment
What major functional segment of rods and cones connect with horizontal and bipolar cells?
synaptic body
What major functional segment of rods and cones contains organelles (especially mito)?
inner segment
What is rhodopsin?
Scotopsin + 11-cis retinal
Rhodopsin + Light»_space;
Scotopsin + All-trans retinal
What is 11-cis converted to in the Rhodopsin-Retinal visual cycle?
All-trans retinal
Dissociates from rhodopsin
What is Metarhodopsin II?
- intermediate
- excites electrical changes in the rods
What is required to convert All-trans retinal to 11-cis retinal?
- isomerase
- metabolic energy
What are 2 alternate pathways besides the Rhodopsin-Retinal visual cycle?
- All-trans RETINAL <> All-trans RETINOL (vitaminA)
- All-trans RETINOL <> 11-cis RETINOL <> 11-cis RETINAL
What does the excitation of rods cause?
increased negativity of the intrarod membrane potential
hyper polarization of rod membrane
What does the decomposition of rhodopsin cause?
decreased rod membrane conductance for Na+ ions in outer segment
(hyper polarization of rod membrane)
Which segment continually pumps Na+ from inside the rod to outside the rod and K+ in the opposite direction?
Inner segment
What happens when the outer segment is in the dark?
- Membrane is leaky to Na+
- Na+ leak into cell, neutralize the inside negativity
**Reduced electronegativity inside the membrane
(-40mV instead of -70 to -80mV)
What happens when the outer segment is in the light?
1- Rhodopsin exposed to light = decomposes
2- Retinal portion of rhodopsin complex activated
3- Transducin (G-protein) stimulated
4- cGMP phosphodiesterase activated
5- cGMP»_space; 5’GMP (reduction in cGMP)
6- Na+ channels close
7- Photoreceptor = hyperpolarized
What are the protein portions of the photo chemicals of cones?
opsins
**instead of scotopsin
What are the same in rods and cones?
Retinal portions
What colors are cones sensitive to?
- blue
- green
- red
What are the 5 layers of the retina?
- photoreceptors
- horizontal cells
- bipolar cells
- amacrine cells
- ganglion cells
- Light
Axons going to the brain
ganglion cells
What do photoreceptors do?
-transmit signals to outer plexiform layer
What is the plexiform layer?
layer of synaptic connections
What do photoreceptors synapse with?
- bipolar cells
- horizontal cells
What do horizontal cells do?
- transmit signals from rods and cones to bipolar cells
- transmit signals to outer plexiform layer
Is the horizontal cell output excitatory or inhibitory?
ALWAYS INHIBITORY
lateral inhibition
What do bipolar cells do?
- transmit signals from rods, cones, and horizontal cells
- transmit signals to inner plexiform layer
What do bipolar cells synapse with?
amacrine and ganglion cells
What do amicrine cells do?
transmit signals:
-directly from bipolar to ganglion cells
-within inner plexiform layer (from axons of bipolar cells) to dendrites of ganglion cells/other amacrine cells
What do ganglion cells do?
transmit signals from retina to brain
Axons of the ganglion cells make up ________
optic nerves
What are the only retina cells that transmit action signals?
axons of optic nerves
**others use electronic conduction (allows graded conduction of signal strength)
What do the interplexiform cells do?
transmit from inner plexiform layer to outer plexiform layer (retrograde)
Are interplexiform cells inhibitory or excitatory signals?
Inhibitory signals (lateral inhibition)
What does the foveal region do?
represents a new type of vision based on cone vision
What 3 neurons are in direct pathway of the foveal region?
- cones
- bipolar cells
- ganglion cells
What 4 neurons does pure rod vision consist of?
- rods
- bipolar cells
- amacrine cells
- ganglion cells
What nt is needed for rods/comes»_space; bipolar cells?
glutamate
What are the 8 nt for amacrine cells?
- GABA
- glycine
- dopamine
- Ach
- indolamine
ALL INHIBITORY
What are some of the types of amacrine cells?
- part of the direct pathway for rod conduction
- responds strongly to onset of continuing visual signal (but fades rapidly)
- respond strongly at the offset of visual signals (but fades rapidly)
- respond when a light is turned on/off
- responds to movement of a spot across the retina in a specific direction
Most amacrine cells are _________
interneurons that help analyze visual signals before they leave the retina
How many rods and cones converge on each ganglion cell and the optic nerve fiber from the ganglion cell?
60 rods
2 cones
Approaching the fovea, _______ rods and cones converge on each optic fiber and rods and cones become more slender
fewer
Approaching the fovea, what happens to the visual acuity in the central retina?
incrases
What are there ONLY in the central fovea?
slender cones (35,000)
NO RODS
What retina is more sensitive to weak light?
peripheral retina
How many rods converge on a single optic fiber in peripheral regions of the retina?
200 rods
What are the 3 types of ganglion cells?
WXY
What ganglion cells make up 40% of all ganglion cells?
W
How fast do W ganglion cells transmit signals?
8 m/sec
W ganglion cell characteristics
- Small
- Receive most of excitation from rods transmitted by way of smaller bipolar cells and amacrine cells
Why do W ganglion cells have broad fields in the peripheral retina?
Their dendrites spread widely in the inner plexiform layer
What ganglion cells make up 55% of all ganglion cells?
X
How fast do X ganglion cells transmit signals?
14 m/sec
X ganglion cell characteristics
- Medium diameter
- Small fields
- Signals = represent discrete retinal locations
- Receives input from at least one cone cell (all color vision)
What ganglion cells make up 5% of all ganglion cells?
Y
How fast do Y ganglion cells transmit signals?
50 m/sec or faster
Y ganglion cell characteristics
- Large diameter (35um)
- Respond to rapid changes in visual image
Where does the Dorsal Lateral Geniculate Nucleus receive input from?
optic nerve
What does the Dorsal Lateral Geniculate Nucleus do?
- Relays information from optic tract to visual cortex using optic radiation (geniculocalcarine tract)
- High degree of spatial fidelity from retina to the visual cortex
What % of decussation in optic chiasm for Dorsal Lateral Geniculate Nucleus?
50%
What nuclear layers in the Lateral Geniculate Nucleus receive signals from the LATERAL half of ipsilateral retina?
2
3
5
What nuclear layers in the Lateral Geniculate Nucleus receive signals from the MEDIAL half of the opposite retina?
1
4
6
What are the Magnocellular layers?
1
2
What do layers 1 and 2 do?
- Receive input from large Y type ganglion cells
- Provides rapidly conducting pathway to visual cortex
Which layers transmit only B&W?
1
2
Which layers contain large neurons?
1
2
Which layers have poor point to point transmission?
1
2
What are the Parvocellular layers?
3-6
What do layers 3-6 do?
- Receive input from large X type ganglion cells
- Provides moderate conducting pathway to visual cortex
Which layers transmit color?
3-6
Which layers have accurate point to point transmission?
3-6
What controls how much of the signal is allowed to pass to the cortex?
Lateal Geniuclate Nucleus
What are the 2 sources of transmission gating control?
- Corticofugal fibers from primary visual cortex
- Reticular areas of the mesencephalon
- *BOTH:
- Inhibitory
- help highlight visual information that is allowed to pass
What is the primary visual cortex also called?
Striate cortex
Where is the primary visual cortex located?
in occipital lobes
Where do signals from Macular area terminate?
near the occipital pole
Where do signals from the more peripheral retina terminate?
At or in concentric half circles anterior to the pole but still along the calcarine fissure
Where do the geniculocalcarine fibers terminate?
Layer 4 of primary visual cortex
Which layer do signals from Y ganglion cells terminate?
IVca
Which layer do signals from X ganglion cells terminate?
IVcb and IVa
What is the diameter of the primary visual cortex?
30-50 um (1000 neurons per column)
What are color blobs?
- Special column-like areas
- Among the columns of secondary visual areas
What do the color blobs do?
- Receive lateral signals from adjacent visual columns
- Activated specifically by color signals
What do signals from the 2 separate eyes enter?
alternating stripes of columns in layer 4
What deciphers whether the respective areas of the 2 visual images from the 2 eyes are in register with each other?
cortical area
What is the deciphered information from the 2 eyes used to adjust?
the directional gaze of the separate eyes
What is cortical area deciphered information also required for?
stereopsis
In children, refractive power of the lens can be voluntarily increased from _____________
20»_space; 34 diopters
accommodation of 14 diopters
In young person, what happens to the lens when it is in a relaxed state with no tension on its capsule?
assumes spherical shape
What creates tension around the lens that causes the lens to remain relatively flat under normal eye conditions?
suspensory ligaments
What do Meridional fibers of ciliary muscle do?
contract and release tension on lens
What do Circular fibers of ciliary muscle do?
decrease tension on lens
Which CN controls both sets of ciliary muscles?
CN III
What happens to the lens in accommodation in older people?
- becomes larger and thicker with age
- becomes less elastic with age
By the age of 45-50 what does the power of accommodation decrease to?
<2 diopters
By the age of 70 what does the power of accommodation decrease to?
0
What is the accommodation in older people called?
Presbyopia
