1
Q
Cornea, lens
A
Transparent, focuses light on the retina
2
Q
Iris
A
controls amount of light entering the eye and size of pupil. It’s transparent.
3
Q
Retina
A
Part of CNS; detects and processes light inputs. Neural tissue lining the back of the eye. It detects light.
4
Q
Aqueous, vitreous humor
A
liquid filling anterior and posterior chambers; maintain shape of eyeball
5
Q
Choroid
A
contains blood vessels
6
Q
Sclera
A
tough protective layer of connective tissue
7
Q
Iris & pupil
A
- Regulates amount of light entering the eye.
- Controlled by autonomic nervous system. Has two sets of smooth muscle.
8
Q
Pupil when bright light
A
It constricts. The inner smooth muscle (the circluar) is activated. It moves in circular motion inwards. The parasymathic (ciliary ganglion) innverates the circular ones.
9
Q
Pupil when dark light
A
It dilates. The outter smooth muscles (the radial) is activated. It moves outward. The sympathetic (superior cervical ganglion) innverates the radial ones.
10
Q
Refractive structures in the eye
A
lens and cornea
11
Q
Cornea (refracftive)
A
- Majority of refractive power (75%)
- But not adjustable
12
Q
Lens refractive power
A
- Less refractive power (25%)
- Adjustable by changing curvature: accommodation
- Accommodation is controlled by the autonomic nervous system
13
Q
Light and focal point
A
Parallel rays focus light to where we want it. Diverging rays don’t, so it fix we make the lens bigger. so it has more reactive power.
14
Q
refractive errors
A
Presbyopia, Myopia (nearsightedness), Hyperopia (farsightedness).
15
Q
Presbyopia
A
The lens ability t accommodate to adjust the focal plane is reduced. Lens loses elasticity with age – most
people in middle age acquire reading glasses. Lens can’t change shape.
16
Q
Myopia (nearsightedness)
A
Eyeball too long or lens
too strong
→ light focused in front of retina
17
Q
Hyperopia (farsightedness)
A
Eyeball too short or
lens too weak
→ light focused behind retina
18
Q
Top to bottom layers
A
Retina pigment epithelium (RPE), Photoreceptor (PR) outer/inner segments, Outer nuclear layer (PR cell bodies), Outer plexiform layer (synapses), Inner nuclear layer (cell bodies), Inner plexiform layer (synapses), Ganglion cell layer (cell bodies), Nerve fiber layer.
19
Q
Simpler layers
A
Stimulus (light) then photoreceptors, then processing, then optic nevre.
20
Q
Cells top to bottom
A
Cones and rods, to bipolar and horizontal cells, to amacrine, to ganglion cell.
21
Q
Optic nerve
A
Made of ganglion cell axons.
22
Q
Ganglion cells
A
Make action potentials.
23
Q
Rods
A
- Vision in dim light.
- High sensitivity (detect single photons!).
- Most abundant photoreceptors (>95%) in most vertebrates.
24
Q
Cones
A
- Vision in room light, daylight
- Color vision
- High-acuity central vision (reading)
25
Scotopic
Only rodes
26
Mesopic
Both rods and cones
27
Photopic
Only cods.
28
Rods and cones have what
Outer and inner segements
29
Outer segments
Specialized for detecting light.
30
Amphibians
Have really big photorecptors (inner segements)
31
Rhodopsin
Opsin (GPCR) + chromophore. it's colvantently attached.
32
Rod phototransduction
Rhodopsin (11-cis-retinal covalently bound). Then Absorption of photon
(light) to isomerization of chromophore to all-trans-retinal then Rhodopsin protein conformational change that activates the heterotrimmeric G protein.
33
Cone phototransduction
About the same as rods but opsin proetins are slightly different. Diffferent cones to different wavelengths is different opsin proteins.
34
Rod phototransduction more steps
Light schanges rhodopsin. Transducion is activated (GTP to GDP). The alpha subunit attaches to Phosphodiesterase which hydrolyzes cGMP into GMP. The Cyclic nucleotide
gated channels close because they need cGMP, so Na+ can't come into the cell so it hyperpolarizes it. Open channels depolarize it. This all happens in a stacked disc in the outter segment.
35
Dark vs light in Rod phototransduction
Dark:
Lots of cGMP.
CNG channels open.
Cell relatively depolarized.
Light:
cGMP gets hydrolyzed.
CNG channels close.
Cell hyperpolarized.
36
Rod phototransduction: Amplification
each activated Rho
activates many transducins; each
PDE hydrolyzes many cGMPs
- One activated Rho leads to
closing of 200 CNG channels. Rods have a lot of discs, dics have a lot of rho.
37
Rod phototransduction:
Abundance
Rho is the most
abundant protein in outer
segments
→ Outer segments specialized for
light detection!
38
Photoreceptor responses to light flashes
Theses are graded potenials. The brighter the flash, the more hyperpolaized you get, the more channels close.Brighter is also more currents. Eventually all of the channels close and you ger saturation.
39
Retinoid cycle
The regeneration of chromophore that gets attached to the opsin protein.multiple enzymes are required for regeneration of the chromophore and that a lot of these steps happen in the RPE cells. Have rhodopsin in rod, when light hits it chromophore actually comes off off of the protein and um eventually gets transported into an RPE cell. And here it goes through a series of steps to remake the um 11 cis retinal that we need to attach to opsin to make rhodopsin.
40
Retina pigment epithelium
Non-neuronl cells that interact closely with outer segments and supports the retina, and mediates the retinoid cycle. outer segment renewal.
41
Photoreceptor outer segment renewal test
- Rats were injected with [3H]-methionine (radioactive)
- 3H detected in eye sections by autoradiography.
- Observed band of 3H signal moving up the outer segments.
42
Photoreceptor outer segment renewal
Photoreceptors are terminally differentiated
neurons - can’t divide.
- But, outer segments are renewed – about 5% of each outer segment replaced every day!
- RPE responsible for phagocytosis of outer
segment tips.
- New discs are formed at the base of the outer segment – how??
43
Color vision
- Three cone types: red, green, blue
(or L, M, S for long, medium, short wavelengths).
- Different cones express different opsin proteins.
- Cones have different absorption spectra.
- Differential responses of the
different cones to a stimulus
results in perception of different colors.
- Humans can detect about 1,000,000 colors.
We are trichromats
44
optic disc
Where ganglion axons go through to the back of the eye. No photoreceptors. - Ganglion cell axons go through the retina to the back of the eye
- Source of the “blind spot."
45
Fovea
Where light hits when you look at something.
-Central area within the macula.
- Highest acuity region of retina.
- In center of fovea (foveal pit), inner retina layers are “pushed away” – less light scattering so nothing in the way of hitting the light receptor.
- Common model organisms (like mice) don’t have a fovea, makes harder to study.
46
Fovea high density of
cones. cones are abundant and densely packed.
47
Peripheral retina
cones are rare.
48
Foveal acuity
Fovea is the highest accuitity regoin because one cone is attached to one ganglion cell.
- Rods have a lot of convergence (up
to ~100 rods → → one ganglion cell)
- Large receptive field
- High sensitivity
- Foveal cones have little or no
convergence (~1 cone → → one
ganglion cell)
- Small receptive field
- Lower sensitivity
49
Rod and cone Ribbon synapses at photoreceptor terminals
LOOK AT SLIDE
50
Cones synapse with what?
On and Off bipolar cells. Example of parallel processing.
51
Rods synapse with what?
On bipolar cells.
52
On bipolar cells
Light increments. depolarize to light.
53
Off bipolar cells
Light decrements. Hyperpolarize to light.
54
sign inverting synapse
In On rod BC and On Cone BC. when you have the rod being hyperpolarized and the down the post synaptic neuron becoming depolarized (in respose to light).
55
Sign preserving synapse
OFF cone BC. Whereas here when you get hyperpolarization in the bipolar cell it's called a sign preserving synapse.
56
Neurotransmitter release at photoreceptor terminals
Dark
Depolarized.
Voltage-gated Ca channels open.
Ca influx → SV exocytosis.
Continuous (tonic) release.
Light
Hyperpolarized.
Voltage-gated Ca channels close.
Reduced release.
Graded responses
57
PR release what
glutamate as their neurotransmitter.
58
In on BC cells from dark to light
Metabotropic glu receptor. Closed in dark, open in light. Cell depolarized. Better at detecting lights turning on.
59
Off BC cells from dark to light.
Ionotropic Glu receptors. Open in dark, closed in light. Cell hyperpolarized. Better at detecting lights turning off.
60
ON-bipolar cell responses to light flashes
Produce graded potenials.
61
Horizontal cells
Mediate lateral inhibition.
Medium center, dark surround
→ center gives response like brighter spot
Medium center, white surround
→ center gives response like darker spot
Enhanced contrast!
62
Positive contract
Having white inside goes up.
Having white surround, it goes down.
63
Negative constrast
Dark center, goes down. Dark surround, does up.
64
Horizontal cells – lateral inhibition
Surround cone depolarizes and release more gluatmate and activites hortizontal cells which inhibits neighbour (bipolar cell) causing it to be more hyperpolarized and produce even less gluatmate, causing a stromger respose from bipolar cell.
65
ON/OFF starts with
Glu detection in bipolar cells
66
Rod have connect to what
Ganglion cells. They connect to amarine cells (a interneuron).
67
Rod pathway
Rod bipolars don’t directly contact RGCs
– how do signals get to the brain?
1. Sign-preserving glutamatergic synapse
between rod bipolar and AII (“A2”)
amacrine cell. It makes two connections.
2a. Glycinergic sign-inverting synapse
between AII amacrine cell and OFF cone
bipolar cell
2b. Gap junction (electrical synapse)
between AII amacrine cell and ON cone
bipolar cell
→ Rod pathway feeds into cone pathway.
At the same time it sends off and on channels.
68
Inner plexiform layer
Has its own layers in it. (Off then on).
69
Intrinsically photosensitive (ip) RGCs
Ganglion cells with own way of detecting light. Can detect light when rods and cones are degenerated. It uses phospholipase C. Depolarization causes
- Circadian clock
- Pupillary light reflex
- Visual perception
- Sleep, Activity
- etc
70
Age-related macular degeneration
Extracellular deposits of proteins and lipids.
Degeneration of RPE and photoreceptors in macula.
Risk factors: age, diet, genetic.
How/why largely unknown.
71
Retinitis pigmentosa
Degeneration of rods and/or RPE.
Affects night vision and peripheral vision first.
Inherited (many different genes/mutations, most common is Rhodopsin).
72
Glaucoma
Degeneration of RGCs, starting with axons.
Risk factors: age, high intraocular pressure.
How/why largely unknown.
73
Geniculate
bent at a sharp angle
74
Thalamus, lateral geniculate nucleus
- Relay station for almost all sensory information.
- Contains many nuclei (nucleus = group of neuron
cell bodies with similar connections and functions).
- The lateral geniculate nucleus (LGN) passes
information from the optic nerve to the visual cortex.
75
Where else does info from the retina go?
The lateral geniculate nucleas.
