1
Q
With one quick look at something, what can we determine about it?
A
- Shape
- Size
- Colour
- Location
- Speed
- Direction
2
Q
LO
A
- Describe the anatomy of the eye, including the structure of the retina
- Explain the function of rods and cones
- Discuss the roles of on center and off-center ganglion cells
- Describe the central visual pathways
- Describe the concept of retinotopic organisation
- Describe the columnar organisation in the visual cortex
3
Q
This is a diagram of the visual system
A
4
Q
Visual field
A
5
Q
What axons form the optic nerve in the eye?
A
The Retinal ganglionic cell axons
6
Q
Where do the optic nerves cross?
A
Optic chiasm
7
Q
Where does the optic nerves terminate?
A
Many terminate at lateral geniculate nucleus => relay to visual cortex (~40% of cortical area)
Others: pretectal nucleus (reflexive eye movements); suprachiasmatic nucleus (sleep – wake cycle)
8
Q
Tell me the rough length of the eyeball?
A
roughly 25mm in length
9
Q
What are the 3 layers of the eyeball?
A
- Sclera- tough and whiteish
- Choroid- all blood vessels
- Retina- photoreceptors present
10
Q
Label the eye
A
11
Q
What fluid is the eyeball filled with?
A
vitrous humour
12
Q
What parts of the eye are transparent?
A
Cornea
Lens
13
Q
Tell me about refraction of light in the eye, whats involved and the light refraction they provide?
A
Refraction of light
- Cornea provides most of refraction
- Lens is adjustable => accommodation
- Lens provides about a 1/3 of refraction. The lens is adjustable which allows us to see near and far. This is what we term as accommodation
14
Q
Where does light collect in order to form the main image you see?
A
Fovea
rest of light collects and forms the peripheral image
15
Q
What is meant by the blind spot?
A
Blind spot is where retina ganglionic axons leave the eye to go towards the brain this is void of photoreceptors
16
Q
What layer of the eyeball forms the cornea?
A
Sclera turns really transparent and forms the cornea. This is the area that does the most refraction of light. Cornea provides most optical power
17
Q
What is meant by the lens be accommodating?
Tell me what is involved in this to help this mechanism work?
A
Accommodation- changing your optical power to focus near and far
18
Q
If your eye can see normally with no visual defects what is this known as?
A
Emmetropia
19
Q
What are the two refractive errors in the eye and why do these occur?
A
- Near sighted= myopia
Nearsightedness usually occurs when your eyeball is longer than normal or your cornea is curved too steeply. Instead of being focused precisely on your retina, light is focused in front of your retina, resulting in a blurry appearance for distant objects
- Far sighted= hyperopia
Farsightedness occurs when light is not refracted properly through an uneven, less smooth cornea or lens. It is a type of refractive error.
20
Q
Which retinal ganglion cell axons cross into the contralateral hemisphere?
- All
- None
- Those coming from the nasal part of the retina
- Those coming from the temporal part of the retina
A
3
21
Q
Where do the retinal ganglionic axons cross?
A
Optic chiasm
22
Q
What structure of the eye provides most of the refraction?
A
Cornea
23
Q
The retina, a view through the pupil
A
24
Q
Is the retina considered part of the CNS or PNS?
A
CNS
25
What is the retina formed from?
The **diencephalon** (optic vesicle)
26
What are the 5 neuronal types in the retina?
1. Photoreceptors
2. Bipolar cells
3. Ganglion cells
4. Amacrine cells
5. Horizontal cells
27
Label the layers of the retina
28
Label these cells of the retina
29
On the electromagnetic spectrum, what do the photoreceptors detect?
Visible light which is 400-700 nm
30
Where are the photoreceptors found in the retina?
Adjacent to RPE (retina pigmented epithelium)
31
What do the outer discs of photoreceptors contain?
Where is this regenerated?
* Outer segment: discs containing pigment for light detection
* Photopigment is regenerated in RPE
32
Are the discs on photoreceptors replaced or do they stay there our entire life?
Discs are turned over (“shed”)
33
Tell me about light detection and the type of response it produced?
Light detected at outer segment leads to hyperpolarization =\> **graded responses** possible
Photoreceptors do not work with action potentials but with graded responses
34
In light and dark conditions
35
Tell me about **opsins role in phototransduction**?
* Opsins sit within the membrane of a disc
* Retinal is bound to opsins
* Opsins tune sensitivity to particular wavelength
* Light absorption leads to cis --\> trans retinal
* Light absorption =\> conformational change =\> activation of transducing =\> activation of **cGMP hydrolysis**
* AMPLIFICATION!!
36
How is the phototransduction signal terminated?
[Ca2+] decreases (channels closed)
* Guanylate cyclase activity increases (cGMP restored)
* Rhodopsin kinase gets activated, phosphorylates rhodopsin arrestin binding/ transducing displacement
* All-trans retinol dissociates
37
Light detection leads to?
1. Depolarisation of the photoreceptor
2. Hyperpolarisation of the photoreceptors
3. Closing of Na+/Ca2+ channels
4. Opening of the Na+/Ca2+ channels
5. Closing of K+ channels
6. Opening of K+ channels
2 and 3
38
What are the Na+/Ca2+ channels gated by (ensures their opening)?
cGMP
39
Tell me about rods and their sensitivity
Rods:
* low spatial resolution
* High sensitivity (1 photon can lead to a change to up to 1mv in membrane potential)
40
Tell me about cones and their sensitivity?
Cones:
* High spatial resolution
* Low sensitivity (\>100 photons)
* Less saturation
* Rapid recovery (deal with brighter light)
* Mediate colour vision
41
Label this diagram of a rod and cone
42
How many light intensities can we see over?
We can see over a range of ~12 logs of intensities!
43
What does loss of rod function lead to?
What does loss of cone function lead to?
Loss of rod function = night blindness
Loss of cone function = legally blind
44
What colours can rods and cones see?
* Trichromatic
* “blue” (short), “green” (medium) and “red” (long wavelength) cones
* Different cones are excited at different wavelengths
=\> each cone is monochromatic!
* Comparison of activity of different classes of cones to extract colour information
* M and L are on X-chromosome
* 3 different types of cones for different colours of light (short, medium and long)
* Only one rod in the green light wavelengths
45
Questions covered at end of lecture
* Myopia linked to dopamine levels during development
* Retina develops from diencephalon, therefore the most accessible part of the CNS, useful for disease diagnosis. As the brain and spinal cord are very well encapsulated
* Spatial resolution is directly related to how well the light focuses on the retina for initial light detection
46
The retinal ganglion axons terminate at the...
1. visual cortex
2. Latera geniculate nucleus
3. optic chiasm
4. suprachiasmic nucleus
5. retina
6. pretecal nucleus
2. lateral geniculate nucleus
47
The lens is adjutable to allow focussing over a range of distances. This is called what?
Accomodation
48
Which statements are correct?
1. light detection leads to hyperpolarisation
2. light detection leads to the opening of voltage-gated channels
3. light detection leads to the opening of cGMP- gated channels
4. the biggest signal amplification upon light detection occurs at the stage of transducin activation
5. the retinal pigmented epithelium regenerates 11-cis retina
1, 4 and 5
49
Which of the following describes cones...
1. provide low spatial resolution
2. are monochromatic
3. show low sensitivity/ low saturation
4. mediate night vision
2 and 3
50
Which three cells make up the simplest retinal circuit?
photoreceptors
bipolar cells
ganglionic cells
51
which statements are true for an on-center bipolar cell...
1. has AMPA/ Kainate receptors
2. depolarises in response to light detection at the cone
3. is sign-inverting
4. silences on-center ganglion cells in response to light
2 and 3
52
Visual acuity (Visual acuity refers to your ability to discern the shapes and details of the things you see)
53
rods, cones and photoreceptors
54
Is photoreceptor density constant across the retina?
55
Differences of rods and cones
56
Contrast and resolution
57
Contrast and resolution- sensitivity
58
What do ganglionic cells detect?
changes in luminance
59
What is the receptive field?
Region in which a stimulus elicits an action potential
60
retinal ganglionic cells
61
Are receptive fields overlapping?
yes
62
Are there equal numbers of on center and off center RGC?
yes
63
Tell me about on/off-center cells
64
RGC and on/off-center
65
Response of an on-center GC
66
edge detection
67
Tell me whats involved in light adaptation?
* Horizontal cells
* Glutamate depolarises --\> sign-conserving
* releasing GABA to hyperpolarise photoreceptors--\> inverting
68
Tell me about horizontal cells
* Connected via gap junctions
* Network over large area of retina
* Allows detection of background illumination
69
dark adaptation
70
Mach bands and Hermann grid
**Mach bands** is an optical illusion named after the physicist Ernst Mach. It exaggerates the contrast between edges of the slightly differing shades of gray, as soon as they contact one another, by triggering edge-detection in the human visual system.
**The Hermann grid** illusion is an optical illusion reported by Ludimar Hermann in 1870. The illusion is characterized by "ghostlike" grey blobs perceived at the intersections of a white (or light-colored) grid on a black background. The grey blobs disappear when looking directly at an intersection.
71
Similar princiople for colour opponent RGCs and Mach and Hermann
72
Rods...
1. mediate achromatic vision
2. are sensitive to scattered light
3. are found at the fovea
4. are much more abundant than cones
5. mediate high acuity vision
1, 2 and 4
73
Which statements are true for off-center ganglion cells?
1. depolarise in response to glutamate
2. activate when the center is brighter than the surroundings
3. show a graded response to light
4. connect to bipolar cells
5. connect to horizontal cells
1 and 4
74
Which cell(s) in the retinal circuit generate action potentials?
ganglionic cells
75
Which cells in the retinal circuit surveys a large area to detect backgroun illumination?
horizontal cells
76
The central visual pathway
77
What do ganglionic cells in the optic tract project into?
When projecting into this area(s) what does it help coordinate?
* **Pretectum:** feeds back to constrictor muscle in iris =\> pupillary light reflex
* **Suprachiasmatic nucleus (hypothalamus):** circadian (day – night) cycle
* **Superior colliculus:** coordination of movement in response to visual cues
* **Dorsal lateral geniculate nucleus:** relayed to visual cortex (“seeing”)
78
Optic nerve
79
Where does the optic nerve project into?
Tell me about this projection?
* Retina projection to dLGN (lateral geniculate nucleus)
* Lateral Geniculate nucleus comes in layers
* The retinal projection to the dLGN is a retinotopic point-to-point projection (no crossing)
80
What arrangement is maintained throughout the visual pathway?
Spatial arrangement of the retina
81
Retinotopic maps
* Visual field maps
* Disproportionate representation
82
Focal lesions lead to focal visual loss
The diagram shows what area (A-E) has an effect on vision
83
Neurons help to detect orientation and the following diagram shows the experiment that was carried out to look at this
84
Explain this diagram with neuronal detection and orientation
* Columns respond to one direction
* Adjacent columns respond to progressive shifts
* Deeper into columns they detect the same orientation
* But as move along the layer they shift slightly and progressively
85
What do oscular dominance columns show?
When one eye is dominant over the other
86
Tell me how input is combined from both eyes?
* dLGN: individual neurons are monocular
* Terminate in alternating ocular dominance columns in cortical layer IV
* Outside layer IV: binocular, but relative strength varies in columns
* stereopsis
87
Tell me about 'hyper column' in the visual cortex
* responds stronger to one eye or the other and have orientation
* as the diagram shows there are all different orientations by the neurons
88
Tell me about higher order visual processing
This is a simple diagram but shows how it branches into different processing sections
* Original input at photoreceptor is pixel-like
* Higher order processing allows robust visual recognition
* And selectivity
