Visual Defects Flashcards
1
Q
What is the ‘lens’? What structures is it suspended by? What muscles is it connected to?
A
- A transparent structure
- Suspended by ligaments (zonule fibres) attached to the ciliary muscles which control the shape of the lens.
2
Q
What is the vitreous humour? Function?
A
Viscous, jelly-like substance that lies between the lens and the retina. Keeps the eye spherical.
3
Q
Which part of the eye keeps it spherical?
A
Vitreous humour
4
Q
Which part of the eye is light transformed into neural activity?
A
Retina
5
Q
Which part of the eye is the point of highest visual acuity? Why?
A
Fovea - in the line of the visual axis where light can reach the photoreceptors directly
6
Q
OPHTHALMOSCOPIC VIEW OF RETINA
A
Macula: central vision, colour vision
Fovea: central/ thinner region of retina
Optic disc: origin of blood vessels, where the optic nerve axons exit the eye (blind spot)
7
Q
Which part of the eye is responsible for colour vision?
A
Macula
8
Q
How is light focused onto the retina?
A
Refraction by the cornea (and lens) - then passes through the vitreous humour to the retina
9
Q
What are the 2 layers of the retina?
A
An outer pigmented layer called the pigment epithelium, which adheres to the choroid, and an inner layer of nerve tissue called the sensory (or neural) retina.
10
Q
What are the cells in the pigment epithelium of the retina filled with? Purpose of this?
A
Cells in the pigment epithelium are filled with melanin which absorbs light which isn’t passed on to photoreceptors (stops damage).
11
Q
What composes the neural layer of the retina?
A
Photoreceptors, bipolar cells, ganglion cells etc
12
Q
What is the other function of the pigment of the retina?
A
Pigment also provides nutrients which are required for photoreceptors to work properly
13
Q
Light passes through all the other retinal cells to reach the photoreceptors at the back of the retina. Then describe the order which light passes between cells
A
- Photoreceptors: receive photons
-
Retinal biopolar cells: connect photoreceptors and ganglion cells
- Receive signals from photoreceptors and excite ganglion cells but releasing glutamate onto the dendrites of ganglion cells
- Ganglion cells: receive signal from bipolar cells, axons form the optic nerve
BUT 2 inhibitory neurons are involved:
- Horizontal cells: inhibitory neurons in the outer retina which help integrate and regulate the input from photoreceptors, also help adapt retina to different light levels
- Amacrine cells: inhibitory neurons in the inner retina which
- modulate transmission from bipolar cells to ganglion cells (form synapses onto the synaptic terminal of the bipolar cells as well as onto the dendrites of ganglion cells)
14
Q
What 2 interneurons (inhibitory neurons) are found in the retina?
A
- Horizontal cells
- Amacrine cells
15
Q
Which neurotransmitter does biopolar cells release?
A
Glutamate (excitatory)
16
Q
What is phototransduction?
A
Conversion of light energy into an electrical signal (how information is moved along neurons).
17
Q
Where does phototransduction occur?
A
Outer segments of photoreceptors are filled with lots of membranous discs – this is where phototransduction happens.
18
Q
Cones vs rods;
a) sensitivity
b) light level
c) number
d) photopigment
e) acuity
f) convergence
A
Rods:
a) Very sensitive to light, can pick up scattered rays
b) low light, nighttime
c) 20x more common
d) High photopigment so can capture more light
e) low acuity
f) High level of convergence
Cones:
a) Not sensitive to light, must have direct ray
b) Day time, bright light
c) uncommon
d) Low photopigment so captures less light
e) high acuity
f) Lower level of convergence: one per ganglion in macula
19
Q
Where are rods found? Where are cones found?
A
Rods: All over the retina except the fovea
Cones: Most dense at the fovea and macula
20
Q
In the dark, are photoreceptors depolarised or repolarised? Describe their glutamate release
A
In the dark photoreceptors are depolarised (to around -30 mV) and continuously release glutamate
21
Q
In the light, are photoreceptors depolarised or repolarised? Describe their glutamate release
A
Light causes these photorecptors to repolarise/hyperpolarise (as depolarising ion channels close) and decrease their glutamate release.
22
Q
What are membranous discs of photoreceptors packed full of?
A
Photopigments
23
Q
What are photopigments?
A
Each photopigments is a single transmembrane protein (G-protein coupled receptors) –> ‘opsin’ proteins
24
Q
What are the photopigments called in rods?
A
Rhodopsin
25
What is found inside each 'opsin' protein?
A central molecule of **retinal**
26
How is retinal synthesised?
Retinal is synthesised from vitamin A, can also be made from alpha and beta carotene (a form of vitamin A found in carrots)
27
What happens when photons hit the photoreceptors (i.e. what happens in phototransduction)?
1. Retinal hit with photon of light and activates the opsin molecule (1 photon is absorbed by 1 opsin protein)
2. 1 opsin then activates transducin molecules
3. Transducin molecules then activates phosphodiesterase enzymes
4. PDE enzymes convert cGMP to GMP
5. As cGMP levels fall, the cGMP sensitive ion channels close (these channels are the ones responsible for depolarisation in the dark)
6. Causes hyperpolarisation and decrease in glutamate release
28
How does the density of photoreceptors affect visual acuity?
Photoreceptors are the retina’s equivalent to pixels; denser photoreceptors = clearer quality
29
How are rods and cones distributed across the retina?
* Generally high density of rod cells in the periphery
* Very low density of cone cells in the periphery
* No rod cells at the fovea
* Many cone cells at the fovea
30
How is daytime vision at the periphery of the retina?
Daytime vision is bad at periphery of retina, good in the centre
31
Can the fovea be used in low light level? Why?
Fovea cannot be used in low light level, due to only cones being present.
32
In the dark, what are photoreceptors continuously releasing?
Glutamate (bipolar cells sense this glutamate via their dendrites)
33
There are 2 types of bipolar cells based on how they respond to light. What are they?
1. OFF bipolar cells
2. ON bipolar cells
34
How do ON bipolar cells respond to light?
depolarise --\> increasing AP firing
35
How do OFF bipolar cells respond to light?
hyperpolarise
36
What causes this difference in bipolar cells?
Types occur due to the differential expression of glutamate receptors they express
37
What glutamate receptors do OFF bipolar cells express?
express AMPA/Kainate receptors (ionotropic receptors)
38
In the dark, describe what happens to OFF bipolar cells
1. Dark = lots of glutamate released by photoreceptors
2. Glutamate binds to AMPA/Kainate receptors --\> causes receptors to open
3. Depolarising current flows into cell and bipolar cell depolarises
39
In the light, describe what happens to OFF bipolar cells
1. Light = no glutamate released by photoreceptors
2. Glutamate unbinds from AMPA/Kainate receptors and ion channels close
3. Bipolar cell hyperpolarises
40
What glutamate receptors do ON bipolar cells express?
express mGluR6 and TRPM1 receptors
41
In the dark, describe what happens to ON bipolar cells
1. Dark = lots of glutamate
2. mGluR6 receptors bind to glutamate
3. This **inhibits** ion channels in the membrane called TRPM1 channel (cation channel)
4. Causes **hyperpolarisation**
42
In the light, describe what happens to ON bipolar cells
1. Light = no glutamate
2. Glutamate can't bind to mGluR6 receptors which prevents inhibition of TRPM1 channels --\> channels open
3. Bipolar cells **depolarise** --\> increase AP firing
43
In the fovea, how many bipolar cells is each photoreceptor connected to?
2 - 1 OFF and 1 ON
44
In the fovea, cone cells are connected to two bipolar cells. In the light, what happens to each bipolar cell?
* ON bipolar cells depolarise. There will be increased glutamate release at the depolarised ON cell, leading to signals at the ON ganglion cell.
* OFF bipolar cell will hyperpolarise.
45
What is each ON and OFF bipolar cell attached to?
An ON and OFF ganglion cell
46
In the dark, what happens to each bipolar cell/ganglion cell?
* ON bipolar cells will hyperpolarise
* OFF bipolar cells will depolarise and release glutamate to the OFF ganglion cell
47
What are ionotropic receptors?
Membrane-bound receptor proteins that respond to ligand binding by opening an ion channel and allowing ions to flow into the cell, either increasing or decreasing the likelihood that an action potential will fire.
48
What is a receptive field?
the area of the retina that causes **any change in response** of a neuron
49
In the outer retina, what shapes the receptive fields of bipolar cells?
horizontal cells
50
In the inner retina, what shapes the receptive fields of ganglion cells?
amacrine cells
51
Ganglion cells have a receptive field centre. What determines this?
The lateral extent of the ganglion cell’s dendrites across the retina (which makes it sensitive to any visual stimuli across this region of photoreceptors) determines the receptive field centre
52
What is lateral inhibition?
* Lateral inhibition occurs from the bipolar cells and amacrine cells on either side of the central receptive field.
* The rods in the centre of the stimulus will transduce the "light" signal to the brain, whereas different rods on the outside of the stimulus will send a "dark" signal to the brain due to lateral inhibition from horizontal/amacrine cells.
* This **contrast** between the light and dark creates a **sharper image**.
53
What is;
a) receptive field centre
b) receptive field surround
a) Receptive field centre is due to direct connection to a **glutamatergic neuron**
b) Receptive field surround is due to ‘lateral inhibition’ from **inhibitory neurons** and is usually much larger than the dendritic field
54
What is the purpose of lateral inhibition and receptive field centre?
Allows comparison of light between centre and surround, so edges of objects can be identified. A shadow around an item will experience decreased firing due to lateral inhibition, bringing the object into clearer view.
55
Example of receptor field:
1. Shadow in ‘surround’ RF only - hyperpolarisation & decreased firing
2. As ‘edge’ moves over ‘centre’ get depolarization & increased firing
3. When shadow completely covers ‘centre’ & ‘surround’ then firing decreases again
4. Conclude that optimal stimulus is dark-light border across ‘centre’ & ‘surround’ RF i.e. edges
56
How is every colour in the rainbow obtained?
* Every colour in the rainbow can be obtained by mixing the proper ratio of **red**, **green** and **blue** light.
* Brain assigns colour based on the comparison of light falling on three cone types: blue, green, red.
57
When all 3 cone types are equally active, what colour do we perceive?
white
58
What are photoreceptor pigments called in;
a) rods
b) cones
a) rhodopsin
b) 3 types of opsins with different spectral sensitivity:
* Blue: 420nm
* Green: 530nm
* Red: 560nm
59
What is 'colour opponency'?
Colour is coded with an opponent process: two colours are compared with one colour **reducing** ganglion cell activity and the other **increasing** it.
60
What are the 2 opposing colour pairs?
* Red vs green
* Blue vs yellow
This produced the colour wheel, as we cannot see a ‘yellowy-blue colour’ but we can see ‘greeny-blue’ or ‘bluey-red’
61
Colour opponency explained:
If there was just red light on the centre of the ganglion’s receptive field, we would get maximal firing. If this red light extended over the surround as well, we are going to get some weak inhibition from the green on bipolar cells (from amacrine cells).
If there was green light in the surround and red light in the centre, this would inhibit the excitation in the centre quite strongly.
62
What is melanopsin? What cell is it expressed in?
* 5th photopigment in the eye
* Sensitive to blue light
* Expressed in intrinsically photosensitive **retino-ganglion cells** (ipRGC)
63
Where do ipRGCs communicate **blue light** information directly to?
Communicate information directly to the **suprachiasmatic nucleus (SCN)** in the hypothalamus (situated directly above the optic chiasm) --\> bypasses the thalamus
64
What is the SCN?
responsible for controlling circadian rhythm
65
How can blue light affect circadian rhythm?
Lots of blue light (e.g. phone screens) can disrupt circadian rhythm
66
What 2 aspects are ipRGCs involved in?
1. Blue light
2. Pupillary light reflex
67
What is the target of ipRGCs in the pupillary light reflex?
ipRGCs ganglion cells project directly to pre-tectal area (bypass thalamus) to the **olivary pretectal nucleus**
68
What occurs at the OPN?
Olivary pretectal nucleus: where the neurons of the ipRCGs converge and then connect to the Edinger-Westphal nuclei (parasympathetic preganglionic of the ciliary ganglia)
69
The lateral geniculate nucleus exhibits a layered structure. Describe this layered structure
There are two **magnocellular** layers, four **parvocellular** layers, and **koniocellular** layers between each of the magnocellular and parvocellular layers
70
Describe the parvocellular layers of the LGN. What ganglion cells do they receive input from?
* Small cell bodies
* Receive input from **P type Retinal Ganglion Cells** (RGC) smaller cell type making up 90% of the population
71
What are the parvocellular layers sensitive to?
Sensitive to stimulus from fine detail
72
Describe the magnocellular layers of the LGN. What ganglion cell do they receive input from?
* large cell bodies
* receive input from M type RGCs (large receptive field)
73
What are the magnocellular layers sensitive to?
Important for detection of stimulus **movement**
74
Describe the koniocellular layers. What ganglion cells do they receive input from?
* Very small cell bodies found in between each main layer
* receives input from K-type ganglion cells
75
Which retinal ganglion cell makes up the majority of the population?
P type RGC
76
how do neurons of the LGN project to the primary visual cortex?
via optic radiations
77
what is the primary visual cortex also known as?
V1 / Brodmann's area 17 / striate cortex
78
Where is V1 located?
Located in the occipital lobe, on either side of the calcarine fissure
79
What are 3 components of the primary visual cortex?
1. Orientation columns
2. Ocular dominance
3. Colour processing - blobs
80
What are orientation columns?
Organized regions of neurons located in the 1ary visual cortex that are excited by visual line stimuli of varying angles
81
What are ocular dominance columns?
Ocular dominance columns are stripes of neurons in the visual cortex that respond preferentially to input from one eye or the other.
* Inputs from the two eyes are still largely separate in V1
* Some inputs will be stronger from one eye over the other
82
What are 'blobs'?
Sections of the visual cortex where groups of neurons that are sensitive to **colour**
* Pillars through the cortex are enriched with **cytochrome oxidase** – a mitochondrial enzyme (cell metabolism)
* Staining with cytochrome oxidase reveals “pillars” running through layers II, III, V & VI
* Each pillar is centred on an ocular dominance column
83
Striate cortex, or V1, is the first region of visual processing in the cortex. There are dozens of other areas of the cortex involved in **extrastriate visual processing**.
There are thought to be two cortical streams of visual processing., What are they?
1. Striate cortex towards **parietal** lobe: *visual motion*
2. Striate cortex towards **temporal** lobe: *recognition of objects*
84
What would a lesion of the visual cortex lead to?
Lesion of visual cortex can lead to complete blindness of **perception**, but not of sight.
85
What is 'scotopic' vision? Which type of photoreceptor is involved in this?
* Vision of eye under low-light levels
* Rods
86
What is 'photopic' vision? Which type of photoreceptor is involved in this?
* Vision of the eye under well-lit conditions
* Cones
87
Which photoreceptor is involved in colour determination?
Cones
88
Which photoreceptor is involved in determining different shades of grey?
Rods
89
The outer segment of a rod is filled with lots of membranous discs which are packed full of what?
A pigment called **rhodopsin**
90
What is rhodopsin made up of?
2 things;
* Retinal
* Opsin
91
What is retinal a derivative of?
Vitamin A
92
What is opsin?
A protein
93
Cones also consists of a pigment. What is this pigment?
Photopsin
94
How many different types of photopsin are there?
3; photopsin I, II and III
95
Photopsin pigment (found in cones) can detect different wavelengths of light to determine which 3 main colours?
* 1) **Blue** part of visible spectrum (lowest wavelength)
* 2) **Red** (highest wavelength)
* 3) **Green**
96
What does photopsin consist of?
* Iodine
* Opsin
97
Which neurotransmitter do horizontal interneurons secrete?
GABA (inhibitory)
98
Function of horizontal interneurons?
Located between photoreceptors and bipolar cells - inhibitory effect to help modulate activity as we move between bright and dark light
99
Describe shape of bipolar cells
1 dendrite extension and 1 axon extension both arising from cell body
100
Which neurotransmitter do bipolar cells secrete?
Glutamate
101
Describe the phototransduction pathway **in the light** **in** **rods** **only** (N.B. is identical in cones)
1. Photon enters eye and hits photoreceptor
2. Hits **rhodopsin** molecule in membranous disc of rod
3. Causes **retinal** to become separated from **opsin** protein
4. **Opsin** goes off and activates **transducin** protein
5. **Transducin** activates **phosphodiesterase** **enzyme** (PDE)
6. **PDE** converts **cGMP** to **GMP** (cGMP is then no longer active so cannot bind to channels in rod membrane)
7. Channels in membrane **close** (Na+ and Ca2+ cannot move into photoreceptor)
8. Cell **hyperpolarises** (more negative)
9. Very few/no APs travel down axon of rod
10. Little/no **glutamate** is released
102
What does opsin activate in the phototransduction pathway?
Transducin protein
103
What does transducin activate in the phototransduction pathway?
PDE
104
Action of PDE in the phototransduction pathway?
Converts cGMP to GMP
105
What is the effect of cGMP being bound to channels on rods membrane?
When bound it opens up the channels and allows influx of Na and Ca (making inside of cell positive response).
106
In the dark, is cGMP bound to channels in rods membrane? What is effect of this?
cGMP is bound to channels in rod membrane; Na+ and Ca2+ entering cell (inside of cell more positive) and **depolarisation** occurs
107
In the light, how much glutamate is released?
Very little
108
What is the effect of very little glutamate being released in the light on **bipolar** and **ganglion cells?**
1. Very little glutamate released by photoreceptor **_stimulates_** bipolar neuron
2. Very few cations leave cell and cell becomes more positive
3. Causes **increase in glutamate** released by bipolar neuron onto ganglion cells
4. **Increase in glutamate** from bipolar cells causes increase in actions potentials moving down optic nerve
109
In the light, describe the glutamate levels released by;
a) photoreceptors
b) bipolar cells
a) very little
b) lots
110
How do horizontal interneurons inhibit bipolar cells?
Glutamate from photoreceptors is released onto bipolar cells but can also be released onto horizontal interneurons as well:
1. Release of **glutamate** onto horizontal cells **stimulates** horizontal cells
2. Horizontal cells then release **GABA**; they are inhibitory interneurons
3. GABA then **inhibits** photoreceptors
111
Purpose of inhibitory effect of horizontal interneurons?
In bright light, horizontal cells function to adapt bipolar cells in depending level of light (e.g. desensitise photoreceptors in bright light)
112
Purpose of inhibitory effect of amacrine cells?
Inhibitory effect on ganglion cells; modulates action potentials and visual pathway (makes sure it’s very precise)
113
Describe the phototransduction pathway in the **dark** in rods only (N.B. is identical in cones)
1. **Retinal** and **opsin** join back together
2. **Transducin** not activated
3. **PDE** not activated
4. **cGMP** not broken down and remains **bound** to channels in rod membrane (levels of cGMP increase)
5. Na+ and Ca2+ enter cell --\> **depolarisation**
6. More **glutamate** released
7. Lots of **glutamate** released by photoreceptor **inhibits** bipolar cell
8. Causes **decrease** in **glutamate** released by bipolar cell
9. Very little action potentials moving down ganglion cells (optic nerve)
114
There are 2 types of bipolar cells based on how they respond to light. What are they? How does each respond to light?
1. ON --\> **depolarise** in response to light, increasing AP firing
2. OFF --\> **hyperpolarise** in response to light, decreasing AP firing
115
Which glutamate receptors do ON bipolar cells express?
Express mGluR6/TRPM1 receptors
116
Which glutamate receptors do OFF bipolar cells express?
Express AMPA/Kainate glutamate receptors
117
In the dark, describe the glutamate levels released by;
a) bipolar cells
b) photoreceptors
a) little
b) lots
118
In the dark, lots of glutamate is released by photoreceptors. Describe the effect of this on ON vs OFF bipolar cells
* ON:
* mGluR6 receptors bind to glutamate
* This inhibits ion channels
* Causes **hyperpolarisation**
* OFF:****
* Glutamate binds to AMPA/Kainate receptors
* Ion channels **open**
* Bipolar cell depolarises
119
In the light, no glutamate is released by photoreceptors. Describe the effect of this on ON vs OFF bipolar cells
ON:
* Glutamate can’t bind to mGluR6 receptors
* Prevents inhibition of ion channels – they open
* Bipolar cell depolarises and increases AP firing
OFF:
* Glutamate unbinds from AMPA/Kainate receptors
* Ion channels **close**
* Bipolar cell hyperpolarises
