The Eye Flashcards
1
Q
What are the first steps of vision?
A
- Transmission and refraction of light by the optics of the eye
- The transduction of light energy into electrical signals by photoreceptors
- The refinement of these signals by synaptic interactions within the neural circuits of the retina
2
Q
Visual processing pathway 3 step diagram
A
3
Q
Full visual processing pathway diagram
A
4
Q
What do photoreceptors in the retina contain?
A
Pigments that absorb light reflected by objects
5
Q
Wavelength defn
A
Distance between two peaks of the electromagnetic wave
6
Q
What does wavelength relate to?
A
Light colour: when white light diffracts through a prism it splits into the separated l (rainbow effect)
7
Q
Transparent media function
A
Will bend the light to focus it in the retina
8
Q
Reflection defn and diagram
A
9
Q
Refraction defn and diagram
A
Bending of light rays when they travel from one media to the other.
10
Q
Absorption defn and diagram
A
Transfer of light energy to a particle. Black objects absorb all the l of visual light, green objects absorb all except green, etc.
11
Q
Anatomy of the eye x 5
A
- Pupil
- Iris
- Cornea
- Extraocular muscles
- Optic nerve
12
Q
What is the pupil?
A
Opening that allows the entry of light
13
Q
What is the iris?
A
Pigmentation that provides eye colour.
14
Q
What other than pigmentation does the iris contain and function?
A
Contain two muscles to regulate the size of the pupil
15
Q
What is the cornea?
A
Transparent surface covering the front eye. It continues in the sclera, the white of the eye
16
Q
Where does the extraocular muscle insert and function?
A
Inserts in the sclera, to control eye movement
17
Q
Where does the optic nerve exit?
A
At the back of the eye
18
Q
Anatomy of the eye diagram
A
19
Q
Optics
A
Cornea and lens will diffract light to focus it in the retina. The lens will accommodate to different distances.
20
Q
Far point optics diagram with focal distance draw
A
21
Q
Near point optics diagram draw
A
22
Q
Myopia diagram
A
Nearsightedness
23
Q
Hyperopia diagram
A
Farsightedness
24
Q
What is the retina a part of?
A
CNS
25
Most direct pathway to the brain
Photoreceptors ➡ Bipolar cells ➡ Ganglion cells
26
What are the only light-sensitive cells in the retina?
Photoreceptors
27
What are the only output cells in retina?
Ganglion cell axons (optic nerve) to higher CNS centres
28
What are horizontal and amacrine cells?
Local interneurons and modulate transmission onto bipolar and ganglion cells
29
Retina pathway to brain and cells diagram
30
Retina laminar organisation and layers
31
What do photoreceptors do?
Transform electromagnetic radiation into electrical signals
32
Rod and cone diagram
33
What does the outer segment of the retina contain?
Membrane disks, with light-sensitive photopigments that absorb light and will trigger changes in membrane potential
34
Rods make up
One photopigmentblack / white vision, lot of membrane disks (more sensitive to light)
35
Cones make up
3 photopigments-colour vision
36
Rods function
Night vision
37
Cones function
Daylight vision
38
What is the fovea enriched in?
Cones
39
What is the periphery enriched in?
Rods
40
Where are cones tightly packed?
In the fovea
41
Where are cones widely spaced?
In the periphery
42
Size of receptor fields in periphery and what does this mean?
Larger - lower resolution
43
Many photoreceptors feeding (convergence) into individual ganglion cells in the periphery, what does this mean?
More rods and greater input in the periphery (higher sensitivity)
44
Lateral displacement of the layers above the photoreceptor at the fovea →
Light hits directly without scattering
45
Describe fovea input
More cones and no convergence of input in the fovea: lower sensitivity
No convergence of input and direct light input in the fovea: higher resolution
(no convergence = one photoreceptor → one ganglion cell)
46
Rods light sensitivity
Low spatial resolution but very sensitive to light; saturated in very bright light → night vision
47
Cones light sensitivity
High spatial resolution but relatively insensitive to light; not operating in dim light → day vision
48
What do rods contain?
Contain photopigment rhodopsin
49
What are rods sensitive to
All visible wavelengths
50
Rods sensitivity to light
High sensitivity
51
When are rods used
Night vision
52
Where are rods located?
All over the retina except fovea
53
Total number of rods
100 million
54
What is each cone sensitive to?
One type of light
55
Do cones or rods require more energy to be activated?
Photopigments of cones require more energy to be activated
56
What do cones contain?
Cone opsins (3 types, with different absorption for red, green or blue light)
57
Sensitivity of cones to light
Low sensitivity to light
58
Function of cones
Daylight sight
59
Location of cones
Located mainly in the macula, and especially the central area of the macula, the fovea
60
How many cones in total?
6 million total
61
What is phototransduction?
Conversion of light energy into membrane potential changes
62
Rods x 6 points
- Contain photopigment rhodopsin
- Sensitive to all visible wavelengths
- High sensitivity to light
- Night vision
- Located over all retina (except fovea)
- 100 million in total
63
Cones x 6 points
- Contain photopigments cone opsins (3 types, with different absorption for red, green or blue light)
- Low sensitivity to light
- Daylight sight
- Located mainly in the macula, and especially the central area of the macula, the fovea
- 6 million in total
64
Describe phototransduction membrane potentials
Graded changes in membrane potentials (the only neurons firing action potentials are ganglion cells) → change in rate of transmitter release
65
Phototransduction in dark
Membrane depolarised, Ca2+ channels open, rate of transmitter release is high
66
Phototransduction in light
Hyperpolarisation, Ca2+ channels close, decrease in rate of transmitter release
67
Stimulus of classic GPCR
Chemical
68
Receptor activation of classic GPCR
G protein binds GTP
69
Enzyme of classic GPCR
Activation
70
Second messenger of classic GPCR
Increase in second messenger levels
71
Ion channel of classic GPCR
Increase / decrease conductance
72
Stimulus of rhodopsin
Light
73
Receptor activation of rhodopsin
G protein binds GTP
74
Enzyme of rhodopsin
Activation
75
Second messenger of rhodopsin
Decrease in second messenger levels
76
Ion channel of rhodopsin
Decrease Na+ conductance
77
Classic GPCR vs Rhodopsin diagram
78
What happens to photoreceptors in darkness and why?
Depolarised (-30mV) due to a “dark current” of Na+
79
Phototransduction of rods in darkness process
80
What happens to rods with light?
Photoreceptors will hyperpolarize with light
81
Rhodopsin =
Opsin + Retinal
82
Rods absorption of light process
83
Rods hyperpolarisation with light process
84
Phototransduction- signal amplification
85
What does light do to photoreceptors?
Hyperpolarises
86
What does darkness do to photoreceptors?
Depolarises
87
What do responses of in the bipolar and ganglion cells varying depend on?
On the receptors they express and lateral modifications (horizontal and amacrine cells)
88
What can bipolar cells be depolarised or hyperpolarised by?
Glutamate
89
Hyperpolarisation expression
AMPA/Kainate R expression
90
Depolarisation expression
mGluR6 expression
91
Depolarisation = mGluR6 expression pathway
Bound to glutamate (during dark), mGluR6 causes closure of Na+ channels. Hyperpolarisation of photoreceptor by light reduces glutamate release, reduces mGluR6 binding and allows Na+ channels to open on the bipolar cell.
92
Dark and light responses diagram
93
What is the bipolar receptive field?
An area of retina where a stimulus will evoke a response in that bipolar cell
94
Receptive field centre =
Direct connection from photoreceptors
95
Receptive field surround =
Connection from photoreceptors through horizontal cells
96
Response triggered in centre =
Opposite will be triggered by the surround
97
How are bipolar cells classified?
According to their responses to light
98
Types of bipolar cells
* On-centre bipolar cells
| * Off-centre bipolar cells
99
ON-centre Bipolar cells
Depolarized by light, expressing mGlur6
100
OFF-centre Bipolar cells
Hyperpolarized by light (as Photoreceptors), expressing AMPA / Kainate
101
What is the area of retina where a stimulus will evoke a response in that ganglion cell called?
The receptive field of a ganglion cell
102
What does each ganglion cell
A ‘centre’ and ‘surround’ to it’s receptive field
103
Effect of centre vs surround
Light in the centre will have the opposite effect to light in the surround
104
What do ganglion cells generate?
AP in the retina
105
What are the only cells that produce APs in the retina?
Ganglion cells
106
When do on-centre ganglions produce APs?
When light is shone on the photoreceptor that directly innervates it.
107
What happens to off centre ganglions when light is shone on photoreceptor that innervates it?
Decreases APs
108
When does the photoreceptor get hyperpolarised?
ALWAYS by light
109
What can happen to AP firing in ganglion cell when hyperpolarised?
May increase or decrease
110
What decreases APs when light is turned off?
On-centre ganglion cells
111
What increases APs when light is turned off?
Off-centre ganglion
112
When is there no change in firing rate?
When both centre and surround are in same level of illumination: A, C and E
113
When is there the greatest difference in firing?
When you have most contrast between the centre and surround: B and E
114
What is the antagonistic centre/surround effect mediated by?
Horizontal cells
115
What do horizontal cells regulate?
Amount of transmitter released by photoreceptor onto bipolar cell
116
Light at centre causes depolarisation of bipolar cell: process
- Light hyperpolarises centre cone
- Decreased release of glutamate from centre cone
- Depolarisation of bipolar cell (mGluR6)
117
What does addition of light to surround cause?
Reduces firing rate of on-centre
118
Horizontal cell hyperpolarisation
Surround cone releases less glutamate onto horizontal cell
119
Addition of light to surround process
* Strong hyperpolarisation of horizontal cells
* Reduced release of GABA from horizontal cell onto centre cone
* Reduces hyperpolarisation of centre cone
120
Reduced hyperpolarisation of centre cone ➡
Increased release of glutamate from centre cone ➡
Hyperpolarization of bipolar cell ➡
Hyperpolarization and reduced firing of GC
121
Opsin ➡
Transducin → PDE → CNG Channel → Glu
122
Light energy conversion
Converted to membrane potential changes in photoreceptor cells, through Opsin → Transducin → PDE → CNG Channel → GluC
123
Membrane potentials in bipolar pathway
Glutamate information will be converted into membrane potential changes in bipolar (centre ON/OFF) and horizontal (surround ON/OFF) cells
