The visual system Flashcards
(103 cards)
1
Q
What is cranial nerve III?
A
Oculomotor nerve
2
Q
What does cranial nerve III control?
A
Movement of the extraocular muscles
3
Q
What is cranial nerve II?
A
Optic nerve
4
Q
What is the sclera?
A
Tough outer protective wall of the eyeball
5
Q
What does binocular vision allow?
A
Depth perception
6
Q
What is the fovea?
A
Thinnest part of the retina with the highest visual acuity because it contains many cone cells
7
Q
What is the optic disk?
A
- Blind spot
- Origin of blood vessels and optic nerve which block vision
8
Q
What is the macula?
A
- Region of the retina for central vision, not blocked by blood vessels to improve vision quality
- Has fovea in the centre
9
Q
What are zonal fibres?
A
- AKA suspensory ligaments
- Suspends the lens by attaching to ciliary muscles which enables stretching of the lens
10
Q
What is aqueous humour?
A
- The fluid between the cornea and the lens which provides nutrients to the cells of the cornea
- Clear to allow refraction
11
Q
Which fluid is located between the cornea and the lens?
A
Aqueous humor
12
Q
What is vitreous humour?
A
Fluid inside the eyeball which maintains the shape and outward pressure
13
Q
Which fluid fills the eyeball?
A
Vitreous humor
14
Q
How much refraction happens at the cornea?
A
80% (most of it)
15
Q
How much refraction happens at the lens?
A
20%
16
Q
What is the refractive index?
A
- Measure of the speed of light within a certain media
- Liquids have a higher refractive index than gases because liquids are more dense
17
Q
What does a large difference in refractive index between two media mean?
A
More refraction
18
Q
What is the degree of refraction determined by?
A
- Difference in refractive index between the two media
- The angle at which light hits the interface i.e. the cornea (perpendicular means no refraction)
19
Q
What is the focal distance?
A
Distance from the refractive surface (i.e. cornea) to convergence of parallel light rays (i.e. retina)
20
Q
Are light rays parallel when coming from distant objects?
A
Yes
21
Q
Are light rays parallel when coming from near objects?
A
No
22
Q
How does the lens accommodate to focus light from distant objects?
A
- The light rays are parallel so the cornea provides enough refraction do focus them on the retina
- Ciliary muscle relaxes, suspensory ligaments contract so lens is flattened
23
Q
How does the lens accommodate to focus light from near objects?
A
- Non-parallel light rays require more refraction than the cornea can provide so the lens fattens
- Ciliary muscle contracts, suspensory ligaments relax so lens is fat
24
Q
What is emmetropia?
A
Normal sight
25
What is hyperopia?
Far-sightedness
26
What does far-sighted mean?
Can see far away but not close up
27
What happens in hyperopia?
- The eyeball is too short so the point where the light rays converge would be behind the retina
- More refraction needed from a convex lens in glasses
28
Which lens is needed for glasses in hyperopia?
Convex
29
What is myopia?
Short-sightedness
30
What does short-sighted mean?
Can see close up but not far away
31
What happens in myopia?
- The eyeball is too long so light rays converge before they hit the retina
- Refraction decreased by a concave lens in glasses which makes the rays more divergent
32
Which lens is needed for glasses in myopia?
Concave
33
What are ganglion cells?
- The output from the retina
| - The only cells which produce action potentials, other cells produce graded potentials
34
What are bipolar cells?
Connect the photoreceptors to the ganglion cells
35
What are photoreceptors?
Rods and cones
36
What are amacrine cells?
Modify energy transfer between bipolar cells and ganglion cells
37
What are horizontal cells?
Modify information transfer between photoreceptors and bipolar cells
38
What is the pigmented epithelium?
Epithelial layer underneath the photoreceptors which nourishes them
39
What is the duplicity theory?
Can't have high sensitivity and high resolution in a single receptor
40
What do rod cells perceive?
Black and white
41
What do cone cells perceive?
Colour
42
What is the structure of rod cells? (3)
- Greater number of disks
- Higher concentration of photopigment
- 1000 times more sensitive to light than cones
43
What does scotopic mean?
Low light
44
Which photoreceptors enable vision in low light?
Rods
45
Which photoreceptors have low visual acuity?
Rods
46
What is visual acuity?
Ability to distinguish shapes
47
Which photoreceptors are more sensitive to light?
Rods
48
What is the structure of cone cells?
- Fewer disks
| - Lower light sensitivity
49
What does photopic mean?
Daylight conditions
50
Which photoreceptors have high visual acuity?
Cones
51
Which photoreceptors are more abundant?
Rods
52
What does mesopic mean?
Intermediate light conditions
53
Which photoreceptors are in the fovea?
Cones - no rod cells present in the fovea
54
Where are rod cells located in the retina?
Around the periphery of the fovea
55
What are the features of rod cells?
- High sensitivity
- Low resolution
- Low visual acuity
56
What are the features of cone cells?
- Low sensitivity
- High resolution
- High visual acuity
57
Why do rods have high sensitivity and low resolution?
- Many rod cells converge onto one ganglion cell
- Increases sensitivity because EPSPs summate which makes it easier to reach threshold for action potential in low light
- Convergence means the brain can't tell which rod cell is being stimulated so can't tell where the light stimulus is coming from = low resolution
58
Why do cones have low sensitivity and high resolution?
- Each cone cell has its own ganglion cell so a higher intensity light stimulus is needed to reach threshold for action potential so lower sensitivity
- One to one organisation means the brain can tell which cone cell has been stimulated so can tell exactly where the light is coming from = high resolution
59
Where in the retina are there no photoreceptors?
Optic disk - blind spot
60
Which photopigments do cones have?
Opsin photopigments (each cone contains 1 of 3 varieties)
61
Which photopigment do rods contain?
Rhodopsin
62
Where are the photopigments in the photoreceptors?
In the membranes of the membranous disks
63
Which photopigment do retinal ganglion cells contain?
Melanopsin
64
Which wavelength of light is rhodopsin (rods) most sensitive to?
500 nm
65
What are the 3 varieties of opsins?
- S = short wavelength
- M = medium wavelength
- L = long wavelength
66
Which wavelength of light are S cones most sensitive to?
420 nm
67
Which wavelength of light are M cones most sensitive to?
530 nm
68
Which wavelength of light are cones most sensitive to?
560 nm
69
Which colour do S cones detect?
Blue
70
Which colour do M cones detect?
Yellow
71
Which colour do L cones detect?
Red
72
What does melanopsin detect?
- Light/dark
| - Important in circadian rhythms
73
What is the resting membrane potential of a photoreceptor?
-30 mV
74
What is the effect of light stimuli on photoreceptors?
HYPERpolarisation
75
What is the dark current?
Influx of Na+ into photoreceptors through cGMP-gated cation channels in the dark, causing depolarisation
76
When are photoreceptors at their resting potential?
In the dark
77
What is happening in photoreceptors at rest?
- cGMP in the cytoplasm binds to cation channels in the cell membrane of photoreceptors which allows influx of Na+
- Causes depolarisation
- K+ channels are open at rest to allow K+ out to prevent too much depolarisation
- Na+/K+ ATPase maintains Na+ gradient
- Ca2+ regulates cGMP levels
78
What happens to photoreceptors in the light?
- cGMP concentration decreases so channels shut = no more Na+ influx
- K+ continues to leave via channels
- Causes hyperpolarisation (more negative)
79
How many photons evoke a sensation of light in humans?
5-7
80
What does each photopigment contain? (2)
- Opsin (varies with rhodopsin, S/M/L cones)
| - Retinal (same in all)
81
What is the opsin part of the photopigment equivalent to?
G protein coupled receptor
82
How does light cause a reduction in cGMP in photoreceptors?
- Light causes retinal to change conformation which causes change in opsin = transducin activated
- Alpha subunit of transducin activates PDE
- PDE cleaves cGMP into GMP
- Reduction in cGMP = channels can't open = no Na+ in = hyperpolarisation
83
Which G protein is the opsin coupled to?
Transducin
84
What is transducin?
G protein which is coupled to the opsin (receptor)
85
What is PDE?
Phosphodiesterase
86
What is the point of having such a complex mechanism for phototransduction?
Allows for signal amplification at every step
87
What is saturation?
Peak hyperpolarisation response to a light source due to all the Na+ channels being shut - limiting factor
88
Why are rods saturated more easily than cones?
Rhodopsin gets bleached in bright light
89
What is adaptation?
- Photoreceptors initially hyperpolarise in light
| - Gradually depolarise with continued bright light
90
Which enzyme makes cGMP?
Guanylyl cyclase
91
What is the role of Ca2+ in photoreceptors in the dark?
- Regulates cGMP levels
- Enters through cGMP-gated cation channels
- Ca2+ blocks guanylyl cyclase to reduce cGMP and prevent too much depolarisation
92
What is the role of Ca2+ in adaptation?
- In the light, cation channels shut = no Ca2+ influx
- Ca2+ can't block guanylyl cyclase so more cGMP is made
- Channels start to open again so cations come in = gradual depolarisation = adaptation
93
What are the 2 types of bipolar cells?
On and off
94
Which neurotransmitter do photoreceptors release?
Glutamate
95
When do photoreceptors release glutamate?
When depolarised i.e. in the dark
96
What happens to the levels of glutamate being released by photoreceptors when it is light?
Reduced (cells are hyperpolarised)
97
Are photoreceptors neurons?
Yes
98
How do off bipolar cells respond to light?
- Light causes photoreceptors to hyperpolarise so they release less glutamate
- Causes off bipolar cells to hyperpolarise because ionotropic glutamate receptors aren't being opened so less cations in
- Switched off by light
99
How do on bipolar cells respond to light?
- Light causes photoreceptors to hyperpolarise so they release less glutamate
- Causes on bipolar cells to depolarise because inhibitory metabotropic glutamate receptors are no longer being stimulated
- Switched on by light
100
What kind of glutamate receptors do off bipolar cells have?
Ionotropic
101
What kind of glutamate receptors do on bipolar cells have?
Metabotropic
102
What happens to on bipolar cells in the dark?
- Glutamate released by photoreceptors, binds to inhibitory metabotropic glutamate receptors
- Causes hyperpolarisation
103
What is the centre-surround organisation of bipolar cells' receptive fields?
Bipolar cells are directly connected to photoreceptors in the centre and indirectly connected to photoreceptors in the surround by horizontal cells
