Vision Flashcards
(76 cards)
1
Q
What is the structure and function of the cornea?
A
- collagen rich connective tissue, blood vessels & nerves
- modified to admit light
2
Q
What is the structure and function of the ciliary muscle?
A
- muscular ring around the lens
- changes pupil diameter & secretes aqueous fluid
3
Q
What is the structure and function of the iris?
A
- pigmented tissue with 2 muscular layers
- adjusts pupillary diameter
4
Q
What is the structure and function of the aqueous humour?
A
- fluid reabsorbed by canal of schlemm
- maintains the shape of the eyeball
5
Q
What is the structure and function of the lens?
A
- transparent tissue rich in elastic fibres
- focusing light
6
Q
What is the structure and function of the vitreous humour?
A
- transparent jelly like material
- maintains the shape of the eyeball
7
Q
What is the structure and function of the retina?
A
- thin transparent layer possessing photoreceptors
- detects light entering the eye
8
Q
What is the structure and function of the fovea centralis?
A
- small area of the retina
- focused image is recreated here
9
Q
What is the structure and function of the optic nerve?
A
- point which this nerve leaves the eye = optic disc
- relays signals from the retina to the CNS
10
Q
What is important to know about the choroid layer?
A
- supplies nutritional needs of the photoreceptors and contains melanin
- which in conjunction with the pigment layer of the retina, helps absorb stray light from being scattered back to the photoreceptor layer
- where it would blur the visual image
11
Q
Why is vision possible?
A
- the eye is capable of detecting light rays which have been emitted / reflected from objects
- when light rays enter the eye they recreate the image on the retina
- with the focused part of the image recreated at the fovea
- to do this, light rays have to be bent so that they converge at the fovea
12
Q
What happens if light strikes an even surface at 90 degrees? = perpendicular
A
- it will travel straight
- no refraction will occur
13
Q
What happens if light strikes at any other angle or if the surface isn’t even?
A
- refraction occurs
- as part of the light ray slows before the rest
14
Q
Where does the greatest amount of refraction happen at in the eye?
A
- the cornea
- as the air and the cornea have the greatest difference in densities
15
Q
Where else does refraction also occur?
A
- between the cornea and the aqueous humour
- the aqueous humour and the lens
- the lens and the vitreous humour
16
Q
Where does the fine tuning of refraction occur?
A
- the lens
- can alter its shape & refractive properties
- allowing us to focus on objects closer to the eyes
17
Q
What is accommodation?
A
- this process by which the lens changes shape
18
Q
How much the light rays need to be refracted in order to be focused, depends on what?
A
- how far the object is away from the eye
19
Q
What happens when emitted or reflected by an object?
A
- light is scattered in different directions
- light rays will order themselves in a parallel fashion when given distance over which to do this
20
Q
The closer the object is…the more
A
- scattered light rays are
- more light needs to be refracted
21
Q
What does the eye do to increase its refractive power?
A
- the lens acquires a thicker shape
- the closer the object is, the thicker the lens
- with distant objects, lens = flat
22
Q
What happens when the image is recreated on the retina?
A
- the image is upside down and reversed left to right
- signals arriving at the visual cortex from the retina are interpreted so the image can be perceived correctly
23
Q
When viewing a distant object, what happens?
A
- ciliary muscles are relaxed
- suspensory ligaments are taut
- lens is thin
24
Q
When viewing an object that is up close, what happens?
A
- ciliary muscle fibres are contracted
- suspensory ligaments are relaxed
- lens is thick
25
What is hyperopia?
- far / long sightedness
26
Why does hyperopia happen?
- an eyeball that it too short
- a lens system that is too weak
27
What happens in hyperopia?
- the light is not refracted enough by the lens system, to come into focus by the time they reach the retina
- people with this condition have difficulty viewing objects close to them
28
How is hyperopia corrected?
- convex lens
29
What is myopia?
- near / short sightedness
30
Why does myopia happen?
- when the ciliary muscles are completely relaxed, the light rays coming from distant objects are focused infront of the retina
- due to an eyeball that is too long
- can also result from too much refractive power of the lens system
- difficulty viewing distant objects
31
How is myopia corrected?
- concave lens
32
Distant object, the light rays are…
PARALLEL
33
Far away object..light rays arrive:
- DIVERGING
34
Other factors that have a role to play in maximizing the focus power of the eye when viewing closer objects:
- constriction of the pupils
- convergence of the eyeballs
35
What are these changes known as?
The NEAR response
36
What does pupil constriction do?
- occurs to limit edge effects
- which is blurring of the image recreated at the fovea around the edges
37
Where is the lens refractive power the weakest?
- at the edges where it is thinner
- so if scattered light rays strike these parts of the lens
- they may not be refracted enough to strike the fovea
- resulting in a blurred image
38
What do the pretectal nuclei in the midbrain control?
- pupillary diameter
39
What do sympathetic nerves innervating the radial smooth muscles of the iris cause?
- cause the pupil to dilate when viewing distant objects
40
What do parasympathetic nerves innervating the circular smooth muscles of the iris do?
- cause the pupils to contract, constricting the pupil
41
Why does convergence of the eyes occur?
- the brain is producing a single image from information gathered by two eyes
- it is important that the image falls on the same part of each retina, to help with integration
- the positioning of the fovea on the retina means they turn inwards when observing a close object
- made possible by the extra cellular muscles of the eye
42
What does the area superior colliculi of the brain do?
- control eye movements
43
What is the pupillary light reflex?
- when light is directed into one eye the pupil constricts
44
What is the consensual light reflex?
- the pupil of the other eye also constricts at the same time
45
Distant object response to light:
- sympathetic motor nerve fibre
- radial muscles contract
- circular muscles relax
46
Close object response to light:
- radial muscles relax
- circular muscles contract
- parasympathetic ganglion
47
What is it possible to happen?
- to lose the light responses but not the near response - and in reverse
48
What is the retina?
- an extension of the brain and the only part which can be viewed without dissection
- light sensitive portion of the eye that contains photoreceptors
49
What do the rods do?
- can detect dim light
- responsible for black and white vision and vision in the dark
50
What do the cones do?
- responsible for colour vision
51
What is the optic disc?
- where the nerve fibres from all over the retina converge & leave the back of the eye as the optic nerve
52
What is the blind spot?
- there are no photoreceptors at this point
53
What happens when rods or cones are excited by light?
- energy is converted into action potentials
- which are transmitted through successive layers of neurons in the retina
- finally into the optic nerve fibres
- onto the cerebral cortex
54
Both photoreceptors have:
- an outer segment
- an inner segment
- nucleus
- synaptic body
55
Where is the light sensitive photochemical found?
- the outer segment
- with large numbers of discs
56
What does the inner segment contain?
- cytoplasm
- with cytoplasmic organelles
- specifically mitochondria
57
What does the synaptic body have?
- portion of the photoreceptor that connects with subsequent neuronal cells
= horizontal and bipolar cells
58
What are rods like?
- narrower
- longer
- contain the light sensitive pigment rhodopsin
- the pigment is the combination of the protein opsin and the vitamin A derivative, retinal
59
What happens when light energy is absorbed by rhodopsin?
- the rhodopsin begins to decompose into opsin and retinal only regenerates in dim light conditions
60
What are cones responsible for?
- detecting colour and detail
61
Why is the focused image recreated on the fovea of the retina?
- their ability to detect detail
- the fovea has the highest concentration of cone cells
62
What are the three different types of cone cells?
- red
- green
- blue
- they contain pigments sensitive to the corresponding wavelength of light
63
What happens when the pigments respond to light?
- this changes the membrane potential of the cone
- eliciting an action potential
64
What is dark adaptation?
- the light sensitivity of the rods and cones increases as the intensity of light decreases
65
What does sensitivity depend on?
- the concentration of pigment in the receptors
- which is reduced in bright light due to bleaching
66
What happens in the dark referring to bleaching?
- bleaching is greatly reduced and pigment concentration rises
- adaptation of rods can increase their sensitivity in 40 minutes
- the reversal of this process = light adaptation
67
What do rods demonstrate x100 greater than cones?
- sensitivity to light when they are both fully dark adapted
- hence their importance in night vision
68
What is the function of the pigmented epithelium?
- absorbs light to prevent it from being reflected back into the eye
69
What is the function of bipolar cells?
- allows the ganglion and photoreceptor cells to synapse & communicate
70
What is the function of the ganglion cells at the front?
- relay signals to the optic nerve, so that they might reach the visual areas of the brain
- not only the area for image formation = visual cortex
- but also the midbrain to help control the pupillary diameter
71
How does stereoscopic vision come about?
- the visual fields of the two eyes overlap
- the slightly different images from both eyes are combined in the brain
72
Why is stereoscopic vision important?
- depth perception
- 3D vision
73
How does depth perception happen?
- judging the position of other points relative to a fixation point
74
What is the fixation point?
- the fovea
- when objects are further away from it, the objects will be seen on the nasal slide of the fovea
- when objects are closer, they will be seen on the temporal side of your fovea
75
What is the central visual pathway?
- the visual information from each eye passes backwards into the optic nerve
- until it reaches the optic chiasma
- they cross over, so that half the nerve signals from the right eye cross to the left & vice versa
- this helps integrate the signals from each eye into a single image
76
Central visual pathway, part II:
- the right brain controls motor responses on the left side of the body & vice versa
- each side of the brain can see what is on the side of the body it controls
