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Y2 CONTROL & MOVEMENT > Visual Defects > Flashcards

Visual Defects Flashcards

1
Q

What is the lens?

A

a transparent structure suspended by ligaments (zonule fibres) attached to the ciliary muscles which control the shape of the lens

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2
Q

What is the vitreous humour?

A

a viscous jellylike substance that lies between the lens and the retina

it keeps the eye spherical

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3
Q

what is the retina?

A

the retina is part of the CNS where light is transformed into neural activity

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4
Q

What is the fovea?

A

the point of highest visual acuity in the retina, where light can reach the photoreceptors directly

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5
Q

Label the components of the eye in cross section

A
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6
Q

What is refraction and when does it occur?

A

refraction is the bending of light rays

refraction occurs when light passes from one transparent medium (e.g. air) to another (e.g. cornea)

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7
Q

What happens during refraction?

A

refraction occurs when light is slowed down as it changes from one media to another

light bends towards a line that is perpendicular to the border between the media

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8
Q

How does refraction occur by the cornea?

A

as light passes through the cornea:

  • light rays that strike the curved surface of the cornea bend so that they converge on the back of the eye
  • light rays that enter the centre of the eye pass straight to the retina
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9
Q

Other than the cornea, what other structure is involved in refraction?

A

the cornea performs a large amount of the refraction that is required

the lens also refracts light rays passing through it to produce a sharp image

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10
Q

What is the problem associated with refraction by the lens and how is this overcome?

A

it is okay if the image is a long distance from the eye as the light rays striking the cornea will be parallel

closer images require greater refractive power to bring them into focus

this focusing is brought about by the lens changing shape (accomodation)

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11
Q

What structures are involved in accomodation?

A

the lens changes shape by the contraction of the ciliary muscles

accomodation is due to contraction of the ciliary muscles, which relieves the tension on the zonule fibres

this allows the lens to become rounder due to its natural elasticity

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12
Q

what is meant by emmetropia?

A

emmetropia refers to an eye that has no visual defects

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13
Q

How does an emmetropic eye focus parallel light?

A

an emmetropic eye focuses parallel light rays on the retina without the need for accomodation

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14
Q

What is meant by hyperopia?

A

farsightedness

this is a vision condition in which you can see distant objects clearly, but objects near by may be blurry

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15
Q

Why does hyperopia occur?

A

when the eyeball is too short from front to back, light rays are focused at some point behind the retina

as a result, the retina sees a blurry circle

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16
Q

How is hyperopia corrected with lenses?

What is the problem with accomodation?

A

accomodation of the lens is needed for distant objects and near objects cannot be brought into focus

a convex lens placed in from of the eye provides the necessary refraction to allow near objects to be brought into focus

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17
Q

What is myopia?

A

nearsightedness

this is a vision condition in which you can see nearby objects clearly, but objects in the distance are blurry

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18
Q

Why does myopia occur?

A

when the eyeball is too long from front to back, parallel light rays converge at some point before the retina

as a result, the retina sees an unfocused blurry circle

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19
Q

How is myopia corrected with lenses?

A

a concave lens placed in front of the eye will provide the necessary refraction to allow distant objects to be brought into focus on the retinal surface

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20
Q

What is photorefractive keratectomy and how does it work?

A

corrective laser eye surgery

it uses a laser to reshape the cornea and increase or decrease the amount of refraction possible

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21
Q

Label the opthalmoscopic view of the retina

A

macula - ‘central’ vision

fovea - central / thinner region of the retina

optic disc - origin of blood vessels and where the optic nerve axons exit the eye (blind spot)

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22
Q

What structures must light pass through to reach the retina?

A

light is focused by the cornea and the lens

it then passes through the vitreous humour to the retina

it passes through all the retinal cells to reach the photoreceptors at the back of the retina

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23
Q

where is the pigment epithelium positioned relative to the retina?

what is its function?

A

the retina lies in front of the pigment epithelium that lines the back of the eye

cells in the pigment epithelium are filled with the black pigment melanin

this absorbs any light that is not absorbed by the retina

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24
Q

What types of cells are present in the retina?

A
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25
what are the 2 different types of photoreceptors?
rods and cones
26
What are the characteristics of rods? What is their sensitivity to light and amplification like?
* high sensitivity to light - specialised for night vision * more photopigment to capture more light * high amplification * low temporal resolution - slower response * more sensitive to scattered light
27
What are the characterisitics of cones? What is their sensitivity and amplification like?
* lower sensitivity - specialised for day vision * less photopigment * lower amplification * high temporal resolution - fast response * more sensitive to direct light rays
28
What is the acuity like of the rod system? What colours can it detect?
* **low acuity** - rods are not present in the fovea and the pathway is highly convergent * **achromatic** as there is only one type of rod pigment
29
What is the acuity of the cone system like? What colours can it detect?
* **high acuity** - cones are concentrated in the fovea and the pathway is dispersed * **trichromatic** - there are three types of cone pigment, each with a different wavelength preference
30
What is meant by phototransduction?
the conversion of light into a change in the electrical potential across the cell membrane
31
What is the state of photoreceptors in the dark? What happens when there is light?
in the dark, photoreceptors are **depolarised** and continuously release **glutamate** light causes depolarising ion channels to close this **hyperpolarises** the membrane potential and **reduces glutamate release**
32
What is rhodopsin and where is it found?
a biological pigment found in the rods of the retina that is a G-protein coupled receptor it is extremely sensitive to light and enables vision in low-light conditions it immediately photobleaches when exposed to light
33
What is retinal? Where does it come from?
it is a form of vitamin A that is bound to proteins called opsins it allows conversion of light into metabolic energy it can be made from alpha or beta carotene
34
What is meant by a photopigment?
opsin-type photoreceptor proteins (specifically rhodopsin and photopsins) in the retinal rods and cones they are responsible for visual perception
35
what are the stages involved in phototransduction?
1. 1 photon is absorbed by 1 opsin 2. 800 transducin molecules 3. 800 PDE enzymes 4. 4800 cGMP converted to GMP 5. 200 cGMP sensitive ion channels close 6. this causes hyperpolarisation and a decrease in glutamate release
36
What is a determinant of resolution and how does this relate to the retina?
density of "pixels" is a determinant of resolution photoreceptors are the retina's equivalent to pixels
37
what are the 2 different types of retinal bipolar cells?
there are two types of retinal bipolar cells depending on how they respond to light **ON bipolar cells:** * these ***depolarise*** in response to light onset **OFF bipolar cells:** * these ***hyperpolarise*** in response to light onset
38
What receptors are expressed by ON bipolar cells? How is hyperpolarisation occur?
ON bipolar cells express **mGluR6** and **TRPM1** receptors when glutamate binds to mGluR6, Gao is activated this inhibits TRPM1 and causes hyperpolarisation
39
40
What receptors are expressed by OFF bipolar cells? How does depolarisation occur?
OFF bipolar cells express **AMPA / Kainate receptors** when glutamate binds, the receptor opens and depolarises the cell
41
What is a receptive field? How does this relate to the retina?
the receptive field of a neurone is the place on a sensory surface that a stimulus must reach to activate that neurone it is the area of the retina that causes any change in response of a neurone
42
What cells in the retina influence the receptive fields?
horizontal cells shape the receptive fields of bipolar cells amacrine cells shape the receptive fields of ganglion cells
43
What is the organisation of retinal - ganglion cell receptive fields like? What does this allow for?
they have a **centre-surround receptive field organisation** this allows ganglion cells to transmit information about whether photoreceptor cells are exposed to light as well as the differences in firing rates of the cells in the centre and surroundings, allowing them to transmit information about contrast
44
What is the purpose of lateral inhibition by amacrine cells?
it is the process in which photoreceptor cells aid the brain in perceiving contrast within an image it increases the contrast and sharpness
45
what is the receptive field centre of the retinal ganglion cell receptive fields due to?
the receptive field centre is due to direct connection to a **glutamatergic neurone** it corresponds to the cell dendritic field
46
What is the receptive field surround of a retinal ganglion cell due to? What is the purpose of this set up?
the receptive field surround is due to **"lateral inhibition"** from inhibitory neurones it is usually much larger than the dendritic field it allows for comparison of light between centre and surround
47
What happens to the retinal ganglion cells receptive fields when there is a shadow in the surround receptive field only? What happens as this shadow moves across?
**shadow in surround RF only:** * hyperpolarisation and decreased firing **as edge moves over 'centre':** * depolarisation and increased firing **shadow completely covers 'centre' & 'surround':** * firing decreases again optimal stimulus is a dark-light border across the 'centre' and 'surround' receptive fields
48
What is the Young - Helmholtz trichromatic theory?
every colour in the rainbow can be obtained by mixing the proper ratio of red, green and blue light
49
How does the Young-Helmholtz trichromatic theory apply to the retina?
at each point in the retina there exists a cluster of **three receptor types** each type is maximally sensitive to either **blue, green or red** the brain assigns colours based on a comparison of the three cone types
50
What colour do we perceive when all three cone types are active?
when all three cone types are equally active we perceive "white"
51
How has the Young-Helmholtz trichromatic theory been proven in blind people?
genes for cone pigments have been identified these genes are missing in some colour-blind patients
52
What is the pigment that is present in all rods?
**rhodopsin** (receptor protein - **opsin**) this is the pigment that is present in ALL rods
53
What are the different types of pigments in cones? How are they different?
in each cone there is one of three types of **opsin**s each cone opsin has a different spectral sensitivity: 1. **blue cones** maximally activated by light of **420 nm** 2. **green cones** maximally activated by light of **530 nm** 3. **red cones** maximally activated by light of **560 nm**
54
What is meant by **colour opponency**?
cone photoreceptors are linked together to form three opposing colour pairs activation of one member of the pair inhibits activity in the other (reduces ganglion cell activity)
55
What are the two main colour opponent pathways? What does this explain?
1. red / green 2. blue / yellow this explains the colout wheel
56
What are the structures involved in the visual pathway?
retina lateral geniculate nucleus in the thalamus primary visual cortex
57
What are luminance encoders? What is the main pigment involved and where is it found?
intrinsically photosensitive retinal ganglion cells involves the photopigment **melanopsin** that is expressed in **ipRGCs** with all synaptic transmission blocked they are still responsive to light
58
What are ipRGCs required for?
ipRGCs are required for normal photo entrainment of the circadian clock they synchronise the body's daily rhythms to the rising and setting of the sun
59
What reflex are the ipRGCs involved with?
they are required for the pupillary light reflex
60
What are the 3 layers of the lateral geniculate nucleus? What type of cell bodies are found in each layer?
**parvocellular layers 3 - 6:** * small cell bodies **magnocellular layers 1 - 2:** * large cell bodies **koniocellular layers:** * very small cell bodies
61
What are the 3 types of retinal ganglion cells? What are their sizes and functions?
**Magnocellular (M-type):** * larger cell type that are 5% of the population * large receptive field * important for detection of stimulus movement **Parvocellular (P-type):** * smaller cell type that are 90% of the population * sensitive to stimulus form and fine detail **Non-M non-P type (K-type):** * medium cell type that make up 5% of the population
62
What types of retinal ganglion cells correspond to which layers of the LGN?
63
Where to the neurones of the LGN project to?
neurones of the LGN project to the primary visual cortex via the optic radiation
64
What is the main target of the LGN?
the main target is the **primary visual cortex** (V1) this is also known as Brodmann's area 17 or the striate cortex located in the occipital lobe
65
What are the 3 components involved in functional organisation of the primary visual cortex?
* orientation columns * ocular dominance columns * colour processing "blobs"
66
What is meant by orientation columns?
all neurones in a vertical column display the same orientation specificity neurones in oblique rows display heterogenous orientation specificity
67
What is meant by ocular dominance?
the inputs from the two eyes are still largely separate in the primary visual cortex (V1)
68
What enzyme is present in blobs? What does staining of this enzyme show?
**cytochrome oxidase** - a mitochondrial enzyme involved in cell metabolism staining with cytochrome oxidase reveals "pillars" running through layers II, III, V & VI of V1 each pillar is centred on an ocular dominance column
69
How do blobs receive P and K-cell input?
they receive P-cell LGN input via layer **IVCB** they receive K-cell input directly from LGN RFs of blobs display **colour opponency**
70
What is represented in this image?
a module comprised of 2 mm3 contains all representations i.e. colour orientations and both eyes
71
What happens after the striate cortex (V1)?
the striate cortex is the first region of visual processing in the cortex there are many other areas of the cortex involved in **extrastriate visual processing**
72
what are the two cortical streams of visual processing?
striate cortex towards ***parietal lobe*** - visual motion striate cortex towards ***temporal lobe*** - recognition of objects

Y2 CONTROL & MOVEMENT (69 decks)

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