Visual Defects Flashcards
What is the lens?
a transparent structure suspended by ligaments (zonule fibres) attached to the ciliary muscles which control the shape of the lens
What is the vitreous humour?
a viscous jellylike substance that lies between the lens and the retina
it keeps the eye spherical
what is the retina?
the retina is part of the CNS where light is transformed into neural activity
What is the fovea?
the point of highest visual acuity in the retina, where light can reach the photoreceptors directly
Label the components of the eye in cross section
What is refraction and when does it occur?
refraction is the bending of light rays
refraction occurs when light passes from one transparent medium (e.g. air) to another (e.g. cornea)
What happens during refraction?
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
How does refraction occur by the cornea?
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
Other than the cornea, what other structure is involved in refraction?
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
What is the problem associated with refraction by the lens and how is this overcome?
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)
What structures are involved in accomodation?
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
what is meant by emmetropia?
emmetropia refers to an eye that has no visual defects
How does an emmetropic eye focus parallel light?
an emmetropic eye focuses parallel light rays on the retina without the need for accomodation
What is meant by hyperopia?
farsightedness
this is a vision condition in which you can see distant objects clearly, but objects near by may be blurry
Why does hyperopia occur?
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
How is hyperopia corrected with lenses?
What is the problem with accomodation?
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
What is myopia?
nearsightedness
this is a vision condition in which you can see nearby objects clearly, but objects in the distance are blurry
Why does myopia occur?
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
How is myopia corrected with lenses?
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
What is photorefractive keratectomy and how does it work?
corrective laser eye surgery
it uses a laser to reshape the cornea and increase or decrease the amount of refraction possible
Label the opthalmoscopic view of the retina
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)
What structures must light pass through to reach the retina?
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
where is the pigment epithelium positioned relative to the retina?
what is its function?
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
What types of cells are present in the retina?
what are the 2 different types of photoreceptors?
rods and cones
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
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
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
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
What is meant by phototransduction?
the conversion of light into a change in the electrical potential across the cell membrane
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
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
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
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
what are the stages involved in phototransduction?
- 1 photon is absorbed by 1 opsin
- 800 transducin molecules
- 800 PDE enzymes
- 4800 cGMP converted to GMP
- 200 cGMP sensitive ion channels close
- this causes hyperpolarisation and a decrease in glutamate release
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
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
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
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
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
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
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
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
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
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
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
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
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
What colour do we perceive when all three cone types are active?
when all three cone types are equally active we perceive “white”
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
What is the pigment that is present in all rods?
rhodopsin
(receptor protein - opsin)
this is the pigment that is present in ALL rods
What are the different types of pigments in cones?
How are they different?
in each cone there is one of three types of opsins
each cone opsin has a different spectral sensitivity:
- blue cones maximally activated by light of 420 nm
- green cones maximally activated by light of 530 nm
- red cones maximally activated by light of 560 nm
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)
What are the two main colour opponent pathways?
What does this explain?
- red / green
- blue / yellow
this explains the colout wheel
What are the structures involved in the visual pathway?
retina
lateral geniculate nucleus in the thalamus
primary visual cortex
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
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
What reflex are the ipRGCs involved with?
they are required for the pupillary light reflex
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
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
What types of retinal ganglion cells correspond to which layers of the LGN?
Where to the neurones of the LGN project to?
neurones of the LGN project to the primary visual cortex via the optic radiation
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
What are the 3 components involved in functional organisation of the primary visual cortex?
- orientation columns
- ocular dominance columns
- colour processing “blobs”
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
What is meant by ocular dominance?
the inputs from the two eyes are still largely separate in the primary visual cortex (V1)
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
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
What is represented in this image?
a module comprised of 2 mm3 contains all representations
i.e. colour orientations and both eyes
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
what are the two cortical streams of visual processing?
striate cortex towards parietal lobe - visual motion
striate cortex towards temporal lobe - recognition of objects
