visual defects 2 Flashcards
what is the pupil
opening that allows light to enter the eye; appears dark because of the light absorbing pigment epithelium in the retina.
what is the iris
pigmented to give the “eye’s colour”; contains two muscles to vary the size of the pupil.
what 2 muscles does the iris contain
sphincter pupillae and sphincter dilator
what is the cornea
glassy transparent external surface of the eye.
what is the sclera
white of the eye
what is the function of the extraoccular muscles.
control movements of the eyeball in the orbit of the skull.
what is the function of the conjunctiva
is a membrane that folds back from the inside of the eyelids and attaches to the sclera.
what is the function of the optic nerve
carries axons from the retina to the brain.
what is the lens
transparent structure suspended by ligaments (zonule fibres) attached to the ciliary muscles which control the shape of the lens.
what is the function of the vitreous humour
is a viscous jellylike substance that lies between the lens and the retina; it keeps the eye spherical
what is the retina
where light is transformed into neural activity
what is the fovea
the point of highest visual acuity in the retina, where light can reach photoreceptors directly.
define refraction
bending of light rays
when does refraction occur
light passes from one transparent medium to another.
which direction does light bend when refraction occurs.
Light bends towards a line that is perpendicular to the border between the media
how to all light rays meet at the fovea (what structure helps to converge them)
Light rays that strike the curved surface of the cornea bend so that they converge on the back of the eye.
is retina part of the CNS
yes
what 2 structures are required in the refraction of light
cornea, lens
once light has been refracted by the cornea it will then be refracted by which other structure
lens
do far distance or close distance objects require a greater refractive power
closer distance.
how is the ability of refraction of the lens changed.
accommodation
This focusing is brought about the lens changing shape
when the lens is rouser are its refractive properties improved
Yes
what muscle helps contract the lens in accommodation
cillary muscles.
relieving tension on the zonule fibres
what is a normal eye aka
An emmetropic (normal) eye
eye focuses parallel light rays on the retina without the need for accommodation.
what is farsightedness aka
Hyperopia
Is the eyeball to short or too long from front to back in hyperopia
to short
light rays are focused at some point behind the retina.
how does the eye adapt to being to short
Accommodation of the lens is needed for distant objects, and near objects cannot be brought into focus.
what type of lens is placed in from to the eye to allow the necessary refraction and enable near objects to be brought into focus.
Convex lens
what is nearsightedness aka
Myopia
Is the eyeball to short or too long from front to back in myopia
too long
parallel light rays converge at some point before the retina.
what type of lens must be placed in front of the eye in order to allow distant objects to be brought into focus on the retinal surface.
concave
what is photo refractive keratectomy.
uses laser to reshape the cornea and increase or decrease the amount of refraction possible.
what is the macula
central vision
what is the fovea
central/ thinner region of the retina.
what is the optic disc
origin of blood vessels; where optic nerve axons exit eye (blind spot)
how does light travel from the cornea to the retina
- Light is focused by the cornea and the lens and then passes through the vitreous humour to the retina.
- 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 which absorbs any light not absorbed by the retina.
- Light passes through all the other retinal cells to reach the photoreceptors at the back of the retina.
what are the 2 main types of photoreceptors.
cones- colour system
rods- no colour.
which one rods, or cones in made up more membranous disks which contain light sensitive photo pigments
rod cells- that is why they look much longer.
what are some of the key features of rod cells
high sensitivity to light more photopigment will capture more light high amplification low temporal resolution more sensitive to scattered light Achromatic- 1 pigment low visual acuity
what are some of the key features of cone cells
lower sensitivity to light. less photopigment lower amplification high temporal resolution]more sensitive to direct rays. Trichromatic- 3 different wavelengths. high visual acuity
define scotopic
night time lighting- rods only
define mespoic
twilight (dawn dusk)- both rods and cones.
define photopic
daytime lighting cones only.
what are the 3 types of cone cells
red- 560 nm
blue- maximally activated by light 430 nm
green-530
what is the young- helmoholtz theory
Every colour in the rainbow can be obtained by mixing the proper ratio of red, green and blue light
At each point in the retina there exists a cluster of three receptor types, each type being maximally sensitive to either blue, green or red.
The brain assigns colours based on a comparison of the three cone types.
what cells is the retina made from
Photoreceptors - convert light energy to neural activity
– Cones – colour vision (trichomatic)
– Rods – achromatic; more sensitive to light
Bipolar cells create the direct pathway from photoreceptors to ganglion cells.
Horizontal / amacrine cells – indirect pathways i.e. modulators
Retinal ganglion cells – axons leave the eye, forming the optic nerve.
Ganglion cells- only cells in eyes which produce an action potential.
which are the only cells in the eyes which produce a retinal reaction
ganglion cells.
does signal transduction in cells occur when the right of the wrong wavelength of light hits the receptors
wrong wavelength- stimulation.
right wavelength - inhibition.
do on bipolar cells depolarise in light or dark
light
do off bipolar cells depolarise in light or dark
dark
do bipolar cells have direct on indirect receptive fields
direct- from receptive field
indirect- from the surround
why do rods have a low visual acuity
only one bipolar cell for a group of rod cell receptors.
what cells are involved in the indirect input of bipolar cells.
horizontal cells (modulatory cells).
Bipolar cell has input from surrounding rods indirectly via horizontal cell synapsing.
what is the mechanism of action in a On- centre bipolar cell (centre)
- LIGHT in centre
- Photoreceptor hyperpolarised
- Less glutamate released from photoreceptor
- mGluR6 on ON-bipolar cell surface LESS active allowing Na channel to open, causing depolarisation.
what is the mechanism of action in a On- centre bipolar cell (surround)
- LIGHT in surround (feed onto horizontal cell); DARK in centre
- Photoreceptor in centre depolarised
- More glutamate released from centre photoreceptor
- mGluR6 on ON-bipolar cell surface MORE active closing Na channel, causing hyperpolarisation
• Inhibitory system- due to difference in light in peripheries and Centre causes a greater effect and hyperpolarization.
what is the mechanism of action in a On- centre bipolar cell (surround)
- LIGHT in surround
- Photoreceptors in surround hyperpolarised
- Less glutamate released from surround photoreceptors
- Less glutamate makes horizontal cell hyperpolarised
- Reduction in GABA release from horizontal cells depolarises central photoreceptor, producing more bipolar cell hyperpolarisation
what are the 3 types of retinal ganglion cells.
Magnocellular (M type)
Parvocellular (P type)
Non M, non-P
which type of ganglion cells carries information about movement
M -type.
what are the characteristic of M cells
– Larger cell type
– 5% of population
– Large receptive field
– Important for detection of stimulus movement.
what are the characteristic of P cells
– Smaller cell type
– 90% of population
– Sensitive to stimulus form and fine detail.
– Rod cells.
what are the characteristic of non m and p cells
– Medium cell type
– 5% of population
which type of ganglion cells carries information fine movement
P- type
how do ganglion cells work
how are they stimulated
differences in illumination that occur within their receptive fields.
(in both On an both cell types- the response is cancelled out by stimulation of the surround)
which type of ganglion is a colour opponent ganglion
P type
how do colour opponent ganglion cells work
- Red in center and surround get lots of AP
- Red in center and surround green (as they have overlap in wavelength).- the inhibitory effect of the green reduced number of AP.
- Light of a particular wavelength (e.g. red) in the centre of the receptive field, will be cancelled out by light of another wavelength (e.g. green) in the receptive field surround.
define retinotopic organisation
throughout the visual pathway e.g. information is based from eyes to brain in the same order that it enters the eyes, information which is received next to each other stays together.
M channel analysis- used for object motion
Simple cells in IVCα respond to light aligned in a particular orientation (orientation selectivity).
P-IB channel – analysis of object shape.
Parvocellular cells in the LGN project to IVCβ, which projects to interblob region in layer III. Complex cells in the interblob region of layer III have small receptive fields that are orientation-selective.
Blobs – analysis of object colour.
Blob cells in layer III are wavelength-sensitive, monocular and have colour-opponent centre surround organisation.- important for colour.
what are orientation columns
Simple and complex cells in layers V and VI are orientation selective. (light coming in at different angles- have different signals from different areas)
what are ocular dominance columns
Cells in layer IVC are monocular i.e. receive information from the left eye or right eye. Information from right and left eye stays sepearte in primary visual cortex it hasn’t been merged
how many parallel pathways are there that reach the brain
3 Magnocellular Pathway (motion) Blob Pathway (colour) Parvo- interblob pathway (shape)
what is structures are involved in extrastriate visual processing.
Striate cortex towards parietal lobe: visual motion.
Striate cortex towards temporal lobe: recognition of objects
