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Flashcards in Visual System 1 Deck (44)

What does the lens do ?

Refracts light onto the retina
Good at this for images further away because light rays that strike the cornea are parallel
For closer images it requires greater refractive power


How do the extraocular muscles of the eye have such fine control ?

Because the neurons innervating them only innervate about 3 muscle fibres


What does the pupil do ?

Allows light to enter the eye


What is the macula ?

It is the central point of visual field


What is the fovea ?

It's the centre/thinner region of the retina
Contains highest concentration of cone photoreceptors to see colour - this is why colour visual field is smaller


What is the optic disk ?

Origin of the blood vessels and where the optic nerves exit - blind spot


What is presbyopia ?

Hardening of the lens with age


What happens when light hits the cornea and refracts ?

Hits the curved surface of cornea and is refracted to hit the back of the eye- light which enters the centre of the eye passes straight through to the retina


What is the term to describe the lens changing shape to focus light rays ?



How does the lens increase its refractive power ?

By increasing its curvature by contracting the ciliary muscles
The contraction of these muscles relaxes the tension on the zonule fibres which makes the les. Rounder


What is hyperopia ?

Eyeball is too short from front to back causing it to focus light rays behind the retina
Treated with a convex lens to bring objects into focus


What is myopia ?

Eyeball is too long from front to back causing light rays to converge before the retina
To fix it a concaved lens is used


How is light converted into neural activity ?

Light is focused and then passes through vitreous humour to the retina
Light passes through all the retinal cells to reach the photoreceptors
Retina is in front of the pigmente tied epithelium which is black and contains melanin which absorbs any light not absorbed by the retina


What are the cells in the retina involved in the pre processing of info before sending it through the optic nerve?

Bipolar cells
Horizontal cells
Amacrine cells
Ganglion cells


What is the difference between the bipolar and horizontal retinal cells ?

Bipolar cells create a direct pathway between photoreceptors and ganglion cells
Horizontal cells feed info laterally in outer plexiform layer to influence neighbouring cells


What are the different types of photoreceptors ?

Rods - no colour- for low light levels used at night time

Cones- colour vision- for daytime


Where does absorption of light occur in photoreceptors and why ?

Occurs in outer segment
Because this is where there are stacks of membranous disks containing light sensitive photopigments - opsin


What do rods and cones look like ?

Rods are long and cylindrical containing many disks
Cones are short and tapering with few membranous disks- why they are no good in night vision


Explain what happens in the signal transduction in photoreceptors ?

Dark- photoreceptors depolarised and release NT
- causes activation of cGMP system which opens sodium channels causing depolarisation
Light- photoreceptors hyperpolarise


What does scotoptic mean

Nighttime lighting so only rods involved


What does mescopic mean ?

Twilight lighting - both rods and cones involved


What does photopic mean ?

Daytime lighting. - only cones involved


What is the pigment in rods called and what are the 3 types in cones ?

Rods contain rhodopsin
Cones contain 3 types of opsin
- blue cones- activation at 430nm
- green cones- activation at 530nm
- re d cones- activation at 560nm


How are we able to see so many different colours ?

Due to the relative contributions of each of the 3 different types of cones


When do we perceive white light ?

When each of the 3 cone types is contributing equally


Why is colour blindness more common in men than women ?

Because the genes for red and green pigments are carried on the X chromosome


What is trichromatic vision ?

Normal colour vision containing all 3 pigments


What is anomalous trichromatic vision ?

Alternate perception of colour


What is dichromatic vision ?

Colour blindness


What is the receptive field ?

Area of retina that alters the bipolar cell membrane potential in response to light


What is the receptive field centre and receptive field surround ?

Centre makes direct contact to the bipolar cells
Surround makes indirect contact with bipolar cells via horizontal cells


What does the RF centre and RF surround make direct contact to ?

Centre- bipolar cells
Surround- horizontal cells which therefore have an indirect connection to bipolar cells


What happens with on-centre bipolar cells when there is light in the centre ?

Light in the centre hyperpolarises the central photoreceptors, reducing the release of glutamate
Less glutamate acts on the mGluR6 channels on the on centre bipolar cell
mGluR6 activation causes closure of sodium channels so the less glum ate means less sodium channel,are closed so on centre bipolar cells is depolarised
This causes it to release glutamate onto AMPA kainate and NMDA causing depolarisation of the on centre ganglion cells


What are the 3 types of ganglion cells ?

Magnocellular ganglion cells - M type
-large cell bodies and large dendritic field, non colour selective and transient response
Parvocellular ganglion cells-P type
- small cell bodies, small dendritic field , colour selective and sustained response
K ganglion cells - nonM and non P type
- small cell bodies medium sized dendritic field and colour selective


What is the distribution of ganglion cells ?

P type - 90%
M type - 5%
K type-5%


What do on centre ganglion cells and off centre ganglion cells do in the light and dark

On centre - depolarise in light and hyperpolarise in dark

Off centre- depolarise in dark and hyperpolarise on light


What happens to off centre bipolar cells when their is light in the centre ?

Photoreceptors is hyperpolarises reducing release of glutamate
Less glutamate released onto AMPA kainate and NMDA receptors on bipolar cell so it causes hyperpolarisation of bipolar cell
This causes less release of glutamate from bipolar cell onto ganglion cell so it is also hyperpolarises


What happens to on centre bipolar cells when the centre is in the dark ?

Photoreceptors are depolarised in the dark so release lots of glutamate
Glutamate binds to mGluR6 on bipolar cells and this causes closure of sodium channels causing hyperpolarisation of bipolar cells
Less glutamate is therefore released from bipolar cells onto ganglion cells so the ganglion cells are also hyperpolarises


What happens to off centre bipolar cells and ganglion cells when the centre is dark ?

Photoreceptor is depolarised so more glutamate is released
More glutamate to act on glutamatertergic receptors on bipolar cell causes bipolar cell to depolarise
This causes it to release flu ate onto ganglion cell and cause it to depolarise


How many inputs from photoreceptors do bipolar cells receive ?

About 10-20


What is the visual system specialised to detect ?

Local spatial variations , not actual light levels


What do horizontal cells do ?

Provide inputs from the photoreceptor receptive field surround


What are the colour opponencies you can have ?

Red-green and yellow-blue


What is colour perception based on ?

Relative activity of ganglion cells