Visual System 1 Flashcards
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 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
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 ?
Accommodation.
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 ?
Farsightedness
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 ?
Nearsightedness
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
