Visual System 1

Question 0

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

Answer 0

Because the neurons innervating them only innervate about 3 muscle fibres

Question 1

What does the lens do ?

Answer 1

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

Question 2

What does the pupil do ?

Answer 2

Allows light to enter the eye

Question 3

What is the macula ?

Answer 3

It is the central point of visual field

Question 4

What is the fovea ?

Answer 4

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

Question 5

What is the optic disk ?

Answer 5

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

Question 6

What is presbyopia ?

Answer 6

Hardening of the lens with age

Question 7

What happens when light hits the cornea and refracts ?

Answer 7

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

Question 8

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

Answer 8

Accommodation.

Question 9

How does the lens increase its refractive power ?

Answer 9

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

Question 10

What is hyperopia ?

Answer 10

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

Question 11

What is myopia ?

Answer 11

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

Question 12

How is light converted into neural activity ?

Answer 12

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

Question 13

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

Answer 13

Bipolar cells
Horizontal cells
Amacrine cells
Ganglion cells

Question 14

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

Answer 14

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

Question 15

What are the different types of photoreceptors ?

Answer 15

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

Cones- colour vision- for daytime

Question 16

Where does absorption of light occur in photoreceptors and why ?

Answer 16

Occurs in outer segment

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

Question 17

What do rods and cones look like ?

Answer 17

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

Question 18

Explain what happens in the signal transduction in photoreceptors ?

Answer 18

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

Question 19

What does scotoptic mean

Answer 19

Nighttime lighting so only rods involved

Question 20

What does mescopic mean ?

Answer 20

Twilight lighting - both rods and cones involved

Question 21

What does photopic mean ?

Answer 21

Daytime lighting. - only cones involved

Question 22

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

Answer 22

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

Question 23

How are we able to see so many different colours ?

Answer 23

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

Question 24

When do we perceive white light ?

Answer 24

When each of the 3 cone types is contributing equally

Question 25

Why is colour blindness more common in men than women ?

Answer 25

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

Question 26

What is trichromatic vision ?

Answer 26

Normal colour vision containing all 3 pigments

Question 27

What is anomalous trichromatic vision ?

Answer 27

Alternate perception of colour

Question 28

What is dichromatic vision ?

Answer 28

Colour blindness

Question 29

What is the receptive field ?

Answer 29

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

Question 30

What is the receptive field centre and receptive field surround ?

Answer 30

Centre makes direct contact to the bipolar cells

Surround makes indirect contact with bipolar cells via horizontal cells

Question 31

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

Answer 31

Centre- bipolar cells

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

Question 32

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

Answer 32

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

Question 33

What are the 3 types of ganglion cells ?

Answer 33

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

Question 34

What is the distribution of ganglion cells ?

Answer 34

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

Question 35

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

Answer 35

On centre - depolarise in light and hyperpolarise in dark

Off centre- depolarise in dark and hyperpolarise on light

Question 36

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

Answer 36

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

Question 37

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

Answer 37

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

Question 38

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

Answer 38

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

Question 39

How many inputs from photoreceptors do bipolar cells receive ?

Answer 39

About 10-20

Question 40

What is the visual system specialised to detect ?

Answer 40

Local spatial variations , not actual light levels

Question 41

What do horizontal cells do ?

Answer 41

Provide inputs from the photoreceptor receptive field surround

Question 42

What are the colour opponencies you can have ?

Answer 42

Red-green and yellow-blue

Question 43

What is colour perception based on ?

Answer 43

Relative activity of ganglion cells