Optical and Vision

Question 1

What type of lens is better for focusing near and far points?

Answer 1

a flat lens for far points

a fat lens for near points

Question 2

what is the refractive index of the cornea?

Answer 2

42 dioptres

Question 3

Which muscles act to increase and decrease the size of the pupils?

Answer 3

circular muscles constrict

radial muscles increase size

Question 4

what is meant by presbyopia?

Answer 4

the hardening of the lens and weakening of the ciliary muscles

Question 5

what is hyperopia?

Answer 5

far sightedness being unable to use refractive power for close objects

Question 6

what is myopia?

Answer 6

near sightedness where the refractive power is too strong for distance objects

Question 7

what is visual acuity?

Answer 7

the ability to distinguish between two nearby points determined largely by photoreceptor spacing and refractive power

Question 8

What is required to see an object?

Answer 8

• visual receptors (rods and cones)
• the right amount of light - too much bleaches signals
• the transduction from photons to electrical
• the brain must receive and interpret the signals

Question 9

What is the pathway for signal transmission?

Answer 9

Photoreceptors-> bipolar cells-> ganglion cells

Question 10

What lateral connections exist and what are their roles?

Answer 10

horizontal cells - receive input from photoreceptors and project to other photoreceptors and bipolar cells
amercing cells - receive input from bipolar cells and project to ganglion, bipolar and other amacrine cells

Question 11

What is the structure of the photoreceptor?

Answer 11

outer segment, inner segment, cell body and synaptic terminal

Question 12

What is the RMP of a photoreceptor?

Answer 12

-20mV - it hyper polarises in light

Question 13

What is the reason for the hyper polarisation and how can the brain interpret this?

Answer 13

-20mV because of a dark current. a cGMP gated Na channel is open in the dark and then closed in the light. the chain in Na is the signal the brain can interpret

Question 14

What are the visual pigment molecules?

Answer 14

Rhodopsin = retinal (vitA derivative) + Opsin (GPCR)

Question 15

How does light convert the visual pigments?

Answer 15

converts 11 cis Retinal to all trans retinal via opsin

Question 16

What is the signal transduction following conversion to all-trans retinal?

Answer 16

all-trans retinal activates transducin a G protein. This activates cGMP PDE where PDE hydrolyses cGMP reducing its concentration and closes the Na channels and hyper polarising the cells

Question 17

What is the neurotransmitter which is steadily released in the light pathway?

Answer 17

glutamate

Question 18

What produces cGMP in the photoreceptor?

Answer 18

guanylyl cyclase

Question 19

How is phototransduction different between day and night?

Answer 19

during the day the rods have become photo bleached as they can no longer become further hyper polarised and the vision is reliant totally on the cones

Question 20

What is the difference between the off and on retinal pathways?

Answer 20

the off pathway works via ionotropic receptors in the bipolar cell and therefore in the light there is a more negative bipolar cell from lack of glutamate.
the on pathway works via metabotropic receptors and therefore produces a positive vm in the bipolar cell as a result of lack of glutamate.

Question 21

How does the metabotropic receptor in the on pathway generate a more positive vm?

Answer 21

the metabotropic receptor responds to glutamate by increasing K and decreasing Ca resulting in hyper polarisation. so less glutamate produced by light results in a more positive membrane potential

Question 22

Why are the on/off pathways important?

Answer 22

they produce graded potentials in response to light and therefore contribute together to the amount of light there is. Only the ganglion cells actually produce the APs

Question 23

Where are rods and cones most concentrated?

Answer 23

rods are more concentrated all around the retina and the cones are focused around the centre of the fovea

Question 24

What does the increased sensitivity in the rods result in?

Answer 24

decreased acuity and high convergence

Question 25

What is the basis of the colour vision?

Answer 25

different opsin for different wavelengths

Question 26

How is the antagonistic centre surrounding the receptive field produced?

Answer 26

by lateral inhibition by the horizontal cells modifying the receptive fields of the ganglion cells to have a centre surround organisation

Question 27

What do the centre-surround organisation and lateral inhibition intend to do?

Answer 27

emphasise areas of difference sharpening the boundary between objects of different luminance detecting local differences in light intensity

Question 28

What are the different types of ganglion cells?

Answer 28

M-type - larger receptive fields and transient activation
P-type - smaller receptive fields and sustained activity and colour sensitive
nonM and nonP- sensitive to wavelength

Question 29

which ganglion cells demonstrate colour-opponent organisation in the receptive feild

Answer 29

P-type meaning to emphasise small differences in spectral absorption on three types of cone

Question 30

Where do optical tracts cross?

Answer 30

the optic chiasm

Question 31

what are the non-thalamic targets of the optic tract?

Answer 31

hypothalamus -sleep and wakefulness
pretectum - controls pupils
superior colliculus - orients head and eyes in response to stimuli

Question 32

What are the what and where pathways following visuotopic organisation?

Answer 32

the where pathway - dorsal pathway projects to parietoccipital association cortex
the what pathway - projects to occipital-temporal associateion cortex

Question 33

What is the role of the lateral geniculate nucleus?

Answer 33

it receives inputs from both eyes and relays these to the primary visual cortex via the optic radiation