Case 3 - Extra

Question 1

what is visual acuity

Answer 1

the ability of the eye to distinguish two nearby points

Question 2

what does acuity depend on

Answer 2

mainly on the spacing of photoreceptors of the retina and the precision of the eye’s refraction

Question 3

how can distance across the retina be described

Answer 3

in terms of visual angle. we can speak of the eyes ability to resolve points that are separated by a certain number of degrees of visual angle

Question 4

what is light energy first converted into

Answer 4

membrane potential changes in the photoreceptors

Question 5

what is photoreceptor membrane potential converted into

Answer 5

a chemical signal (the neurotransmitter glutamate) which is again converted into membrane potential changes in the postsynaptic bipolar and horizontal cells

Question 6

this process of electrical-to-chemical signalling repeats again and again until

Answer 6

the presence of light or dark or colour is finally converted to a change in the action potential firing frequency of the galngion cells. this is done through a complex process.

Question 7

the information from the 125 million photoreceptors is funnelled into how many ganglion cells

Answer 7

1 million

Question 8

what happens in the central retina

Answer 8

relatively few photoreceptors feed each ganglion cell.

Question 9

what happens in the peripheral retina

Answer 9

thousands of receptors feeding the ganglion cell. they develop a one-to-one relationship at the fovea

Question 10

what does this specialisation ensure

Answer 10

high acuity in central vision but also requires that eyes move to bring the images of objects of interest onto the fovea

Question 11

what is the blind spot

Answer 11

the optic disc - contains no photoreceptors

Question 12

secondary visual pathway

Answer 12

Question 13

the fibres containing melanopsin (fibres to pretectum and hypothalamus) are capable of what

Answer 13

modulating their response to changes in light levels in the absence of signals from rods and cones. therefore the circadian rhythms are maintained even after the degeneration of photoreceptors

Question 14

models of disabilities

Answer 14

Question 15

transduction signal from photons into action potential summary

Answer 15

1. photon absorption - retinaldehyde (retinal)
2. 11-cis retinal converted to all-trans retinal by light
3. opsin activated
4. G protein (transduction) dissociation
5. alpha subunit interacts with cGMP phosphodiesterase
6. hydrolyses cGMP to reduce local concentration of cGMP
7. cGMP cation channels close
8. reduced cation inflow
9. hyperpolarisation
10. reduction in glutamate release