vision, the eye

Question 1

outer-segment:

Answer 1

discs and invagination of the plasma membrane in the outer segment

Question 2

importance of discs and invagination of the plasma membrane in the outer segment

Answer 2

increases the effective concentration of plasma membrane in comparison to a normal neurone

important as the protein and cofactor component that absorbs light is a membrane associated protein [opsin (rhodopsin, cone opsin).

Light may not be absorbed at the shallowest part of the photoreceptor, but at a deeper part

Question 3

how many protein opsins per rod?

Answer 3

10 ^8

Question 4

what occurs in the outersegment?

Answer 4

light absorption

Question 5

what part of opsin photopigment is responsible for its colour

Answer 5

chromophore

Question 6

what cofactor does light absorption rely on?

Answer 6

organic co-factor: retinaldehyde

Question 7

origin of retinaldehyde

Answer 7

derivative of retinol, which comes carotene

Question 8

when does an opsin protein form a photopigment

Answer 8

when it binds retinaldehyde

Question 9

role of retinaldehyde

Answer 9

absorbs light

Question 10

key feature of retinaldehyde

Answer 10

exists in multiple structural isoforms

Question 11

what happens to retinaldehyde when it absorbs light

Answer 11

it drives the isomerisation of 11-cis retinaldehyde to all-trans retinaldehyde

Question 12

what is an opsin protein

Answer 12

a 7 transmembrane domain G protein coupled receptor that binds retinaldehyde

Question 13

general concept GPCR

Answer 13

• Sit in plasma membrane
• Detect something extracellularly that activates them
• Change their conformation and can then interact with heterotrimeric g protein
• This interaction causes Galpha and Gbeta/gamma to separate, each component can then interact with effector enzymes to regulate second messenger systems

Question 14

opsin as a GPCR

Answer 14

Doesn’t detect an extracellular change, detects a change in the retinaldehyde (which it is always bound to)

because the all-trans acts as an agonist, whilst 11-cis acts as an inverse agonist

Question 15

inverse agonist

Answer 15

very successfully suppresses signalling from the receptor

Question 16

phototransduction cascade

Answer 16

• Photon is absorbed by rhodopsin
• Rhodopsin interacts with g protein transducin
• Transducin separates the domains
• The G alpha domain can then activate cyclicGMP phosphodiesterase (effector enzyme)
• Activated cGMP PDE hydrolyses cGMP (reduction in the local concentration of cGMP)
• Cell membrane has cyclicGMP gated cation channels
  when cyclicGMP concentration reduces cation channels change (close), changing the neurone conductance of the cell
• Causes graded hyperpolarisation (the brighter the light the more hyperpolarised the photoreceptor becomes, and the bigger the reduction in neurotransmitter release)
• Neurotransmitter response is reduced
  depolarised when dark causing neurotransmitter release (glutamate)

Question 17

g protein phototransduction cascade

Answer 17

transducin

Question 18

g protein subunits activated phototransduction cascade

Answer 18

alpha domain

Question 19

effector enzyme phototransduction

Answer 19

cyclicGMP phosphodiesterase

Question 20

role of phosphodiesterase

Answer 20

hydrolyses cGMP causing a reduction in the local concentration of cGMP

Question 21

what does reduction of cGMP do in the phototransduction cascade

Answer 21

causes cyclicGMP gated cation channels to close, causing hyperpolarisation

Question 22

feature of hyperpolarisation

Answer 22

graded- the brighter the light, the more hyperpolarised the cell becomes and the bigger the reduction in neurotransmitter release

Question 23

what is the neurotransmitter in photoreceptors?

Answer 23

glutamate

Question 24

what feature of the phototransduction cascade allows extreme sensitivity in rod vision

Answer 24

signal amplification at each stage, a single photon on rhodopsin can induce a large signal

Question 25

what keeps photoreceptors in a generally depolarised state?

Answer 25

open cyclicGMP gated cation channels

Question 26

structural and physiological differences in rod cells

Answer 26

capture more photons as they have a larger membrane surface area have a larger signal amplification

Question 27

cones

Answer 27

* adjust their sensitivity (adaption) to become active under any light level: can detect light across the huge range of brightness we encounter in the daytime * have a higher acuity * provide colour vision

Question 28

fovea

Answer 28

area of all cones on retina, provides highest acuity vision

Question 29

what do photoreceptors do?

Answer 29

translates light into a biological signal

Question 30

inner nuclear layer

Answer 30

extracts visual information

Question 31

retinal ganglion cells

Answer 31

transmits signal to the brain

Question 32

outer plexiform layer

Answer 32

the point at which the photoreceptors synapse with neurones in the inner nuclear layer

Question 33

inner plexiform layer

Answer 33

where ganglion cells and neurones from the inner nuclear layer synapse

Question 34

optic nerve

Answer 34

where retinal ganglion axons bundle send signal to the brain

Question 35

the first steps in seeing

Answer 35

* eye projects an image onto photoreceptors * photoreceptors translate into a spatial pattern of glutamate release * Bipolar cells convey a signal from the cones to RGCs: separate On vs Off signals * retinal ganglion cells send information to the brain using action potentials

Question 36

2 types of bipolar cells

Answer 36

* On: depolarised by flash * Off: hyperpolarised by flash

Question 37

on bipolar cells

Answer 37

depolarised by flash, signal inverting synapse

Question 38

off bipolar cells

Answer 38

hyperpolarised by flash, signal conserving synapse

Question 39

receptor on bipolar cell

Answer 39

metabotropic glu receptors

Question 40

receptor off bipolar cell

Answer 40

ionotropic glu receptor

Question 41

mechanism on bipolar cells

Answer 41

metabotropic Glu receptors: Glu activates signalling cascade closing cation channels hyperpolarisation in photoreceptor translated into depolarisation in the postsynaptic neurone

Question 42

mechanism off bipolar cell

Answer 42

ionotropic glu receptors: cation channels opened by Glu hyperpolarisation in photoreceptor transmitter as hyperpolarisation in the postsynaptic neurone

Question 43

what do on and off bipolar cells allow

Answer 43

parallel positive and negative representations of the scene to be sent to the brain

Question 44

horizontal cells

Answer 44

provide lateral inhibition in the retina link photoreceptors in the retina

Question 45

location of horizontal cells

Answer 45

* Have their cell bodies in the inner nuclear layer * Make connections in the outer plexiform layer between photoreceptors

Question 46

horizontal cells receiving inputs from local cones via sign conserving synapses

Answer 46

hyperpolarised by light

Question 47

horizontal cells receiving inputs from local cones via sign inverting synapses

Answer 47

antagonise the light response (depolarise)

Question 48

what is the role of centre surround organisation caused by horizontal cells?

Answer 48

amplifies local differences in light intensity pull out edges, enhances contrast of the visual information

Question 49

role of amacrine cells

Answer 49

perform horizontal information transfer at the inner plexiform layer

Question 50

where are amacrine cell bodies?

Answer 50

the inner nuclear layer

Question 51

what are the layers of the eye starting with the photoreceptor layer

Answer 51

photoreceptor layer, outer plexiform layer, inner nuclear layer, inner plexiform layer, ganglion cell layer

Question 52

what do amacrine cells link, what is the nature of this link?

Answer 52

amacrine cells provide an inhibitory response between bipolar cells and retinal ganglion cells

Question 53

what types of synapse do amacrine cells usually have with a bipolar cell?

Answer 53

sign conserving

Question 54

what type of synapse do amacrine cells usually have with ganglion cells?

Answer 54

sign inverting

Question 55

what unique function do amacrine cells allow

Answer 55

directional selectivity, cells respond to dark spots moving left to right, but not right to left

Question 56

features of amacrine cell involved in directional selectivity

Answer 56

amacrine cell only appears on one side of the circuitry and has a long lasting response

Question 57

what does directional selectivity allow?

Answer 57

allows us to understand the direction of a movement without higher encoding

Question 58

what does the pattern of ganglion cell responses show?

Answer 58

encodes visual information, doesn't simply report amount of light falling on the photoreceptors