Vision Lecture 11-12

Question 1

What structure is the eye part of?

Answer 1

It is part of the CNS as it is connected to the visual cortex by the optic nerve

Question 2

What is the function of the pigment epithelium?

Answer 2

Dark to avoid the scatter of light

Question 3

Label the retina:

Answer 3

Insert Diagram

Question 4

What are the types of cells used by the retina?

Answer 4

Light sensitive: Rods, Cones Action Potential generating: amacrine and ganglion cells Plexiform layers containing synapses

Question 5

Describe the light sensitive cells- Rods?

Answer 5

Very sensitive to light so are used in dark conditions and are used to see fainter objects. Concentrated in the periphery not the centre - able to see fainter objects which are off centre. Achromatic

Question 6

Describe the light sensitive cells- Cones?

Answer 6

Concentrated in the fovea and used in normal light Allow for colour vision

Question 7

What is the difference in cilium between rods and cones?

Answer 7

Cilium: region where the outer segment is connected to the inner segment

Discs pinch off of cones but not from rods

Question 8

What are rods and cones sensitive to light?

Answer 8

Plasma membrane discs which contain opsin (cones) and rhodopsin (rods)

Question 9

What range of wavelength and colours can humans see?

Answer 9

380nm-750nm

Violet - Indigo - Blue - Green - Yellow - Organe - Red

Question 10

What types of opsin (photopigment) are there?

Answer 10

Red, Green, Blue

Rhodopsin (rods) which is achromatic

Question 11

What is the link between rhodopsin and GPCRs

Answer 11

Rhodopsin is coupled to G alpha T

This has a binding site for 11 cis retinal at residue 296

Activated by light: conforomational change from 11 cis retinal to all trans retinal- This is known as the phototransfuction step

Question 12

What is the difference in sensitivity of the different photopigments

Answer 12

Rods / rhodopsin has a broader spectrum than any individual opsin.

Question 13

What is the mechanisms for activation?

Answer 13

Light is absorbed causing a signalling cascade

GTP binds to G-Potein –> activates cGMP phosphodiesterase –> breakdown of cGMP (produced from the conversion of GTP by guanylyl cyclase) into 5’-GMP (inactive form) –> prevents the gating of the Na+ cGMP-gated channel

Question 14

Why is cGMP vital?

Answer 14

Regulates the opening of sodium channel, therefore controlling membrane potential

Channel is naturally open in dark conditions as cGMP is in high concentrations

Question 15

Describe the rhodopsin visual cycle

Answer 15

Rhodopsin is converted by light energy (Phototransduction) into bathorhodpsin –> lumirhodopsin –> metarhodopsin I –> metarhodospin II –> either opsin or all trans-retinal

Opsin –> Rhodopsin

All trans-retinal –> 11-cis-retinal –> rhodopsin

Question 16

What is the purpose of this mechanism?

Answer 16

Allows for amplification of the signal

reduction in cGMP causes hyperpolarisation due to lack of Na+ entry.

Question 17

What are the mechanisms at action in a rod cell in the dark?

Answer 17

cGMP gated channel is open –> Na+ influx which travels through outer segment and down through cilium into inner segment –> Na+ K+ exchanger swaps Na+ for K+

Influx of K+ causes a release of glutamate –> graded potential change

K+ leaves through K+ selective background leakage channels –> current flux –> Na+ enters

Question 18

What is the affect of a light stimulus?

How is this terminated?

Answer 18

cGMP gated channel closes –> hyperpolarisation

This leads to decreased glutamate release

This action is terminated by opsin kinase and arrestin which are activated by the photopigment

Question 19

How do the cells adapt to the light response?

Answer 19

Ca2+ flows through the Na+ channel

This inhibits guanylyl cyclase moderately preventing an excessive increase in cGMP concentration

If no Ca2+ can enter –> rapid hyperpolarisation

Question 20

Comparison between rods and cones?

• Light detection
• Temporal resolution
• Response and integration times
• Sensitivity
• Acuity

Answer 20

Rods: single photon detection / Cones: saturate in really intense light

Rods: low temporal resolution (12Hz) / Cones: high (55Hz)

Rods: Fast response, Slow integration / Cones: opposite

Rods: sensitive to scattered light / Cones: direct light

Rods: low acuity, highly convergenet retinal pathway / Cones: high acuity, less convergent retinal pathway

Question 21

Describe ganglion cells?

Answer 21

Mediated by horizontal and bipolar cells in the retina

Graded change in membrane potential from horizontal and bipolar cells feed in

Ganglion cells are the first to fire action potentials

Question 22

What types of ganglion cells are there?

Answer 22

M-type

P-type

Others

Question 23

Describe M-type Ganglion Cells

Answer 23

Large receptive fields detecting gross features

On and off centre

Not wavelength selective

Question 24

Describe P-type ganglion cells

Answer 24

Small receptive fields, detect fine features

On and off centre

Wavelength selective

Question 25

Describe other types of ganglion cells

Answer 25

Circular receptive fields with a centre and antagonistic surrounding Process information via 2 parralel pathways Cause dilation of pupils in response to light

Question 26

What are receptive fields?

Answer 26

Areas of sensitivity for light which extra information about size, form and shape of light and dark in a visual field. Usually circular with centre and antagonistic surround, meaning information is processed in parralel There are on centre and off centre receptive fields

Question 27

What are on-centre receptive fields / how do they respond?

Answer 27

Stimulated by light spot on Adapt to light by decreasing the firing of A.P bursts Light not straight on --\> prevention of APs Illuminate centre --\> powerful stimulation Illuminate off centre --\> prevents A.Ps but rebound excitation occurs Diffuse illumination --\> normal AP firing

Question 28

What are off centre receptive fields and how do they respond

Answer 28

Apply light spot on centre --\> prevents AP's Apply light spot off centre --\> stimulation then adapts Illuminate centre --\> prevent APs but then rebound Illuminate off centre --\> powerful stimulation (continous AP firing) Diffuse illumination --\> normal AP firing

Question 29

How do ganglion cells respond to bipolar cells

Answer 29

On and off centre bipolar cells With receptors on bipolar cells

Question 30

Ganglion cells respond to on centre bipolar cells?

Answer 30

mGluR6 receptors DARK Increased glutamate release --\> binding to receptor --\> release of second messenger --\> closing of TRPM1 channels --\> hyperpolarisation LIGHT Decreased glutamate release --\> decreased binding to receptor --\> decreased second messenger --\> TRPM1 channels open --\> inward cationic current --\> depolarisation

Question 31

How do ganglion cells respond to off centre bipolar cells?

Answer 31

AMPA receptors instead of mGluR6 DARK: Increased glutamate released --\> AMPA receptors open --\> sodium influx --\> depolarisation LIGHT: Decreased glutamate release --\> AMPA receptors closed --\> no sodium influx --\> hyperpolarisation

Question 32

What is centre surround inhibition?

Answer 32

Light activates seperate cone --\> glutamate release --\> activation of AMPA receptors at inhibitory interneuron --\> glutamate release from horizontal cell --\> binds to GABAa receptors on central cone --\> decreased glutamte release --\> binds to mGluR6 --\> open TRPM1 channels --\> depolarisation

Question 33

What is the role of inhibition in ganglion cells?

Answer 33

Where does the edge of the ganglion cell fall / off centre? If the edge is exposed to light --\> inhibition = -1 This allows for extraction of shape information This is enhanecd by the overlapping of receptive fields

Question 34

How do rods cooperate with cones?

Answer 34

When there is decreasing brightness the electrical synapse opens to allow ionic movement between rods and cones. This helps image detection because it increases the sensitivity to light When it is completely dark the synapses close again and only rods are used. This is regulated by amacrine cells

Question 35

Describe the causes and symptoms of colour blindness

Answer 35

Occurs when one pigment (red green or blue) is abnormal or absent Most common to be affected is green (blue is least) Causes the loss of the ability to detect colour differences due to lack of contrast Females are carriers and males affected.