5. Retinal Photoreceptors

Question 1

What part is the inner retina?

Answer 1

The part which lies against the vitreous

Question 2

What part is the outer retina?

Answer 2

The part which lies against bruch’s membrane of the choroid.

Question 3

Are photoreceptors found in the inner or outer retina?

Answer 3

Outer retina

Question 4

What happens to the retinal cells at the foveola and why?

Answer 4

The retinal cells are swept aside so that light can travel directly to the foveal cones. There are only cones and muller cells. Thinnest part of retina.

Question 5

What are photoreceptors?

Answer 5

Rods and cones. Highly specialised nerve cells.

Question 6

Draw and label rod cell

Answer 6

1. Discs
2. Connecting cilium
3. Cell body
4. Axon
5. Nucleus

1 is part of outer segment
2 is part of inner segment

Question 7

Draw and label cone cell

Question 8

What are the types of cones?

Answer 8

S cone (blue) - short wavelength
M cone (green) - medium wavelength
L cone (red) - long wavelength

Question 9

What are the differences between rods and cones?

Answer 9

Rods- 2um thick and 50um long
Cones- 3-5um thick and 40um long

Rods- rod shaped outer segment
Cones- cone shaped outer segment

Rods- tips embedded in microvilli of RPE
Cones- located just close to RPE

Rods- Discs are pinched off the infoldings to form free discs in the outer segment
Cones- discs remain as infoldings and do not be free

Rods- high turnover of discs
Cones- turnover is less

Rods- no rods at foveola
Cones- foveal cones are specialised

Question 10

When light hits photoreceptors do they hyperpolarise or depolarise?

Answer 10

Photoreceptors hyperpolarise when stimulated by light. This is unlike most other neurones which depolarise when stimulated.

Question 11

What photopigment do rods contain?

Answer 11

Rhodopsin

Question 12

What photopigment do cones have?

Answer 12

Photopsin

Question 13

Why do the photopigment need to be tightly packed?

Answer 13

There is a high density of photopigments which allows a high proportion of the photons of light reaching the photoreceptors to be absorbed.

Question 14

What are the photopigments made from?

Answer 14

Protein- Opsin
&
A chromophore- a derivative of vitamin A called retinal.

Question 15

What happens once the photopigment has absorbed light?

Answer 15

The photopigment breaks down and is recycled. Known as visual cycle.

Question 16

Opsins have different amino acids. What does this then determine?

Answer 16

The amino acids determine which wavelengths of light they absorb.

Question 17

What can happen if there is genetic changes to opsin proteins?

Answer 17

Might cause shift in their spectral sensitivity (due to the changes in amino acids which determine which wavelength of light they absorb) causing milder forms of colour deficiency.

Question 18

What is the purpose of phagocytes in the RPE?

Answer 18

Light damages the outer segments, so the tips of rods and cones undergo destruction by the lysosomes to then be recycled.

Question 19

What are the 2 roles of RPE?

Answer 19

1. Convert vitamin A to 11-cups retinol
2. Phagocytoses the worn out outer segment discs.

Question 20

Under mesopic conditions how are cones not in competition with rods?

Answer 20

The muller cells provide the cones with 11-cis retinol

Question 21

What other roles does the RPE have?

Answer 21

1. Contains melanin to absorb stray light
2. Basement membrane forms part of Bruch’s membrane
3. Forms the blood retinal barrier
4. Regenerates the photopigments after bleaching
5. Antioxidant properties
6. Liposuction deposits in the RPE with age.

Question 22

At what part of the fovea are each of the cone cell types found? And what does this mean?

Answer 22

Red and green - central
Blue- towards the edge of the fovea

This means that the maximum a cutout for blue light is lower than that of other colours and so our visual perception of intensely blue objects is less distinct.

Question 23

What are after images?

Answer 23

Bleaching causes a bright stimulus to appear as a dark after image.
Red image causes green after image
Blue image causes yellow after image.

Question 24

Why does Red image cause green after image?

Answer 24

Due to colour opponency and the L cones would be less responsive after prolonged exposure to a red image due to adaptation.

Question 25

Define phototransduction

Answer 25

Cascade of events of how photons of light are converted into an action potential by the photoreceptors

Question 26

Define bleaching

Answer 26

When 11-cis retinal is in the TRANS configuration and can’t respond to any further light stimulation

Question 27

What happens in dark state?

Answer 27

1. In the dark, photoreceptors are depolarised
2. Glutamate is continuously released
3. Glutamate inhibits bipolar cells
4. No neurotransmitter released by bipolar cells so no signal at optic nerve.

Question 28

What happens in light state?

Answer 28

1. Photon of light hits 11 cis retinal
2. Proteins active : phosphodiesterase and transducing
3. Decreased CGMP so Na+ and Ca+ channels close
4. Hyperpolarisation from -40Mv to -70Mv
5. No glutamate
6. Bipolar cells are active and release neurotransmitter to generate signal in optic nerve.

Question 29

What is dark current?

Answer 29

Na+ moves out of inner segment and into outer segment
Photoreceptors have resting membrane potential of -40Mv