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 brutes 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

Draw a diagram of how rods interact with the RPE

Question 10

Draw a diagram of how cones interact with RPE

Question 11

What are the differences between rods and cones?

Answer 11

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 12

When light hits photoreceptors do they hyperpolarise or depolarise?

Answer 12

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

Question 13

What photopigment do rods contain?

Answer 13

Rhodopsin

Question 14

What photopigment do cones have?

Answer 14

Photopsin

Question 15

Why do the photopigment need to be tightly packed?

Answer 15

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

Question 16

What are the photopigments made from?

Answer 16

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

Question 17

What happens once the photopigment has absorbed light?

Answer 17

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

Question 18

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

Answer 18

The amino acids determine which wavelengths of light they absorb.

Question 19

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

Answer 19

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 20

What is the purpose of phagocytes in the RPE?

Answer 20

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

Question 21

What are the 2 roles of RPE?

Answer 21

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

Question 22

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

Answer 22

The muller cells provide the cones with 11-cis retinol

Question 23

What other roles does the RPE have?

Answer 23

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 24

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

Answer 24

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 25

What is the dark state?