Physiology

Question 1

Absorption spectrum of rhodopsin

Answer 1

498nm

Question 2

Absorption spectrum of red opsin

Answer 2

564nm

Question 3

Absorption spectrum of green opsin

Answer 3

534nm

Question 4

Absorption spectrum of blue opsin

Answer 4

420nm

Question 5

Absorption spectrum of melanopsin

Answer 5

480nm

Question 6

What type of protein are opsins?

Answer 6

G protein coupled receptors

Question 7

What is the visual cycle?

Answer 7

Regeneration of 11-cis-retinal after its conversion to all-trans retinal on exposure to light

Question 8

What triggers the shedding of rod and cone outer segments respectively?

Answer 8

Rod OS - shed at first light

Cone OS - shed at darkness

Question 9

State in brief the steps in the visual cycle.

Answer 9

1. Rhodopsin –> light –> Opsin + all-trans-retinal
2. All-trans-retinal complexes with lipoprotein and is transported into rod cytoplasm by ABCR
3. All-trans-retinal in rod cytoplasm –> retinol dehydrogenase –> oxidised to all-trans-retinol
4. All-trans-retinol is transported into RPE by IRBP
5. All-trans-retinol complexes to CRBP –> transferred to LRAT which stores it as all-trans-retinyl esters
6. All-trans-retinyl esters –> RPE65 –> 11-cis-retinol
7. 11-cis-retinol complexes to CRALBP –> transferred to retinol dehydrogenase which reduces it to 11-cis-retinal
8. 11-cis-retinal is transported back into cones by IRBP and combines with opsin to regenerate rhodopsin

Question 10

Visual cycle protein implicated in Stargardt’s disease + mode of inheritance

Answer 10

ABCR, autosomal recessive.
Retinal-lipid complexes are not transported from the rod membrane discs into the cytoplasm –> accumulate in RPE cells after phagocytosis –> toxic to RPE

Question 11

Visual cycle protein implicated in retinitis pigmentosa and Leber’s congenital amaurosis

Answer 11

RPE65 -accounts for 2% of retinitis pigmentosa spectrum and 16% of LCA spectrum. Autosomal recessive

Question 12

What is the volume of aqueous humour in the eye? What is the rate of secretion of aqueous humour?
How long does it take for the aqueous humour to be completely replaced?

Answer 12

Normal vol. = 250ul in anterior chamber
Normal rate of secretion = 2.5ul / min

Hence, normal turnover = every 100 minutes

Question 13

What is the blood supply of the ciliary body?

Answer 13

Ophthalmic artery –> long post. and ant. ciliary arteries –> Major arterial circle –> supplies the highly vascular nonfenenstrated capillaries ciliary stroma

Question 14

What are the components that make up the blood-aqueous barrier?

Answer 14

Non-fenestrated capillaries in the iris stroma

Tight junctions in the ciliary epithelium (nonpigmented epithelium)

Question 15

What is the main GAG in the corneal stroma?

Answer 15

Keratan sulphate

Question 16

What is the main collagen in the corneal stroma?

Answer 16

Type 1 / Type 5

Question 17

What are the factors that:

a) tend to cause corneal hydration
b) tend to cause corneal dehydration?

Answer 17

Corneal hydration:
- Imbibition pressure
- Intraocular pressure
Corneal dehydration:
- Endothelial cell pump
- Epithelial cell pump
- Tear film evaporatioin

Question 18

What are the organisms that can penetrate an intact corneal epithelium?

Answer 18

CHNLS

Corynebacterium
Haemophilus influenzae
Neisseria meningitides / gonorrhoeae
Listeria
Shigella

Question 19

What are some drugs that cause drug-induced keratopathy?

Answer 19

Amiodarone
Indomethacin
Tamoxifen

Question 20

At what age does the human eye usually stop growing?

Answer 20

Age 7-10

Axial length is 17mm at birth and 25mm in the adult

Question 21

What is the rate of basal tear production?

Answer 21

1.2ul/min

Question 22

What chromosome and arm is the HLA gene complex located on?

Answer 22

6p21