How we see - part 2

Question 1

What is visible light?

Answer 1

Visible light is an electromagnetic wave

Question 2

What is phototransduction?

Answer 2

Phototransduction is defined as the conversion of light energy to an electrochemical response by the photoreceptors (rods and cones)

Question 3

What activates optic nerve cells?

Answer 3

The phototransduced rods and cones need to activate optic nerve cells (generate an action potential)

Question 4

What are 2 photoreceptors present in the retina?

Answer 4

rods

cones

Question 5

What is the function of rods?

Answer 5

function in less intense light

responsible for vision at low light levels (scotopic vision). They do not mediate color vision, and have a low spatial acuity

entierly responsible for night vision

Question 6

What is the function of cones?

Answer 6

responsible for colour vision

function best in brighter light

Question 7

What is the fovea centralis?

Answer 7

small, central pit in the retina composed of closley packed cones

responsible for sharp central vision

Question 8

What are the different types of cones?

Answer 8

S-cones

M-cones

L-cones

each cone is sensitive to visible wavelengths of lgiht that corrospond to short, medium and longer wavelength

colour blindness is due to the abscence of one of the following types of cones

Question 9

What is the strucutre of rods and cones?

Answer 9

Question 10

Where is the visual pigment found in photoreceptors?

Answer 10

Outer segments

Question 11

What are the photosensitive chemicals in rods and cones called?

Answer 11

The outer segment of a rod or a cone contains the photosensitive chemicals:

in rods, this chemical is called rhodopsin

in cones, these chemicals are called color pigments

Question 12

What makes up rhodopsin?

Answer 12

Opsin + 11-cis Retinal

Question 13

What is shown here?

Answer 13

protein in the membrane of the outer segement of a photoreceptor

Question 14

What is 11-cis retinal?

Answer 14

a vitamin A compound

Question 15

Where does the vitamin A compound of 11-cis retinal sit?

Answer 15

inside opsin

Question 16

What happens when light hits the vitamin A?

Answer 16

When light falls on 11-cis retinal, it isomerises to all-trans retinal

it becomes straight and no longer sit in the opsin t so it releses it and no longer looks purple as rhodopsin looks purple but now looks bleeched

Question 17

What happens when light hits rhodopsin?

Answer 17

when light hits retina, vitamins A in opsin changes shape and rhodosin slipts and bleeches and when rhodopsin splits this causes a who series of reactions

ultimatley leading to a Na+ channel in the cell membrane to close and when this closes this causes hyperpolarisation and by the time it reaches the optic nerve it becomes depolarisation and this is the action potential which is taken to the brain to see the outside world

then all resets for next photon of light

Question 18

what happens to all-trans retinal after rhodopsin splits?

Answer 18

leaves the photoreceptor

Question 19

Where is all-trans retinal taken up?

Answer 19

taken up by a layer underneath the rods and cones called the pigment epithelial layer

Question 20

Where is all-trans retinal changed by to 11cis-retinal?

Answer 20

pigment epithelial layer

Question 21

During the process of all-trans retinal changing back to 11cis-retinal some vitamin A is lost, what does it go on to form?

need to be replenished in order to continue to see normally and comes form the diet

Answer 21

retinyl ester

Question 22

How does bleaching of the visual pigment result in phototransduction?

Answer 22

Phototransduction cascade

Question 23

What role does Vitamin A play in the visual pigment?

Answer 23

visual pigment regeneration

Question 24

How does vitamin A deficiency affect the eyes?

Answer 24

Vitamin A is supplied through the diet, any condition that affects vitamin A absorption will affect vision - (night)blindness

Vitamin A is also essential for healthy epithelium so conjunctiva and corneal epithelium are also abnormal

Vitamin A deficiency can occur in conditions such as malnutrition, malabsorption syndromes such as coeliac disease, sprue

Question 25

What things can vitmin A deficiency cause in the eye?

Answer 25

Bitot’s spots in conjunctiva are sometimes the first indication of Vitamin A deficiency - silver traingles

Corneal ulceration ( green colour is dye to show up extent of ulcer)

Corneal melting which leads to future opacification of the cornea

Question 26

What is the main difference between rods and cones?

Answer 26

they respond to different wavelengths

Question 27

What is the process of an action potential forming?

Answer 27

Cascade of reactions within rod/cone - hyperpolarises - impulse is transmitted - becomes an action potential in the optic n cell

Question 28

How is cis-retinol reformed?

Answer 28

All trans-retinol converted into cis-retinol in RPE and will use up some Vitamin A

Cis-retinol then transported back to rod/ cone to reform “opsin”