Optics and Physiology of Vision

Question 1

What are the two part of the visual pigment molecule? What are their roles?

Answer 1

Opsin - determines the wavelength of light that the photopigment will absorb, thus determining color viison

Visual chromophore/retinal - uses the energy of the photon particle to undergo isomerisation and so starting conversion of light stimulus into electrical signal

Question 2

what is the different between luminous intensity and luminance? And what are the units for each?

Answer 2

Luminous intensity - intensity of light source (candela)

Luminance - brightness reflected from a surface (candela/m2)

Question 3

What are the 3 changes than light travelling in a vacuum undergoes when it strikes denser media?

Answer 3

1. velocity is reduced
2. wavelength shortens
3. it bends (unless it struck the surface of the medium at a 90 degree angle)

Question 4

what is an object that bends (or refracts) light called?

Answer 4

a lens

Question 5

what happens to light rays when they strike a convex lens?

Answer 5

convergence

Question 6

what happens when ray of light strike a concave lens?

Answer 6

divergence

Question 7

what is unit for vergence power (amount of bending) of a lens?

Answer 7

diopters

Question 8

what is the focal length of a lens?

Answer 8

the distance between the centre of the lens and the point at which parallel rays of light are brought into focus by that lens

Question 9

True or false: the focal lens of a lens is directly proportional to it curvature radius

Answer 9

true

Question 10

true or false: the vergence powers of lenses in a optical system are not addivitive

Answer 10

false, they are additive

Question 11

what is accommodation?

Answer 11

rapid change in refractive power of the eye, intended to bring images of objects at different distances into focus on the retina

Question 12

give the 4 mechanisms by which vertebrate eyes can accommodate

Answer 12

1. changing the curvature or position of the lens
2. changing the corneal curvature
3. changing the distance between the cornea and the retina
4. having 2 or more separate optical pathways of different refractive powers

Question 13

true or false: accommodation is achieved in humans, primates and cats by changing the curvature of the lens

Answer 13

false: the feline lens capsule only has 5% of the elasticity in humans so cats are incapable of changing the lens curvature for accommodation

Question 14

how do feline eyes accommodate and how is this possible?

Answer 14

translation - anteroposterior movement of the entire lens

abundance of meridonal (longitudinal) fibres in the feline ciliary body muscle with low amount of circular fibres

Question 15

true or false: viewing distant objects is stimulated by the sympathetic nervous system

Answer 15

true

Question 16

what 3 ciliary muscles control accommodative mechanisms in the chicken?

Answer 16

1/ anterior (cramptons muscle)

2. intermediary (mullers muscle)
3. posterior (bruckes muscle)

Question 17

what are the unique anatomic adaptations that significantly increase the lenticular accommodative capability in chickens?

Answer 17

very large ciliary processes
ring of columnar epithelial cells at equatorial periphery of the lens (annular pad)
lenses are soft and malleable
corneoscleral sulcus permits a greater range of movement

Question 18

true or false: accommodation in chickens occurs from direct squeezing of the lens

Answer 18

true

Question 19

true or false: chickens cannot accommodate by changing the corneal curvature

Answer 19

false

Question 20

how is corneal accommodation mediated in chickens?

Answer 20

anterior ciliary muscle (cramptons muscle)

Question 21

true or false: chickens can accommodate independently in both eyes

Answer 21

true

Question 22

Apart from changing corneal and lens curvature, what else contributes to accommodation in chickens?

Answer 22

changing choroidal thickness

Question 23

does mydriasis increase or decrease the depth of focus of the eye?

Answer 23

decrease

Question 24

what is an emmetropic eye?

Answer 24

one in which parallel light rays from a distant object are focused on the outer segments when the eye is disaccommodated

Question 25

what is a myopic eye?

Answer 25

one in which parallel light rays from a distant object are focused on anterior to the retina (nearsighted)

Question 26

what is hypermetropic/hyperopic eye?

Answer 26

one in which parallel light rays from a distant object are focused posterior to the retina (farsighted)

Question 27

true or false: cycloplegia has a significant effect on refraction in most veterinary patients

Answer 27

false: it has no significance

Question 28

true or false: aphakia leaves the eye severely myopic

Answer 28

false: severely hypermetrophic

Question 29

what diopter power should the canine IOL have?

Answer 29

40-41.5D

Question 30

why is a 41D IOL needed to correct 15D of hypermetropia?

Answer 30

IOL is placed in a capsular bag filled surrounded by aqueous humour. This environment results in a reduction of its overall refractive power due to the small difference in refractive indices between the AH and the IOL

Question 31

what diopter power should the feline IOL have?

Answer 31

52-53D

Question 32

what is scotopic vision?

Answer 32

dim-light vision

Question 33

what are the characteristics of cone systems? (4)

Answer 33

high resolution of fine details rapid responses colour perception low sensitivity to small fluctuations in light intensity

Question 34

what are the characteristics of rod systems? (4)

Answer 34

poor visual resolution no colour perception extremely sensitive to minute changes in light levels detection of movement

Question 35

what is the fovea centralis in haplorrhine primates and many avian species?

Answer 35

a specialised, avascular area of the retina with higher cone density providing higher spatial resolution

Question 36

what are the four morphological parts of rods and cones?

Answer 36

1. outer segment 2. inner segment 3. nucleus 4. synaptic terminal

Question 37

what is the name of the rod photopigment?

Answer 37

rhodopsin

Question 38

what is the function of opsin?

Answer 38

determines the wavelengths of light that the photopigment will absorb

Question 39

briefly describe the phototransduction cascade

Answer 39

energy from photons transferred to chromophore decrease in cyclic GMP levels leading to closure of the ion channels hyperpolarisation of the photoreceptor glutamate release decreased or terminated which affects second order neurons

Question 40

true or false: photoreceptors are hyperpolarised in the dark causing them to release glutamate

Answer 40

false: they are depolarised

Question 41

what are horizontal and bipolar cells?

Answer 41

second order neurons of the retina that connect the first order (photoreceptors) and third order (RGCs) neurons