Lecture 28-Visual System I

Question 1

Which two parts of the eye control focusing power?

Answer 1

cornea and lens

Question 2

Which part of the eye accounts for 2/3 of the focusing poers?

Answer 2

cornea

Question 3

In addition to the cornea, which structure is involve in “fine tuning” the focus of an image based on distance from the eye?

Answer 3

lens

Question 4

How is the curvature of the lens controlled?

Answer 4

contraction/relaxation of the ciliary muscle

Question 5

What is the function of the ciliary muscle?

Answer 5

controls curvature of the lens (accommodation)

Question 6

How do the ciliary muscle and lens adjust when looking at distant objects?

Answer 6

• ciliary muscle relaxes

- lens flattens

Question 7

How do the ciliary muscle and lens adjust when looking at near objects?

Answer 7

• ciliary muscle contracts

- lens is rounded

Question 8

When is the ciliary muscle at its largest diameter?

Answer 8

when looking at distant objects (flatten lens-tighten suspensory ligaments)

Question 9

What happens as the lens looses elasticity with age?

Answer 9

-focusing power on nearby objects declines (presbyopia)

Question 10

What is presbyopia?

Answer 10

• happens as the lens looses elasticity w/age

- focus on nearby objects declines

Question 11

What are the 2 types of photoreceptors?

Answer 11

Rod and Cones

Question 12

Which photoreceptors are more receptive to lights?

Answer 12

rods

Question 13

Which photoreceptors are more receptive to color?

Answer 13

cones

Question 14

What is scotopic vision?

Answer 14

night vision

Question 15

What would happen if you only functioned on “rod” vision?

Answer 15

you would only see in shades of gray

Question 16

What is photopic vision?

Answer 16

day time vision

Question 17

Which photoreceptors have a higher density in the peripheral retina?

Answer 17

rods

Question 18

Which photoreceptors have a higher density in the fovea?

Answer 18

cones

Question 19

Where in the retina are the cones more highly concentrated?

Answer 19

cones-FOVEA

Question 20

What is the light absorbing pigment found in photoreceptors?

Answer 20

rhodopsin

Question 21

What are the 2 parts of the rhodopsin protein?

Answer 21

• 11-cis retinal (chromopore-Vit A derivative)

- opsin (protein part)

Question 22

Which part of the rhodopsin molecule is responsible for catching light?

Answer 22

11-cis retinal (chromopore)

Question 23

What happens to the 11-cis retinal part of the of the rhodopsin molecule in the presence of light?

Answer 23

isomerizes to all trans-retinal

Question 24

How does light affect the photoreceptor cells

Answer 24

causes isomerization of the chromopore (11-cis retinal) to all trans-retinal

Question 25

What happens to the all-trans retinal portion of the rhodopsin molecule after photoisomerization?

Answer 25

It detaches from the opsin protein

Question 26

What causes the G-protein transducin to be activated?

Answer 26

activated rhodopsin (after photoisomerization and separation of the 2 parts)

Question 27

What does the G-protein transducin activate? What does this cause?

Answer 27

Transducin activates phosphodiesterase (PDE) which hydrolyzes cGMP (decreased cGMP)

Question 28

How does the phosphodiesterase (PDE) affect cGMP levels?

Answer 28

decreases them

Question 29

What is the role of cGMP in darkness?

Answer 29

keeps ion channels in outer segment open

Question 30

What is the "dark current"?

Answer 30

constant flow of ions in the dark that keeps the photoreceptor more polarized and leads to a continuous release of glutamate

Question 31

What is the resting membrane potential during the dark current for a photoreceptor? Why is this so?

Answer 31

-35mV, more depolarized because cGMP keeps ion channels open and maintains a constant ion current

Question 32

What neurotransmitter is released at the synaptic ending of a photoreceptor?

Answer 32

glutamate

Question 33

What happens to the photoreceptor membrane potential in the presence of light?

Answer 33

hyperpolarizes

Question 34

What does the reduction in cGMP by the PDE cause in the photoreceptor?

Answer 34

-closure of membrane channels-> reduction in dark current -> hyper polarization -> decreases glutamate release

Question 35

Do photoreceptors generate action potentials?

Answer 35

No- they send receptor potentials based on the intensity of light

Question 36

What are the 2 advantages to having a phototransduction cascade?

Answer 36

1. amplification | 2. modulation

Question 37

How does the phototransduction cascade provide amplification?

Answer 37

1 rhodopsin activates 100s of transducins which activate 1000s of PDEs which significantly affect cGMP levels

Question 38

Why is modulation of the phototransduction cascade important? How is it controlled?

Answer 38

- important for light and dark adaptation | - controlled by Calcium feedback mechanism

Question 39

Where do photoreceptors send info to be pre-processed before signals reach the brain?

Answer 39

bipolar cells

Question 40

What are the two types of bipolar cells?

Answer 40

``` H= off-center bipolar cells D= on-center bipolar cells ```

Question 41

What type of receptors do H-bipolar cells express? How do they respond when stimulated?

Answer 41

- ionotropic GLU receptors | - hyperpolarize when stimulated

Question 42

What type of receptors do D-bipolar cells express? How do they respond when stimulated?

Answer 42

- metabotropic GLU receptors | - depolarize when stimulated

Question 43

Where do bipolar cells synapse?

Answer 43

retinal ganglion cells

Question 44

What is contained within the optic nerve?

Answer 44

axons of many retinal ganglion cells

Question 45

Where do reinal ganglion cells project?

Answer 45

to the brain (lateral geniculate nucleus of the thalamus) via the optic nerves/tract

Question 46

In what plane would the horizontal cells be found?

Answer 46

Between the photoreceptors and bipolar cells

Question 47

What cells mediate lateral inhibition in the retina?

Answer 47

horizontal cells (and amacrine cells)

Question 48

What is the purpose of the lateral inhibition provided by the horizontal cells?

Answer 48

enhance contrast

Question 49

Which cells provide lateral inhibition at the bipolar retinal ganglion cell synapse?

Answer 49

amacrine cell

Question 50

What are the 3 forms of rhodopsin that cones contain?

Answer 50

blue (short-420nm) green (medium-496nm) red (long-560nm)