L15 - Vision and the Eye

Question 1

Properties of Light

Answer 1

Wave: Colour (spded/frequency)

Amplitude: intensity

380 to 750 for humans

Short – blue / Long – red

Question 2

structure of the vertebrate eye

Answer 2

Light enters the eye through the cornea (42D) and the lens (change shape/12 D), which together bend the light onto the retina (processing) that has photoreceptors → resolution

Question 3

What does the optic nerve do?

Answer 3

Nerve brings blood and takes axons of the ganglion cells, which has the info for the brain.

Question 4

Whats the lens attached to?

Answer 4

Zonule fiberes to ciliary muscle (relaxed - long) contracted (short)

Question 5

Problems in eye sight

Answer 5

Short sighted – resolving power of lens is too strong and get cross over of focal point. (Myopia)

Long sighted – focal point further

Astigmatism is another common problem caused by irregularities of cornea and lens

Question 6

Order of lamina structure and corresponding cells (6)

Answer 6

1. Photoreceptor outer (Rods)
2. Outer nuclear layer (cones)
3. Outer proximal layer (horizontal)
4. Inner nuclear (BipolaR)
5. Inner plexiform layer (Amacrine
6. Ganglion cell layers (Retinal ganglion)

Question 7

What is the retail pigment epithelium? (5)

Answer 7

Makes pupil look black - absorbs most light

Back of the retina

Recycling of retinaldehyde, thus maintaining function of rods and cones

Helps rods and cones to cope with oxidative stress

Reflective surface

Question 8

Cones and properties

Answer 8

Colour/daytime vision. Humans have 3 cones - detect activation of just 1 cone!

Blue: 430
Green: 530
Red: 560

Question 9

Rods and properties

Answer 9

Night vision and peripheral vision

1000X more sensitive than cones

1 type

Question 10

What are photopigment?

Answer 10

Opsin:
GPCR with 7 transmembrane domains – binds to retinal
Different opsin

Retinal (same in all)
• Vitamin A derivative
• Absorbs light and changes conformation (=bleaching)

Question 11

Brief description of photoreceptors detect darkness

Answer 11

• In the dark they are depolarised with sodium channels open.
• In the light, Na+ channels close, leading membrane hyperpolarisation.
• This is a neuron - it releases glutamate. No AP, but in the dark releases glutamate. In the light glutamate is not released.

o In the invertebrates – the opposite. But it’s the change that matters!

Question 12

Steps in Phototransduction (7)

Answer 12

1. Retinal absorbs photo
2. Conformational change Rhodopsin
3. Shape change in opsin
4. activation of complex transducing (G-PROTEIN)
5. Activates phosphodiesterase (alpha unit)
6. reduces cGMP (needed to open)
7. Close sodium channels

Question 13

Different number of cones

Answer 13

• 1 type of cone (monochromatic): dolphins, wales, seals, etc
• 2 type of cones (dichromate): dogs, reptiles, mice, cats, horses
• Human, apes: 3 types of cones (trichromatic)
• Mantis Shrimp: 12-16 cone types

Question 14

Colour blindness

Answer 14

X chromosome: red and green opsins

Chromosome 7: blue opsin

Question 15

Horizontal cells (4)

Answer 15

Prevent brain responding to individual cones/rods being activated/inhibited

Light intensity adaptation

Spatial processing

Colour processing (opponency)

Question 16

Amacrine cells (4)

Answer 16

Directional motion

Modulate light adaptation

Modulate circadian rhythm

Sensitivity of night vision

Question 17

2 types of bipolar

Answer 17

1) OFF bipolar cells have glutamate-gated Na channels. When a cone detects light, it releases less glutamate, leading to closure of these Na channels and subsequent hyperpolarization of the OFF bipolar cell.

ON bipolar cells have G-protein coupled receptors (GPCRs) and de-polarize in response to glutamate.

Question 18

Retinal ganglion cells (RGC) 2

Answer 18

Further process colour, motion and shapes

THE ONLY output cells, fire action potentials

Question 19

Melanopsin doesn’t contribute to image formation, but affects: 4

Answer 19

Circadian rhythms

Pupil size

Body temperature

Contrast

Question 20

Pathway

Answer 20

Pathway – 2 eyes, allows us to see depth -

Image goes to LGN, and then to primary visual cortex – this is where most of the processing goes on.

Left side of the retina goes to the left side of the brain. Not a complete cross over – but occurs in optic chiasm.

Info joins together in the LGN.

V1 – ocular dominance columns and orientation selectivity (have a read).

Vision is a very important sense: in primates more than 50% of the cortex is devoted to processing visual information.

Question 21

Evolution of vision: 2 and 2

Answer 21

least twice.

• Type I rhodopsin (ion channels): prokaryotes, algae, fungi, amoeba
• Type II rhodopsin (GPCRs): animals

Question 22

2 types of photoreceptors

Answer 22

Ciliary (vertebrate) and

Rhabdomeric (invertebrate) photoreceptors coexist: early origin of both types