L15 - Vision and the Eye Flashcards
Properties of Light
Wave: Colour (spded/frequency)
Amplitude: intensity
380 to 750 for humans
Short – blue / Long – red
structure of the vertebrate eye
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
What does the optic nerve do?
Nerve brings blood and takes axons of the ganglion cells, which has the info for the brain.
Whats the lens attached to?
Zonule fiberes to ciliary muscle (relaxed - long) contracted (short)
Problems in eye sight
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
Order of lamina structure and corresponding cells (6)
- Photoreceptor outer (Rods)
- Outer nuclear layer (cones)
- Outer proximal layer (horizontal)
- Inner nuclear (BipolaR)
- Inner plexiform layer (Amacrine
- Ganglion cell layers (Retinal ganglion)
What is the retail pigment epithelium? (5)
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
Cones and properties
Colour/daytime vision. Humans have 3 cones - detect activation of just 1 cone!
Blue: 430
Green: 530
Red: 560
Rods and properties
Night vision and peripheral vision
1000X more sensitive than cones
1 type
What are photopigment?
Opsin:
GPCR with 7 transmembrane domains – binds to retinal
Different opsin
Retinal (same in all)
• Vitamin A derivative
• Absorbs light and changes conformation (=bleaching)
Brief description of photoreceptors detect darkness
- 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!
Steps in Phototransduction (7)
- Retinal absorbs photo
- Conformational change Rhodopsin
- Shape change in opsin
- activation of complex transducing (G-PROTEIN)
- Activates phosphodiesterase (alpha unit)
- reduces cGMP (needed to open)
- Close sodium channels
Different number of cones
- 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
Colour blindness
X chromosome: red and green opsins
Chromosome 7: blue opsin
Horizontal cells (4)
Prevent brain responding to individual cones/rods being activated/inhibited
Light intensity adaptation
Spatial processing
Colour processing (opponency)
Amacrine cells (4)
Directional motion
Modulate light adaptation
Modulate circadian rhythm
Sensitivity of night vision
2 types of bipolar
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.
Retinal ganglion cells (RGC) 2
Further process colour, motion and shapes
THE ONLY output cells, fire action potentials
Melanopsin doesn’t contribute to image formation, but affects: 4
Circadian rhythms
Pupil size
Body temperature
Contrast
Pathway
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.
Evolution of vision: 2 and 2
least twice.
- Type I rhodopsin (ion channels): prokaryotes, algae, fungi, amoeba
- Type II rhodopsin (GPCRs): animals
2 types of photoreceptors
Ciliary (vertebrate) and
Rhabdomeric (invertebrate) photoreceptors coexist: early origin of both types
