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Excitable Cells > Sensory pathways: vision > Flashcards

Sensory pathways: vision Flashcards

1
Q

How can light be described?

A

both as an electromagnetic wave, and as a bunch of photons

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2
Q

What is wavelength?

A

colour of light (shorter – violet, longer – red)

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3
Q

What is amplitude?

A

intensity of light (large amplitude – bright light)

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4
Q

What can we see?

A

a narrow wavelength range of electromagnetic radiation

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5
Q

What in the eye is involved with light focussing?

A
  • Lens: crystalline protein structure – 12D – accommodation
     Bulges to look at close objects
  • Cornea – 42D – fixed structure, greatest refracting power in they eye
  • Measure the ability to bend light in D
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6
Q

What does the iris do?

A

Constricts or expands to allow more or less light through

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7
Q

What are the vitreous and aqueous humors?

A

fluid filled areas which allow the light through but give pressure to the eye to keep the structure intact to allow the retina to be held in the right distance and the shape behind the lens

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8
Q

What do all the fibres from the retina converge into?

A

the optic nerve

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9
Q

what is the retina considered?

A

Part of the brain

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10
Q

What is myopia and what do we need to correct it?

A

short sightedness: focal point of light is in-front of the retina. Need a concave lens to correct it

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11
Q

What is hyperopia and what do we need to correct it?

A

far sightedness: focal point of light is behind retina. Need a convex lens to correct it

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12
Q

What is astigmatism?

A

another common problem caused by irregularities of cornea and lens

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13
Q

What is the back of the retina made up of?

A

rod and cone cells

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14
Q

What does light go through before it reaches the rod and cone cells?

A
  • several layers of retinal cells:
  • Retinal ganglion cells
  • Bipolar cells
  • Horizontal cells
  • Amacrine cells
  • Layers in front of cells reasonably transparent
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15
Q

Where does visual information go in the eye?

A

from top to bottom

Photoreceptors -> other cells

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16
Q

What is the order in which light is passed through the cells and what happens as light passes through?

A

rods and cones -> bipolar cells -> retinal ganglion cells -> CNS (convergence of information)

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17
Q

What type of neurons are rods and cones?

A

glutamatergic excitatory neurons

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18
Q

Why is there convergence of information throughout the sensory system?

A

To cut down noise

19
Q

What does the sclera on the inside of retinal epithelial cells have the ability to do and why?

A

• The sclera on the inside of retinal epithelia has ability to bounce back light into the eye

  • Allows you to maximise amount of light at night
  • Tapetum lucidum (eyeshine) – layer just behind the RPE
20
Q

What is the RPE?

A

pigmented layer at the back of the retina

21
Q

what is the RPE essential for?

A
  • Essential for recycling of retinaldehyde, thus maintaining function of rods and cones
  • Helps rods and cones to cope with oxidative stress
22
Q

What does pigment epithelium do?

A

Makes the pupil look black

23
Q

where are most cones?

A

in the fovea, where we have maximum visual acuity

24
Q

what are rods used for and why?

A

night vision and peripheral vision. 1000 times more sensitive than cones

25
Q

Where are rods and cones located?

A

In the outer nuclear layer of the retina

26
Q

What is photopigment made up of?

A

Opsin + retinal

27
Q

What is opsin?

A
  • GPCR with 7 transmembrane domains
  • Different opsins in 3 types of cones, rods and melanopsin (in ganglion cells, helps with ability to get contrast) RGCs (=5 opsins, all rhodopsin based)
28
Q

What is retinal?

A
  • same for every opsin
  • Vitamin A derivative
  • Absorbs light and changes conformation (=bleaching)
29
Q

How do photoreceptors work?

A
  • When they detect light cGMP -> GMP
  • Cyclic GMP only is able to activate sodium channel:
  • In the dark they are depolarised when sodium channels open
  • In the light, Na+ channels close, leading membrane hyperpolarisation
  • Presence of stimulant = hyperpolarisation
  • Reduction of glutamate is the signal for light being perceived
30
Q

How does phototransduction take place in rods and cones?

A
  • Retinal absorbs light and changes conformation (from 11-cis retinal to all-trans retinal).
  • Activates rhodopsin molecule.
  • Rhodopsin linked to G protein assembly in ground state, allows phosphesterase to be inhibited and so you have cGMP
  • when light binds, GDP is ejected and replaced by GTP.
  • GTP causes release of phosphoesterase which breaks down cyclic GMP into GMP which can’t open sodium channel. This results in the closure of sodium channels and hyperpolarisation
31
Q

How many opsins are there in humans?

A

• There are 5 opsins in humans

  • 3 in cones
  • 1 in rods (peaks around 500nm)
  • 1 in melanopsin (peaks around 475nm)
32
Q

What are the three types of cones in humans?

A
  • S, L and M type cones
  • Blue
  • Green
  • Yellow/orange
33
Q

what are animals that only have 1 cone and 2 cones called?

A

monochromats and dichromats?

34
Q

Which animal has 16 types of cone?

A

Mantis shrimp

35
Q

How do cones need to be activated for the brainn to create the perception of colour?

A

they need to be differentially activated

36
Q

Give features of colour blindness

A
  • X chromosome: red and green opsins
  • Chromosome 7: blue opsin
  • 6% men have anomalies in colour vision
  • 2% of men lack a gene for red or green opsins
  • 1% of women have colour vision anomalies
  • < 0.001% of people lack all colour vision
37
Q

Give features of horizontal cells

A
  • Light intensity adaption
  • Spatial processing
  • Colour processing (opponency)
38
Q

Give features of amacrine cells

A
  • Directional motion
  • Modulate light adaption
  • Modulate circadian rhythm
  • Sensitivity of night vision
39
Q

Give features of bipolar cells

A
  • OFF-bipolar cells: hyperpolarisation
  • On bipolar cells: depolarise
  • Centre-surround receptive field of an ON bipolar cell
40
Q

Give features of retinal ganglion cells (RGC)

A
  • Further process colour, motion and shapes
  • The only output cells, fire action potentials to brain
  • Some RGC (intrinsically photosensitive retinal ganglion cells, ipRGC) can detect light via melanopsin
41
Q

What does Melanopsin affect?

A
  • Circadian rhythms
  • Pupil size
  • Body temperature
42
Q

What is the downstream visual pathway once information leaves the retina?

A
  • Nasal retina projects information to opposite site of brain
  • Temporal retina sends information to the same part of the brain
  • Optic nerve
  • Optic chiasma
  • Lateral geniculate nucleus (LGN) – relay area
  • Primary visual cortex (v1, striate cortex – located in occipital lobe)
  • Orientation selectivity in V1
43
Q

What is orientation selectivity in V1?

A
  • Found by imagining

- Different cells have different orientation selectivity

44
Q

What is melanopsin?

A

a protein

  • Light sensing
  • Not in rod or cone
  • Non-visual
  • Found in ganglion cells

Excitable Cells (22 decks)

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