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Neuroscience > Photosensation > Flashcards

Photosensation Flashcards

1
Q

why is light abosrbed?

A

circadian rhythm, vitamin production and metabolism

- detected to impart information from the world - forms visions

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2
Q

how can light be detected?

A

ocular (dedicated eye) or extraocular

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3
Q

what is vision?

A
  • the state of being able to see or form image
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4
Q

what aspects can we can sense?

A

colour, shape, movement and dimension

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5
Q

what are the key points of ocular?

A

decdicated structure requiring lenses in combination with light gathering structures

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6
Q

what is the primitive eye?

A
  • simple, evolution from primitive cilia on protozoans

- evolved into structures with more cilia that have a bigger surface area for efficient absorption of light

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7
Q

what are the 2 different classes that primitive structures have evolved into?

A
  • invertebrate photoreceptors

- mammalian photoreceptors

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8
Q

what is the different between mammalian and invertebrate photoreceptors?

A
  • orientation of cilia polarity
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9
Q

what are the main types of photoreceptors?

A

rhabdomeric and ciliary photoreceptors

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10
Q

what are camera eyes?

A

have smalle aperture with a larger light harvesting structure behind

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11
Q

what are the properties of a lens eye?

A
  • everything in reverse
  • the lens allows the focusing of light in a smaller area
  • more efficient detection and clarity
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12
Q

what does a corneal eye do?

A
  • need focuse to help catch prey
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13
Q

what are the features of the compound eye?

A
  • limited focus
  • gives speed (can pick up speeds we wouldnt be able to)
  • as an object moves across the field of vision it triggers individuals photoreceptors
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14
Q

what do arrays of photoreceptors sgguest?

A

an increasing developmental sophsistication

- development relfects increasing sensory sophistication

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15
Q

what is the difference between a convex and a concave eye?

A

-convex eyes harvest and focus more light than concave

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16
Q

how did the concave camera eye develop?

A

evolving complexity of the eye separates chambers allowing specialisation and optimising of funtions:

  • colour filtering
  • higher refractive index
  • blocking UV radiation
  • ability to operate in and out of water
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17
Q

what does the lens allow?

A
  • allows focus and to see further with less light

same amount of light focused on a smaller area

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18
Q

what detects light?

A

rhodopsins

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19
Q

what are rhodopsins?

A
  • from the family of opsins
  • light harvesting
  • visual pigmemnt
  • a PGCR embedded in the photoreceptor membrane in the villi or cilia
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20
Q

what is the ligand of the rhodopsin GPCR?

A

ligand is the activation of II cis retinal (sits in the middle of the rhodopsin) to ‘all trans-retinal’ (absorption of light)

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21
Q

what are the two types of rhodopsins?

A

C; cilliary

R; rhabdomeric

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22
Q

how does the retianl harvest the light?

A
  • photon converts II cis reitnal to all-trans-retinal (in the heart of the rhodopsin protein)
  • decay of the molecule that convertes all-trans to II-cis activates rhodopsin
23
Q

what is retinal produced from?

A

vitamin A derived from dietary beta carotene (in carrots)

24
Q

what at are the steps that rhodopsin activation follows?

A
  1. rhodopsin
  2. bathorhopsin
  3. lumirhodopsin
  4. meta I rhodopsin
  5. meta II rhodopsin
  6. back to rhodopsin
25
Q

what happens as rhodopsin converts to meta II rhodopsin?

A

as you move through each step there is a drop in energy

- at meta II rhodopsin: the structure activates the associated G protein

26
Q

what downstream signalling occurs in vertebrates?

A
  • rods hyperpolarise light

- vertebrate ciliary (C) opsins act through phosphodiesterase

27
Q

what downstream signalling occurs in insects?

A
  • rhabdomeres (R) depolarise to light

- insects: rhabdomeric (R) opsins act through phospholipase C (PLC)

28
Q

what are the structures detecting light in insect photosensation?

A
  • cystalline array of ommatidia
  • each ommatadia contains photoreceptor cells R1-R8 is under R7
  • each photoreceptor has a rhabodmere expressing a rhodopsin tuned to the light wavelength
29
Q

in insects what are channel openings in the photoreceptor caused by?

A

single photos

in the photoreceptor 1 photon generates 1 ‘quantum bump’

30
Q

what is meant by the quantum bump?

A
  • around 20ms duration with ~10pA amplitude when Ca2+ is present
  • if theres no calcium = no bump
  • represents the opening of ~15 TRP cahnnels within on villus
  • there is a small gap (latency) of 20-100ms which is the time for signalling molecules to accumulate and activate TRP channels
31
Q

how do the TRP/TRPL channels function?

A

in the dark they are in the villi

  • in the light they move down to the cell body
  • TRP1 channels move down the rhabdomere toward SMC on light activation
  • this adpats the rhabdomere to light
32
Q

what is the SMC?

A
  • the sub-microvillar cisternae at the base of the rhabdomere
33
Q

what is the role of the SMC?

A

originally thought to ba Ca2+ store but more likely to be involved in phosphoinositide regulation

34
Q

what is the sigla complex?

A

tightly coupled G protein coupled cascade and motor complex

- represents a really tight biophysical coupling

35
Q

what is the process of how the sigla complex functions?

A
  • light hits the microvilli on the rhabdomere
  • converts rhodopsin to meta rhodopsin
  • activates the G protein (G alpha) which activates PLC beta
  • PLC beta cleaves PIP2
  • results in IP3 and DAG
  • at the same time PKC is activated and phosphorylates NINAC which is attached to the cytoskeleton
36
Q

what is the compex held together by?

A

INAD so theres a really intense coupling

37
Q

how are the TRP/TRPL channels activated in the sigla complex?

A
  • by DAG and PUFFA

- causes influx of Ca2+ and Na+

38
Q

what molecules are activated by dissocation from NINAC in the sigla complex?

A
  • Arr2

- Goes back to stop the reaction

39
Q

how is TRP activation triggered in insects?

A
  • insertion of DAG into the membrane and (PUFA) physically deforms the membrane and it deflects
  • the movement opens the channel
  • in invertebrates photosensation is mediated by membrane based mechanotransduction
40
Q

what are the two types of photoreceptors in vertebrate photosensation?

A

rods and cones

41
Q

what are the properties of rods?

A
  • detect degree of lightness
  • easily bleached by light (high sensitivity)
  • sensitivity determined by rhodopsin
  • low spatial resolution
  • work in low light
  • limited colour resolution
  • can be activated by 1 photon
42
Q

what are the properties of cones?

A
  • sensitive to light but retain function in high illumination
  • can express one of 3 C opsins
  • 3 types: S,M,L (different spectral sensitivites)
  • high spatial resolution
  • activated by 100 photons
43
Q

when are rods and cones both used?

A

in moderate light

44
Q

what is the organisation of a vertebrate eye?

A
  • it is convex
  • light travels to contact the rods and cones through the retinal layer
  • information is processed by the retinal layers
45
Q

what happens when vertebrate rod photo receptors are hyperpolarised?

A

they dont generate action potentials

46
Q

what happens when vetobrates are in the dark?

A
  • in the dark there are constant action potentials
  • current travelling down the photoreceptor due to the opening of cyclic nucelotdie gated channels
  • cGMP, they open in the dark and shut off in the light
47
Q

how does the vertebrate photoreceptor response compare to the invertebrate?

A
  • much slower in vertebrate eyes
  • insects can respond to flickering light at up to 300Hz while vertebrates cannot respond to any light flicker above 10Hz
48
Q

how do photoreceptors work in vertebrates?

A
  • light hits rhodopsin
  • stimulates a G protein, alpha transducin
  • alpha tranducin stimulates phosphodiesterase which takes the inhibtory subunit of phosphodiesteraseE (PDE)
  • PDE hydrolyes cGMP to GMP
  • No cGMP left to gate the channel so the action potential turns off
49
Q

how is phototransduction activation a diffusion process in vertebrates?

A

not untile the concentration of the cGMP drops below , that the cGMP bound to the channel diffuses off and closes the channels

50
Q

what is the cGMP concentration with no light in vertebrates?

A
  • cGMP concentration is high
  • cGMP synthesis low
  • CNG channel activated
51
Q

what is the cGMP concentration with light in vertebrates?

A
  • cGMP to GMP
  • CNG channel closes
  • cGMP synthesis activated
52
Q

what is the importance of ‘off’ being a signal?

A
  • the dark current (no signal from the rod cell to the optic nerve)
  • could explain why vertebrate species have moved into caves
  • lose eyes quickly and eyes need energy (not much food)
53
Q

how is there deep evolutionary function between the vertebrate and the invertebrate eye?

A
  • second order neuronal organsiation mediating vision in mammals and insects is very similar
  • Pax6

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