Photosensation Flashcards
why is light abosrbed?
circadian rhythm, vitamin production and metabolism
- detected to impart information from the world - forms visions
how can light be detected?
ocular (dedicated eye) or extraocular
what is vision?
- the state of being able to see or form image
what aspects can we can sense?
colour, shape, movement and dimension
what are the key points of ocular?
decdicated structure requiring lenses in combination with light gathering structures
what is the primitive eye?
- simple, evolution from primitive cilia on protozoans
- evolved into structures with more cilia that have a bigger surface area for efficient absorption of light
what are the 2 different classes that primitive structures have evolved into?
- invertebrate photoreceptors
- mammalian photoreceptors
what is the different between mammalian and invertebrate photoreceptors?
- orientation of cilia polarity
what are the main types of photoreceptors?
rhabdomeric and ciliary photoreceptors
what are camera eyes?
have smalle aperture with a larger light harvesting structure behind
what are the properties of a lens eye?
- everything in reverse
- the lens allows the focusing of light in a smaller area
- more efficient detection and clarity
what does a corneal eye do?
- need focuse to help catch prey
what are the features of the compound eye?
- 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
what do arrays of photoreceptors sgguest?
an increasing developmental sophsistication
- development relfects increasing sensory sophistication
what is the difference between a convex and a concave eye?
-convex eyes harvest and focus more light than concave
how did the concave camera eye develop?
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
what does the lens allow?
- allows focus and to see further with less light
same amount of light focused on a smaller area
what detects light?
rhodopsins
what are rhodopsins?
- from the family of opsins
- light harvesting
- visual pigmemnt
- a PGCR embedded in the photoreceptor membrane in the villi or cilia
what is the ligand of the rhodopsin GPCR?
ligand is the activation of II cis retinal (sits in the middle of the rhodopsin) to ‘all trans-retinal’ (absorption of light)
what are the two types of rhodopsins?
C; cilliary
R; rhabdomeric
how does the retianl harvest the light?
- 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
what is retinal produced from?
vitamin A derived from dietary beta carotene (in carrots)
what at are the steps that rhodopsin activation follows?
- rhodopsin
- bathorhopsin
- lumirhodopsin
- meta I rhodopsin
- meta II rhodopsin
- back to rhodopsin
what happens as rhodopsin converts to meta II rhodopsin?
as you move through each step there is a drop in energy
- at meta II rhodopsin: the structure activates the associated G protein
what downstream signalling occurs in vertebrates?
- rods hyperpolarise light
- vertebrate ciliary (C) opsins act through phosphodiesterase
what downstream signalling occurs in insects?
- rhabdomeres (R) depolarise to light
- insects: rhabdomeric (R) opsins act through phospholipase C (PLC)
what are the structures detecting light in insect photosensation?
- 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
in insects what are channel openings in the photoreceptor caused by?
single photos
in the photoreceptor 1 photon generates 1 ‘quantum bump’
what is meant by the quantum bump?
- 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
how do the TRP/TRPL channels function?
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
what is the SMC?
- the sub-microvillar cisternae at the base of the rhabdomere
what is the role of the SMC?
originally thought to ba Ca2+ store but more likely to be involved in phosphoinositide regulation
what is the sigla complex?
tightly coupled G protein coupled cascade and motor complex
- represents a really tight biophysical coupling
what is the process of how the sigla complex functions?
- 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
what is the compex held together by?
INAD so theres a really intense coupling
how are the TRP/TRPL channels activated in the sigla complex?
- by DAG and PUFFA
- causes influx of Ca2+ and Na+
what molecules are activated by dissocation from NINAC in the sigla complex?
- Arr2
- Goes back to stop the reaction
how is TRP activation triggered in insects?
- 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
what are the two types of photoreceptors in vertebrate photosensation?
rods and cones
what are the properties of rods?
- 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
what are the properties of cones?
- 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
when are rods and cones both used?
in moderate light
what is the organisation of a vertebrate eye?
- it is convex
- light travels to contact the rods and cones through the retinal layer
- information is processed by the retinal layers
what happens when vertebrate rod photo receptors are hyperpolarised?
they dont generate action potentials
what happens when vetobrates are in the dark?
- 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
how does the vertebrate photoreceptor response compare to the invertebrate?
- 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
how do photoreceptors work in vertebrates?
- 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
how is phototransduction activation a diffusion process in vertebrates?
not untile the concentration of the cGMP drops below , that the cGMP bound to the channel diffuses off and closes the channels
what is the cGMP concentration with no light in vertebrates?
- cGMP concentration is high
- cGMP synthesis low
- CNG channel activated
what is the cGMP concentration with light in vertebrates?
- cGMP to GMP
- CNG channel closes
- cGMP synthesis activated
what is the importance of ‘off’ being a signal?
- 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)
how is there deep evolutionary function between the vertebrate and the invertebrate eye?
- second order neuronal organsiation mediating vision in mammals and insects is very similar
- Pax6
