Vision and eyes Flashcards Preview

3rd year - Sensory Ecology > Vision and eyes > Flashcards

Flashcards in Vision and eyes Deck (36):

how do you calculate the energy per photon?

E= hv = hc/WL
h= Planck's constant
v = freq
c/WL = freq


what is the average natural irradiance at these levels?
full moon

sunlight 10^20
sunrise/set 10^18
twilight 10^16
full moon 10^14
starlight 10^12


what are two outcomes of a molecule absorbing a photon?

1. high energy - vibration, electrons jump to higher energy levels. this can result in fluorescence, change of shape, chemical reaction, or break apart molecules. (night is essential for bodies molecules to replenish)
2. low energy - Electron is stripped off and the molecule is ionised.


what is the basis of 3 types of cones in the retina?

long wave pass/red filters
short wave pass/blue filters
intermediate - green filters


what cells are in the retina and in what order?

central - ganglion cells
acramine cells
bipolar cells
horizontal cells
cone and rods - flat discs contain the photoreceptors, containing rhodopsin


what causes rhodopsin to convert into what?

Cis isomer under light converted into a trans isomer. Enzyme used to regenerate cis, this is a costly process. regeneration involves pigment migration and metabolic energy in the night to restore eyes in a fresh state.


how is luminance and different colours measured in the human eye?

luminance - overall light level = R+B+G
colour eg green: R+B-G


what are 2 steps in the process of vision?

1. Trichromatic stage
trichromatic cone cells respond positively to one of 3 freqs of photons arriving at cell surface.
2. Opponent process stage
Opponent cells are near cone cells and detect 3 colour channels. Cg - excited by R, B, inhibited by green etc.


What is photon univariance?

photons have the same effect in transduction, regardless of the WL.
therefore, sensitivity of a photoreceptor is determined by its absorbance.


Describe the structure of rhodopsin

Made of 7 Trans-membrane alpha helices, with mainly non polar AAs so can float in the cell membrane easily, allows for high density in stacking.
retinal attachment site on 7th helix.


are opsins common in the animal kingdom?

hundreds of vertebrate opsins have been sequenced and has helped in understanding vertebrate vision and its evolution. opsins are highly conserved and probably homologous throughout the animal kingdom,
Common molecular ancestor.


describe the structure of a rod cell

scotopic vision
inner segment contains cell organelles and specialist features of neurones.
outer segment contains stacks of discs of lipid membrane - large SA and volume of visual pigments embedded in the membranes. 50% of outer segment is filled w rhodopsin.
Remnant cilium joins inner and outer segment = rods and cones evolved from an ancestral ciliated cell type.


what are some physiological differences between rods and cones?

rods - more sensitive, longer time constants for internal physiological processes (work slower), in most vertebrates they work at low light levels.
cones - opposite.


what is a pigment made of?

opsin (7 thrans membrane helice structure) + chromophore (aldehyde of vit A. In rhodopsin it is called retinal {A1}).


what is one cause of blindness?

vit A deficiency, as retinal cannot be synthesised (retinal,A1, is the chromophore in rods, alternatively in very few terrestrial animals, it is A2, 3,4,di-dehydroretinal).


which animals use 34 di-dehydroretinal in their chromophore?

in very fewterrestrial, but common in aquatic FW, mainly fish.
opsins based on 3,4di-dehydroretinal are called porphyropsins.


describe the vertebrate photoreceptor response to a flash of light

arrival of light is encoded by hyperpolarisation.
transduction leads to the a change in the photoreceptor potential caused by ion movement, causing release of NT.
- amount of change in membrane potential is related to the brightness of the light (graded response) – how many
photons fall on a photoreceptor and are absorbed in a period of time.
- the transduction cascade is an amplifier, as one photon can isomerise one rhodopsin, which then can activate around 700 molecules of transducin.


What causes light adaptation

transduction system is being regulated.
in the dark - more amplification of the light signal. down regulated in brighter conditions. Involves calcium.


How many eye types are there?

Vertebrates have 2
inverts have all 10


When did eyes evolve and what pushed this?

530mya earliest fossils with large eyes
Cambrian explosion - 5m animals w complex eyes appeared. increase in oxygen allowed transition from crawling on sea floor to swimming.


describe an early compound eye

Trilobite - hexagonal design
lenses made of calcite, not protein as insect eyes are.
v large eyes, anomalocaris had 15000 facets.


where did camera eyes first evolve?

in jawless fish
Haikouichthys are earliest fossil evidence.


How fast did eyes evolve?

Nilsson and Pegler: very fast. suggested 40,000 generations for an eye like ours to evolve from just a light sensitive patch.


Do all animals require good vision?

beh function determines why different eye types have evolved.
visually guided beh needs acute vision whereas others work with low resolution vision.
the world is blurry to the majority of animals.


what are 4 stages of eye evolution? examples of each ad a new invention which allowed it.

1. Non directional monitoring of ambient light. drives circadian rhythm, control of pupil size and melatonin release eg in cephalopods.
2. Directional light sensitivity - phototaxis and alarm responses. Screening pigment and membrane stacking.
3. low spatial resolution - pinhole camera nautilus eyes, habitat selection and orientation.
4. high spatial resolution, for predation. compound eyes. Focusing optics


5 groups which have simple/camera eyes and 5 with compound eyes

simple/camera: Vertebrates,
annelids, cnidaria,
Compound: Platyhelminthe, echinoderms


what types of eyes do inverts have?

10 types:
8 compound - 3 superposition, 5 apposition
2 - mirror and simple.


whats the difference between apposition and superposition compound eyes?

- Apposition - focus all light from one lens onto 1 cell. , 'fused rhabdom' of 7 cells, microvilli interdigitated. pigment around cells screen light between ommatidia.
- Superposition - clear zone beneath lenses focuses light from multiple lenses to one cell, either use a telescope, mirror or lens mirror combination.


whats the structure of an ommatidia

top: lens
crystalline cone
visual cells with microvilli pointing into the centre lined with rhodopsin, forming the rhabdom.


Describe the eye structure of shrimp

reflecting mirror eye type
square facet structure so easy to spot
design inspired x ray satellite telescope


Whats strange about scallop eyes?

approx 100 eyes, very distributed visual system.
2 retinas in each eye, unknown why. mirror at the back of the eye
sensitivity and lack of chromatic aberration


what is chromatic aberration?

effect produced by refracting different wavelengths of light through slightly different angles, resulting in a failure to focus.


describe the eyes of a spookfish

tubular eyes, with increased sensitivity.
secondary eye at a right angle to the primary and of a very different structure, focuses a second image on the retina.


which is the tiniest eye?

Fairy fly
facets 5-10microns
worse acuity than shrimp but can see well enough to fly.


which is the largest eye?

Giant squid.
27cm across with a 9cm pupil. bog size advantageous for spotting dim bioluminescence created by a predator moving through the water.


fan worms (Sabellids)

unusual - has distributed eyes on many radiols, or alternatively 2 consolidated eyes on modified upright radiols.
display a progression of eye designs, from separate ocelli to clusters of small lenses to compound eyes