11c photoreception Flashcards
photoreception
a sensory cell’s response to light
three functions of the simplest type of eye
photoreception
shading by dark pigment to allow detection of light direction
connection to motor structures to allow phototaxis
define phototaxis and what a pos or neg phototaxis would mean
the bodily movement of a motile organism in response to light, either toward the source of light ( positive phototaxis ) or away from it ( negative phototaxis ).
visible spectrum
Electromagnetic waves falling in the range that can stimulate an animal’s photoreceptors comprise that animal’s
why does the visible spectrum vary between animal species?
bc it depends on the spectral sensitivities of their photoreceptors
visible spectrum range
390nm - 720nm, on average.
pigment spot
simplest type of eye
adaptive for a facultative autotroph
allowed simple organisms to find light and dark with the help of the flagelllum
found in some protists (e.g., Euglena, Chlamydomonas)
pigment cup
allows detection of light stimulus direction
in comparison to the pigment spot, it has more nerve fibers and the pigment cells are bended
optic cup
encloses a water filled cavity (pinhole opening)
reduces light scatter
increases resolution
found in many flatworms, rotifers, polychaetes, gastropods, chitons, and echinoderms
deeper cup with a clear epithelial covering
further increases resolution
eye is protected from environmental toxins, pathogens
has a photoreceptor layer that will create the future retina
found in Chambered Nautilus (VERY SIMPLE CEPHALOPODS)
eye with primitive lens
eye is able to focus incoming light eye can form clearer images has a now formed retina has refractive lens found in lampreys, fossil hagfishes
complex eye aka camera eye
high resolution
has lens and a cornea
ability to form clear images
additional anatomical adaptations for color vision, etc.
compound eye found in insects and some crustaceans
camera eye found in all craniate vertebrates and cephalopod mollusks like an octupus
spot vs cup
the cup has less of an opening which increases resolution and reduces distortion
and allows only a certain amount of light to come in
based on video… the cup also helps to avoid predators
lens helps by
changing the curvature to adapt to n ear and far vision
why do slow moving or sessile creatures have poor eyesight?
it is more dangerous to move so fast when practically blind so you might as well stay in place to protect yourself to avoid bumping into things
eyesight of slow moving or sessile creatures
simple eyes detect shadows and close movement
eye sight of slow mobile
eyes that aid in navigation, simple image formation
sight of the very fast
high visual acuity for accurate navigation, predation, predator avoidance.
vestigial remnants
primitive structure and no longer believed to be important for survival
define metazoans
any of a group (Metazoa) that comprises all animals having the body composed of cells differentiated into tissues and organs and usually a digestive cavity lined with specialized cells.
Hox gene, PAX6
encodes a transcription factor essential for brain and eye development.
does loss of function in the pax gene effect as homozygous or heterozygous?
effects both and two pox genes that are loss will cause failure of eyes to develop at all
rhabdomeric photoreceptors - (for protostome invertebrates)
maximize membrane surface area with microvilli projecting from the apical surface
apical surface is rotated 90o with respect to ancestral position
This lateral bed of (dendritic) microvilli form the rhabdom.
can vary in shape
second messenger signal transduction via phospholipase C (PLC)/inositol phosphate (IP3) pathway
what are the specialized phtotoreceptor cells that detect and transduce light?
rhabdomeric and cilliary
ciliary photoreceptors
maximize membrane surface area via highly folded cilium (outer segment) projecting from the apical surface of the cell body
form rod and cone photoreceptors
use glutamate as a neurotransmitter
second messenger signal transduction via
phosphodiesterase (PDE) altering [cGMP] pathway
cillary vs rhabsomeric
rhabodemeric
- used for invertebrates
- SA increases with microvilli
- secondary messenger is phospholipase C (PLC)/inositol phosphate (IP3) pathway
ciliary
- used for vertebrates
- SA increases with cilium
- phosphodiesterase (PDE) altering [cGMP] pathway
macula degeneration
can be wet or dry
wet= get vascularization of retina which u don’t want blood getting in the way of where light is coming from
definition: if u lose all of the photoreceptors, u will go blind
define rhabdom
This lateral bed of (dendritic) microvilli which act as the receptors
cause of blind spot
The natural blind spot (scotoma) is due to lack of receptors (rods or cones) where the optic nerve and blood vessels leave the eye.
what creature has both rhabdom and ciliary photoreceptors?
Ancestral bilaterians and larva platenera
role of each photoreceptor in planaris larva
The eyes contain rhabdomeric photoreceptors. These enable the larvae to detect and swim towards light sources.
ultraviolet light activates ciliary photoreceptors, whereas cyan, or blue-green, light inhibits them. Shining ultraviolet light onto Platynereis larvae makes the larvae swim downwards. By contrast, cyan light makes the larvae swim upwards. In the ocean, ultraviolet light is most intense near the surface, while cyan light reaches greater depths. Ciliary photoreceptors thus help Platynereis to avoid harmful ultraviolet radiation near the surface.
ommatidum
contains six to eight sensory receptors arranged under a cornea and refractile cone and is surrounded by pigment cells, which adjust the intensity of light. Each ommatidium can act as a separate eye and is capable of responding to its own visual field.
describe the arthropod/insect compound eye
Insect eyes can contain 3000 (fruitfly) - 25,000 (dragonfly)
rhabdomeric photoreceptors per eye.
The compound eye is composed of multiple ommatidia,
each containing multiple photoreceptor cells.
describe the Cephalopod Camera Eye
The cephalopod retina contains 20,000 - 50,000
rhabdomeric photoreceptor cells/mm2.
describe the vertebrate camera eye
The vertebrate retina contains millions of ciliary photoreceptor cells.
Many vertebrates have a recessed structure, the fovea,
which is the point of highest visual acuity.
The human fovea has ~200,000 photoreceptors/mm2 .
The avian (bird) fovea may have 400,000 - 1,000,000 photoreceptors/mm2, depending on species.
Raptors (birds of prey) have two foveas (shallow and deep foveas where the deep one helps with acuity) . Lots of dots!
define fovea
a small depression in the retina of the eye where visual acuity is highest
pigment
is a substance that changes the color
of reflected or transmitted light via wavelength-selective absorption.
photopigment
biological pigment used by a living organism to absorb light.
absorption spectrum
The range of wavelengths that a photopigment can potentially absorb
comprise the
action spectrum
The range of wavelengths capable of driving a biological process
is the
the pigment is responsible for facilitating that process IF
Close correspondence between the absorption spectrum of a pigment and
action spectrum of a biological process associated with that pigment
what is the peak of the wavelength of the relative absorption?
500 nm
chromophore (11-cis retinal)
Vitamin A (all-trans retinol) is converted in the retina to the 11-cis-isomer of retinaldehyde or 11-cis-retinal. 11-cis-retinal functions in the retina in the transduction of light into the neural signals necessary for vision.
The 11-cis retinal in photoreceptors is covalently bound to an opsin signaling protein to form a visual pigment molecule. In the presence of light, 11-cis retinal is isomerized to all-trans retinal, and the straightening of the polyene chain activates the opsin
opsin
a protein which forms part of the visual pigment rhodopsin and is released by the action of light.
determines the wavelength of light to which pigment is sensitive
is a protein - encoded by DNA, so variable…
among species
among individuals of the same species
within a single individual
All are members of the G protein-coupled receptor (GPCR) superfamily.
chromophore
The prosthetic group (chromophore) confers light sensitivity to the protein.
Type I opsins
bacteria and some algae (all-trans chromophore)
Type II opsins
all eumetazoans (11-cis chromophore)
Upon isomerization
visual pigments trigger a metabotropic
second-messenger cascade, eliciting a receptor potential
rhodopsin
ancient G-protein coupled receptors.
vertebrate rod photoreceptor pigment
extremely light sensitive
photobleaches upon exposure to light
returns to unbleached isomer in absence of light
confers high sensitivity to light/night vision
blue cone
Short Wavelength Senstive (SWS) Opsin has a λmax of 430nm.
green cone
Medium Wavelength Sensitive (MWS) Opsin has a λmax of 530nm.
red cone
Long Wavelength Sensitive (LWS) Opsin has a λmax of 560nm.
blue photopsin vs rhodopsin
many amino acid substitutions
green photopsin vs red photopsin
many amino acid substitutions
red photopsin vs green photopsin
many identical amino acids
plural of rhabdomere
rhabdome
what insect has more ommatidia/unit area than any other arthropod.
dragon fly compound eye
As aerial predators, they have evolved high visual acuity that allows them to localize and capture small, flying prey in mid-flight.
light gathering in the ommatidium
outer/top section
-cornea, derived from cuticular corneagen (epidermal cells that take mucus from cornea mold formed) cells
-crystalling cone, derived from cuticular Semper’s (cone cells) cells
These focus incoming light onto the photoreceptor of the crystal part
light-sensing segment (photoreceptor)
lower/bottom section
-light goes to 7-8 rhabdomeric retinula cells (photosensitive cells)
-Microvilli (dendritic processes) form the rhabdomere of each cell.
Rhabdomeres converge in the center to form the rhadom.
iris
prevent light from scattering among ommatidia, isolating each ommatidium from its neighbors.
optic nerve aka cranial nerve II
transfer visual information from the retina to the vision centers of the brain via electrical impulses.
Retinula cell axons pass through a basement membrane
protocerebrum
(visual center), located in the anterior portion of the brain
where visual stimuli are processed and output is generated.
location of primary and secondary iris/pigment cells
closest to the cornea: primary iris/pigment cells
closest to the base:
what two types of ommatidum compound eye have?
photopic (apposition)
scotopic (superposition)
photoptic
have a crystalline cone touching (in apposition to) the rhabdome
are the most common type of compound eye
are found in diurnal insects
have low sensitivity to light
confer high resolution
have relatively short ommatidia, each of which
has a relatively small field of view
produces an inverted image
The rhabdom averages the light received by the eye.
Multiple, inverted images are combined (and flipped) in the brain
scopotopic
have a “clear zone” between crystalline cone and rhabdom
The “clear zone” contains crystalline tracts
These arise from retinula cells and act as “light guides”
phototopic vs scopotopic
phototopic is during the day time so it is diurnal, very common, has low sensitivity, high resolution, short ommitilda, and has inverted images
scopotopic is the opposite
causes of photopic having short ommatilda?
has a relatively small field of view
produces an inverted image
The rhabdom averages the light received by the eye.
eff of scopotopic having long ommatilda?
At night, pigments in secondary pigment cells migrate to cell apices
Incoming light can stimulate unshielded rhabdomes of neighboring ommatidia
Images from each ommatidium are superimposed on each other.
Visual input to the brain comprises a single, un-inverted image.
cornea
transparent, colorless “window” transmits light into the eye helps focus light onto the retina.
pupil
aperture (hole/opening) through which light enters the eye
iris
pigmented “shutter” contracts or relaxes to change aperture, controlling light admission.
lens
transparent, colorless crystalline disc fine-focuses light onto the retina.
anterior chamber / segment
fluid-filled space between cornea and iris
anterior humor
fluid in the anterior chamber
posterior chamber/segment
fluid-filled space between lens and retina
posterior humor
aqueous gel in the posterior chamber
retina
layer of specialized neurons that lines proximal interior of the eyeball
senses light, generates RPs and APs that travel through the optic nerve to the brain
fovea
small, recessed retinal spot with the highest cone density and thus, highest visual acuity
macua
(primates only) unrecessed retinal spot with a high density of cone photoreceptors, and housing the fovea.
facet
corneal lens for individual ommatidium
primary iris/pigment cells
closest to the cornea
secondary iris/pigment cells
closest to the base:
