Photoreception Flashcards
What is the range of electromagnetic spectrum that can be seen by animals?
How do they compare to aquatic animals?
350 nm to 750 nm (visible light)
Very long wavelengths of EM radiation and visible light can penetrate deeper water, which may show aquatic animals have evolved photoreceptors
Photoreceptors:
rhabdomeric photoreceptors
Ciliary photoreceptors
Mainly in insects
- Has microvillar projections (looks like comb)
Mainly in vertebrates
- Single folded cilium
- Disks that contain photopigments
Eyespots
Eyes
Eyespots
- Basic type of eye
- Single cell or regions of cell containing photosensitive pigment
Eyes
- Complex organs w/ groups of specialized cells
- Photoreceptor cells and pigment cells
- Senses light direction + contrast
- Some can form focused images
Types of animal eyes:
Flat-sheet
Cup-shaped
Vesicular
Convex
Flat-sheet:
- Primitive
- Senses light direction and intensity
- Seen in larval forms or as accessory in adults
Cup-shaped:
- Retinal sheet folded to form narrow aperture (limits light entering eye)
- Too much light creates fuzzier image, less light creates clearer image but darker
- Better discrimination of light direction and intensity
Vesicular:
- Lens in aperture to improve clarity and intensity (Refracts light to focus onto singly point on retina)
- Found in mollusks, but more complex ver in vertebrates
Convex:
- Photoreceptors radiate outwards and are exposed
- Annelids, mollusks, arthropods
- Most complex is compound eyes
Compound eyes
- Apposition
- Super-position
(Convex eyes)
Consists of ommatidia (detects light)
- Cornea (like the lens), crystalline cone, rhabdomeric photoreceptors, rhabdom
- Effective at detecting movement
Apposition:
- Ommatidium operate independently (1:1 ratio w/ afferent neurons)
- Diurnal insects
Super-position:
- Ommatidia work together (Many project to single afferent neuron)
- Works well in dim light (Nocturnal insects and crustaceans)
How do you improve resolution of compound eyes?
Increase number of ommatidia
Or reduce size of ommatidia
Parts of vertebrate eye: Anterior chamber (6)
Sclera: White of eye
Cornea: Transparent later
- Helps w/ directing light
Iris: Two laters of pigmented smooth muscle
- Regulate size of pupil
Pupil: Opening in iris
Lens: Focuses light on retina
Ciliary body: Muscles for changinf lens shape
Parts of vertebrate eye: Posterior chamber (3)
Retina: Contains photoreceptors
Choroid: Vascular layer of eye
- Gives pro-survival cues to photopigment cells, degrades cells that have died, provides source of blood to retina
Tapetum: Layer in choroid that reflects light
- Only in nocturnal animals
Blind spot
Aqueous humor
Vitreous humor
(Vertebrate eye)
Blind spot: Where optic nerve exits eye
- No rods or cones here
Aqueous humor: Watery fluid in anterior chamber
Vitreous humor: Gelatinous mass in posterior chamber
- Holds eye in round shape
Light refraction
Bending of light
- When light waves pass at oblique angle into mediums of diff densities
Not so useful in aquatic animals bcuz density of water and liquid in eyes is already similar
- Not much refraction between the mediums
Concave lens
Convex lens
- What happens to image on retina?
Concave:
- Scatters light rays
Convex:
- Causes light rays to converge into focal point
- Focal length is distance from centre of lens to focal point
Image on retina is upside down and reversed from left to right
Accomodation
- Ciliary muscle relaxed vs contracted
Process by which eye adjusts shape of lens to keep objects in focus
Relaxed - Ligaments pull and flatten lens
Contracted - Ligaments release tension and round lens
Visual defects:
Hyperopia
Myopia
Hyperopia:
- Far-sightedness; focal point falls behind retina
- Can be corrected w/ convex lens
Myopia:
- Near-sightedness; focal point falls in front of retina
- Corrected w/ concave lens
Retina
- Rods and cones
- Ganglion cells
- Optic disk
Arranged into several layers:
- Rods and cones at back (tips face backwards)
- Axons of ganglion cells form together to form optic nerve
- Optic nerve exits retina at optic disk (blind spot!
Macula lutea (ML)
Fovea
ML:
- In retina
- Produces sharp visual images
Fovea:
- In middle of ML (where overlying bipolar and ganglion cells are pushed to side)
- Only cones
- Provides sharpest images
Age-related macular degeneration (AMD)
Development of blood vessels under macula (over top of fovea)
- Reduces quality of images received
Loss of pigment epithelium (and choroid)
- No more pro-survival cues, which causes photoreceptors to die
Both cause loss of photoreceptors in macula lutea
Mammalian (ciliary) photoreceptor structure (rods and cones)
Outer segment: Series of disks containing photopigments
Inner segment: Cell body
Synaptic terminal: Makes connections w/ neurons in retina
- Uses glutamate between bipolar cell and photoreceptors (using graded potentials since they’re close together)
Photopigments:
- Chromophore
- Opsin
Steps in photoreception
Bleaching
Photopigments: Molecules that absorb energy from photons
- Chromophore: Pigment derivative if vitamin A (retinal)
- Opsin: G-protein-coupled receptors
1) Chromophore absorbs energy from photon
2) Chromophore changes shape
3) Photoreceptor protein changes shape
4) Signal transduction cascade
5) Change in membrane potential
Bleaching: When activated retinal no longer bonds to opsin
- Activates opsin
Rhodopsin (pigments in rods)
Erythrolabe, chlorolabe, cyanolabe (pigments on cones)
Rhodopsin:
- Decomposes in presence of light
- Triggers reactions that increase/decrease AP firing rate of ganglion cells (impulses travel along optic nerve)
Pigments on cones:
Sensitive to diff wavelengths bcuz of diff amino acid sequences
- Erythrolabe: Responds to red
- Chlorolabe: Responds to green
- Cyanolabe: Responds to blue
Phototransduction pathway in vertebrate photoreceptors (6)
1) 11-cis Retinal absorbs light and isonerizes into all-trans retinal
2) All-trans retinal dissociates from opsin
3) Activates G protein transducin
4) Activates PDE, converting cGMP to GMP
5) Decrease cGMP closes Na+ channel
6) Na+ entry decreased, causes hyperpolarization of bipolar cells (no glutamate release)
*In dark, rods depolarize, increasing glutamate release
Phototransduction in invertebrates (rhabdomeric photoreceptors) (6)
1) 11-cis 3-hydroxy retinal absorbs light, isomerizing into all-trans 3-hydroxy retinal
2) All-trans 3-hydroxy retinal dissociated from opsin
3) Activated opsin activates Gq protein
4) Activates PLC, converting PIP2 into DAG and IP3
5) DAG activates TRP (nonselective) cation channel
6) Ca2+ and Na+ enter cell, depolarizing it
True or false
Nocturnal animals have more rods
True
Cones vs Rods
Cones:
- 1:1 ratio (1 cone w/ 1 bipolar cell w/ 1 ganglion cell)
- Small visual field
- High resolution image
- Concentrated in fovea
Rods:
- Principle of convergence (Many rods w/ 1 bipolar cell, many bipolar w/ 1 ganglion cell)
- Large visual field
- Fuzzy image
Retinal ganglion cells receptive fields:
ON-center vs OFF-center
ON-center:
- Light increases APs in ganglion cells in middle
- Surrounding decreases APs
- If light diffused all over, weak response
OFF-center:
- Middle, decreases APs
- Surroundings, increases APs
- Diffused all over, weak response
