Vision Flashcards
What is Photoreception?
The ability to detect a portion of the electromagnetic spectrum (300 nm – 1000 nm)
What range can animal detect light?
700nm to 400 nm
What do vertebrates and invertebrates have for photoreceptors?
Vertebrates- Ciliary
Invertebrates- All over
Are photoreceptors phylogenetic?
No.
What are the two types of vertebrate photoreceptors? Define each
Rods- Low light; black/ white images; peripheral vision; high sensitivity; not sharp
Cones- Hight light; colour images; central vision; low sensitivity; sharp images
How are vertebrate photoreceptors defines?
Not on shape but function as shapes vary greatly
What is visual transduction?
Light must be converted by photopigments in the photoreceptors into signals the brain can interpret
What are photopigments?
- Convert light into signals.
- Chromophore (e.g. retinal) and an associated protein (an opsin)
- Ex: rhodopsin, iodopsin, melanopsin, etc.
What are animal opsins type? Bacterial and algal?
Animal
Type II; G-coupled receptors (proteins bind and respond to ligand)
Bacterial and algal
Type I
What is the chromophores shape in darkness?
Bent= cis conformation
What are the chromophores shape changes in light?
-Isomerization
Absorption of a photon of light causes cis- to straighten into the trans-
-Bleaching
Trans-chromophore completely separates from the opsin
-Regeneration
An enzyme converts trans-chromophore→ cis-chromophore
What are the steps of visual transduction? (Light to dark)
- Light*
1. Rhodopsin (opsin + cis-retinal) absorbs light rays
2. Cis-retinal transformed to trans-retinal (Isomerization)
3. Trans-retinal disassociates from opsin as opsin becomes activates (bleaching) - Dark*
4. Trans-retinal is converted to cis-retinal in pigmented epithelium using ATP
5. Cis-retinal associates with opsin to reform rhodopsin
Describe Phototransduction in rhabdomeric photoreceptors.
AP increases NT release that goes to the nervous system and is interpreted as light
- Cis-retinal absorbs light and transforms to trans-retinal
- Trans-retinal dissociates from opsin
- Opsin activates a G protein
- G protein activates PLC converting PIP to DAG and IP
- DAG activates TRP cation channel
- Ca2+ and Na+ enter cell and depolarize it.
Describe Phototransduction in vertebrate (ciliary) photoreceptors.
G protein inhibitory: causes Na+ channel inhibition, so reduced Na+ influx with continued K+ efflux – cell hyperpolarizes and NT release decreases. Graded response.
- Cis-retinol absorbs light and transforms to trans-retinol
- Trans-retinal dissociates from opsin
- Opsin activates a G protein transduction
- Transduction activates PDE converting cGMP to GMP
- Decreased cGMP closes Na+ channel
- Na+ entry decreases and hyperpolarizes the cell.
Describe transduction in dark.
- Photoreceptor is depolarized
- Ca2+ channels ope and inhibitory neurotransmitter is released to bipolar cell
- Bipolar cell is hyper polarized and inhibited
- Ca2+ channels close in bipolar cell NT not released
- No signal to ganglion
Describe transduction in light.
- Photoreceptor is hyper polarized
- Ca2+ channels close and inhibitory NT is not released
- Bipolar cell not inhibited and depolarizes
- Ca2+ channels in bipolar cell open and excitatory NT released
- Signal sent to brain (nerve signal initiated)
What makes a structure an eye/eyespot?
Must detect direction of light
What are eyespots? What are they found on?
- Single cells or part of a cell
- Sea Stars, unicellular organisms, animal groups
What are eyes?
- Consist of cells specialized for different functions
- Can provide light direction, light dark contrast, form focused images, and detect colour
Describe 4 types of eyes
- Flat sheet (ex: Patella)
- Layer of photoreceptor cells
- Form a primitive retina
- Light direction
- Light dark contrast
- Often in larval forms or as accessory eyes - Cup shaped (ex: jellyfish, insects, slugs and snails)
- Retinal sheet folded forms narrow opening
- Better light direction and intensity discrimination
- pinhole-sized opening illuminates one spot on the retina.
- No lens.
- Compromise between image clarity and brightness. - Vesicular (Vertebrates and cephalopods
- Lens in place- takes many light sources and focuses them - Convex (annelids, molluscs and arthropods)
- Photoreceptors radiate from a base, forming convex surface
- Compound eye made of ommatidia
What is the convex eye shape in hermit crabs?
- Ommatidia (parts of compound eye) organized for light detection, not visual acuity
- Compound eye like insects
- On stalks: wider field of vision and binocular vision
What are scallops eyes? How many?
- 10-100 Concave, parabolic mirror eyes (light isn’t focused with a lens)
- Double layer retina (outer for light, inner for darkness)
What does the cornea do?
Bends light entering the eye (refraction)
What does the lens do?
Change shape (accommodation) -functions to change the focal distance of the eye
How many photoreceptors do cephalopods have?
One photoreceptor (rods) No blindspot- retina in front of nerves
Describe the function of the:
Lens, Iris, Pupil, Cornea, Retina, Choroid, Fovea, Optic disk, and Optic nerve
- Lens-changes shape to allow a sharp image
- Iris- Controlling the diameter and size of the pupil and thus the amount of light going in
- Pupil- Control amount of light going into eye
- Cornea- Protection and focus or bend light
- Retina- Contains photoreceptors; at back of eye; receives light and converts to signals
- Choroid- provides oxygen and nourishment to the outer layers of the retina
- Fovea- only cones; produces the sharpest vision and greatest colour discrimination
- Optic Disk- the point of exit for ganglion cell axons leaving the eye.
- Optic Nerve- transmits electrical impulses from the retina to the brain
Describe the visual pathway of the human eye
Cornea-> Pupil -> Aqueous Humor -> Lens -> Vitreous humor -> Retina -> Horizontal cells -> bipolar cells -> amacrine cells -> ganglion cells -> optic disk -> optic nerve -> brain
Describe signal processing in rods and cones. (do they converge?)
Rods- Converge
Cones- Don’t Converge
Whats lateral inhibition?
Excited horizontal cells inhibit neighbouring bipolar cells to reduce signal in diffuse light
Whats the optic chaism?
Where some axons cross to opposite side.
Where do visual axons enter the brain?
- Axons enter the brain matter as the optic tracts
- Synapse with neurons that project to the visual cortex in the occipital lobes
