Vision Flashcards
What is photoreception?
The ability to detect a portion of the electromagnetic spectrum between UV and IR.
What is the minimal criteria to be considred an eye?
- Photoreceptors
- Determine directionality of light
Despite the minimal criteria to be an eye, they are usually a complex organ and…
- include receptor cells
- pigmented epithelium
Where are flat sheet eyes seen?
In larval eyes or as accessory eyes.
Describe cup shaped eyes.
Curved eye creates a narrow aprture (allows less light into the eye)
Describe convex eyes.
Present in annelids, molluscs and arthropodia.
What are compound eyes composed of?
Ommatidia
What are ommatidia?
- modified region of cuticle (cornea) overlying a crystalline cone that forms a lens
- below lens, retinular cells (rhabdomeric cell receptors)
What are vesticluar eyes?
A lens help solve the compromise between image clarity and image intensity.
List the structures related to function in the vertebrate eye.
- Sclera
- Cornea
- Iris
- Ciliary body
- Lens
- Fovea
- blindspot
What does the vertebrate retina look like?
What are the two types of photoreceptors?
Ciliary and Rhabdomeric
What do photoreceptors do?
Transduce light signal
Describe ciliary photoreceptors.
Single cilium with highly-folded ciliary membrane (disks containing photopigments). Vertebrates.
Describe rhabdomeric photoreceptors.
Microvillar - projections contain photopigments. Jellyfish, drosophila.
Describe signalling in rhabdomeric photoreceptors.
- Gq protein activates PLC pathway
- IP3 and DAG pathways open nonselective cation channels
- Ca2+ and Na+ enter the cell
- Cause a depolarizing receptor potential
- NT release is increased
What are the two types of photoreceptors in vertebrates?
Rods and cones
What is a photopigment?
Chromophore associated with specific receptor protein.
How are the chromophore and receptor protein binded?
Covalently
Receptor proteins in photopigments are…
GPCR and members of the opsin family
Give an example of a photopigment.
Retinal + opsin = rhodopsin
Describe
Describe the unactivated state of rhodopsin.
- chromophore in cis conformation
- bound to opsin
Describe the activated state of rhodopsin.
- chromophore undergoes conformational change to trans
- released from opsin (process called bleaching)
- recycled (trans –> cis) in the RPE
What enzyme reverses the isomerization of retinal?
Isomerase
Describe ciliary photoreceptor transduction.
What does bright light cause?
- Lowest levels of cGMP
- Most Na+channels
What happens to photoreceptors in the dark?
- cGMP levels are high; cGMP binds to ion channels
- depolarization is triggered
What happens to photoreceptors in dim light?
- lower levels of cGMP present
- some ion channels close
Show rhodopsin inactivation.
Describe how the use of glutamate (inhibitory NT) is utilized in darkness and in light by photoreceptor cells.
How do vertebrates recycle chromophores?
- released trans form exported to adjacent epithelial cells
- converted back to cis form
- exported back to photoreceptor cells
Horizontal cells ensure…
that only signal from intensely lit photoreceptor cells reaches the ganglion (lateral inhibition).
How do horizontal cells inhibit the activity of the bipoalr neurons connected to the photoreceptors at the center of the receptive field?
Photoreceptors synapse with bipolar neurons and with horizontal cells. Conflicting signals to these bipolar neurons ersults in reduced signal to retinal ganglion cells.
Show the difference between light on the centre of the receptive field and light on surround.
What is the pathway of APs from the retina to the brain?
Optic nerves –> optic chiasm –> optic tract –> lateral geniculate nucleus –> visual cortex
Describe binocular vision.
- eyes have overlapping visual fields
- binocular zone
- combine and compare information from each eye to form a 3D image
- depth perception
