How the eye sees Last time Anatomy of the eye Rods and cones Visual receptors Color Vision This time...
-
date post
22-Dec-2015 -
Category
Documents
-
view
225 -
download
1
Transcript of How the eye sees Last time Anatomy of the eye Rods and cones Visual receptors Color Vision This time...
How the eye sees
Last timeAnatomy of the eyeRods and conesVisual receptorsColor Vision
This timeVisual transductionEye to brain
1
Rods and cones have different visual receptorsThe visual receptors are G protein-coupled receptors
• seven transmembrane regions
• hydrophobic/ hydrophilic domains
• conserved motifs• chromophore stably
attached to receptor
(Schiff’s base Lys296 in TM7)• thermostable
2
Nomenclature for visual receptorsReceptor == GPCR, opsinLigand == chromophore, retinal, pigmentReceptor bound to ligand == rhodopsin
The visual cascade is a G protein-coupled cascade
Rhodopsin Gtransducin phosophodiesterase cGMP to GMP close cGMP channels
4
Negative regulation of phototransduction
Rhodopsin Gtransducin phosophodiesterase cGMP to GMP close cGMP channels
7
Closing cGMP channels causes a decrease in Ca2+
Decrease in Ca2+ activates
1. Rhodopsin kinase === deactivate receptor
2. Guanylate cyclase === converts GTP to cGMP === opens cGMP channels
Ca2+ independent deactivation also occurs
1. GTPase activating protein
Turning ON Guanylate Cyclase
Guanylate cyclase GTP--- cGMP opens cGMP channels GCAP (less Ca)
Phosphodiesterase cGMP--- GMP closes cGMPchannels transducin
ENZYME REACTION ACTION on CHANNELS ACTIVATED BY
Ca-dependent,Decrease in Caactivates GC
9
Dark noise is very low
1 rhodopsin/minute108 rhodopins/ photoreceptor1000 years for all rhodopsins to turn over
14
High amplification increases signal size and reliability
Rhodopsin Gtransducin phosophodiesterase cGMP to GMP close cGMP channels
1 100 100 100,000 ~1000
15
Properties of phototransduction
• responds to 1 photon of light• responses are extremely reliable• high amplification of signaling• low dark noise
•1000s of discs maximize surface area of light detection• high concentration and thermostability of rhodopsin means high detection, low noise
Photoreceptors are highly specialized to detect light!
16
Phototransduction: Differences between rods and cones
Rods ConesVery sensitive to light 30x less sensitive to light
each rhodopsin activates 30x less G proteins
17
The signaling pathway for Drosophila phototransduction
Fastest GPCR cascade measuredNo amplification
19
Structure of the eye
The Basic Retinal Circuit
1. Receptor Cells(rods and cones)
2. Bipolar Cells
3. Ganglion Cells
Connectivity in the retina
Back of eye
Front of eye
4. Horozontal Cells
5. Amacrine Cells
6. Pigment cells
20
Flow of visual information in the retina
Vertical Connections
Back of eye Front of eye back of eyePhotoreceptor Cell---Bipolar Cell---Retinal Ganglion Cell---Brain
Horozontal Connections
Horozontal Cells- connect photoreceptors and bipolar cellsAmacrine Cells- connect bipolar cells and retinal ganglion cells
light
22