59. Retinal Anatomy/Physiology Flashcards
What are the functions of the following retinal cell types:
- Photoreceptors
- Horizontal Cells
- Bipolar Cells
- Amacrine Cells
- Ganglion Cells
Photoreceptors: outer retina, transduce light into current
Horizontal Cells: outer retina, lateral INHIBITION
Bipolar Cells: carry excitatory signals from outer to inner retina
Amacrine Cells: INHIBITORY processing/modulation in inner retina
Ganglion Cells: sole output cells, axons form optic nerve
What is the difference between cones and rods?
What is the name for vision that is all rods/all cones/mixed
Cones: bright light, insensitive; color vision (R, G, B cones = L,M,S), resolution and acuity, 100% fovea (no blue cones though)
Color blind = missing one cone type
Rods: dim light, sensitive; black/white vision, detection of objects, periphery around fovea
Scotopic: rods only - lowest intensity light
Mesopic: mixed
Photopic: cones only - highest intensity light
Describe the pathway for Phototransduction in Rods
GPCR: Rhodopsin (Rh)
Ligand: all-trans-retinal chromophore (changes from 11-cis retinal with photon)
G-protein: Transducin (Amplification: 1 Rh activates 1000 transducin)
Effector mLc: PDE - hydrolyzes/inactivates cGMP (Amplification: 1 PDE hydrolyzes 50 cGMP)
2nd Msgr: cGMP (lack of)
Target: cGMP cation channels (Amplification: close 1 channel blocks 10000 ions/sec of entry
Active Rh closes channel - hyperpolarizes rod - decreased sustained photocurrent
(in darkness, rods constantly releasing Glu)
Describe the ON and OFF pathways for Cones
Dark: all cones release Glu to both paths
OFF: mGluR on OFF bipolar cells (excitatory in dark)
light - less Glu - inhibits OFF bipolar cell - causes inhibition of OFF RGC
ON: sign-inverting mGluR on ON bipolar cells (inhibitory in dark)
light - less Glu - disinhibits ON bipolar cell - excites ON RGC
What is the midget path in fovea?
Single Red/Green Cone has 1:1 contact with ON and OFF midget bipolar cell - 1:1 contact with ON and OFF midget ganglion cells
supports high acuity and sole red/green color vision depending on cone
How do Rods signal?
Piggyback off Cone pathway
Only “ON” - contact A2 Amacrine cells
Excited A2 amacrine cells = excite “ON” bipolar cells on cones (Glu) and inhibit “OFF” bipolar cell of gones (Gly)
“ON” and “OFF” RGCs handle BOTH rod and cone vision!
