Chapter 15 Flashcards
Pigment part of eye functions
Prevent light scattering
Renews photoreceptors
Neural part of eye cells
Photoreceptors: rods + cones
Bipolar cells
Ganglion cells (give rise to optic nerve)
Optic disk (blind spot)
Photoreceptor structure
Outer segment: many disks for light, many visual pigment complexes in each disk
Inner segment: organelles
Synaptic terminal: neurotransmitter release
Capturing light structures
Plasma membrane: phospholipid bilayer
Visual pigment: retinal and opsin
Opsin: transmembrane G protein
Retinal: light absorbing chromophore (vitamin A)
Resting state (dark)
Cyclic guanosine keeps cGMP-gated channels open
Na+ ions free to enter cell
Photoreceptors depolarize
Neurotransmitters continuously released
Bipolar cells inhibited
Steps of phototransduction
1: Opsin activation: light photon absorbed by photopigment, isomerization of retinal (11-cis to 11-trans)
2: Activation of transduction and PDE: Opsin activates transducin (G-protein, “on” switch)
3: cGMP levels drop and Na+ channels close, PDE breaks down cGMP to GMP
Active site in the light
Na+ channels in outer segment closed
Membrane is hyperpolarized
Rate of NT release drops, as light increases glutamate release decreases
Regeneration of photopigment
Bleaching (light): cis to trans-retinal, the eye is exposed to bright light, causing rhodopsin to split
Regeneration (dark): cis-retinal binds to opsin, reforming functional photopigment
Color vision
3 cone opsins (red, blue, green cones)
Retinitis pigmentosa
Tunnel vision
Degeneration of photoreceptors
Color blindness
missing cones
Night blindness
Poor vision in low light
Vitamin A deficiency
