Vision Flashcards
Three layers of the eye, outside to inside
sclera, choroid, retina
Developmental history of the eye
Part of the diencephalon, called to optic cup, anatomically part of the CNS
outer segment
where the detection of light occurs
outer nuclear layer
alignment of the nuclei for the receptor
outer plexiform layer
first synapses with the bipolar and horizontal cells
inner nuclear layer
soma and nuclei of bipolar cells
inner plexiform layer
bipolar cells synapses with ganglion cells (also contains amincrine cells)
ganglion cells
projection neurons with long axonal projections that travel in the nerve fiber layer and exit the eye at the optic disc to form the optic nerve
phototransduction
the conversion of a light stimulus to a voltage response. in the dark - 40 mV. light causes a HYPERPOLARIZATION. light causes the cGMP-gated channel to close, this hyperpolarizes the membrane because the K+ outflow channel is still active
photopigments
11-cis retinal bound to an opsin (G protein coupled membrane receptors in the outer discs). activated when converted to all-trans retinal (retinal comes from beta carotene in the diet –> cleaved to all-trans retinal then can be cleaved to retinol or vitamin A)
Opsins
Rods: rhodopsin (peak absorption is blue/green). Cones: 3 types, L (red), M (green), S (blue)
rod outer segment 1 photon
can cause 1 mV hyperpolarization
retinal pigment epithelium
outermost layer of the retina, has special supportive roles for photoreceptors
retinoid recycling
all-trans retinal dissociates from the activated opsin —> becomes all trans retinOL –> binds to IRBP –> endocytosed by the retinal pigment epithelium –> conversion to 11-cis retinal and oxidation to retinal —> 11-cis IRBP complex is then shuttled to a photoreceptor
disc removal
lifetime of a disc: 12 days. phagocytosed by the retinal pigmented epithelium
L and M opsins
closely related on the x-chromosome (high chance of crossover, deletion, or duplication). red/green colorblindness is very common in men. perception of red versus green is ambiguous
Cones
location: central (fovea), abundance: 5%, opsins: L, M, S, convergence on to bipolar cell: 1 to 1, sensitivity: low (bright light), response time: fast, functional role: central vision, color discrimination in bright light, fine spatial resolution 1:1 relation with bipolar cells.
retinitis pigmentosa
degenerative disease caused by the progressive loss of rods over decades causing pigment changes in the retina. symptoms include poor vision in low light conditions or “night blindness” and peripheral vision restriction
rods
location: periphery, abundance: 95%, opsins: rhodopsin, convergence on to bipolar cell: 100 to 1, sensitivity: high (dim light), response time: sluggish, functional role: peripheral vision, monochromatic, sensitive to dim light, poor spatial resolution
receptive field
the region of the body that when appropriately stimulated elicits a action potential
ganglion cell receptive field
the location on the retina that elicits a action potential when appropriately stimulated by light. the receptive field is smallest in the fovea
simulation of the surround…
produces a response that opposes the center
photoreceptors release what, when?
glutamate, in the dark
the determination of on/off behavior is set by…
the bipolar cell based on it’s response to glutamate (which is high in the OFF state). On center bipolar cells has mGluR6 (cause cGMP gate closure when stimulated by glutamate). OFF center bipolar cells have AMPA and kainite receptors (outer plexiform synapse) which causes depolarization in response to high glutamate.
lateral inhibition: horizontal cells
projections in the outer plexiform layer, in response to glutamate, release at inhibitory GABA-ergic synapses. Inhibition by one cells to it’s cells on opposite sides
two benefits of lateral inhibition
- amplifies local differences 2. provides a mechanism to adapt to the background level of illumination (extend the range of brightness level based perception levels)
rod based vision, cone based vision, mix of both
scotopic, photopic, mesopic (moonlight)
change in surround inhibition
allows for sensory adaptation
two ways for sensory adaptation to occur in the eye
- changes in surround inhibition 2. modulation of the phototransduction process via a constant increased light level –> causes a decrease in intracellular Ca2+ which increases cGMP affinity, a higher level of cGMP (through increased guanylate cyclase activity), increased rhodopsin kinase activity
Images are…
FLIPPED as they enter the retina (plz don’t forget this)
retino-geniculo-calcarine pathway
retina —> lateral geniculate —> primary visual cortex
