Anatomy, Biochemistry, & Physiology of Photoreceptors
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General Structure of Rods and Cones
"pigmented base" of retina -
outer segment (pigmented discs)
connecting stalk
inner segment (mitochondria)
outer fiber
cell body (nucleus)
inner fiber
synaptic ending -
General Structure of Rods and Cones
"neural layer" -
bipolar cell
ganglion cell (axons carried to brain by optic nerve) - outer segment
- contain membrane-bound discs with pigments that absorb and react to light
- rods
- pigment discs stacked like pennies all the way to the base, membranes are DISTINCT from the plasma membrane
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1. sensitive to dim light (night vision)
2. respond to ALL wavelengths (colors)
3. only "grey" information to the brain
4. 100 rods per ganglion cell to brain
5. widely spread throughout the retina
6. not good for visua - rods
- cones
- pigment discs taper off toward the base, membranes are CONTINUOUS with the plasma membrane
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1. require bright light for stimulation
2. different cones have different pigments specific for certain wavelengths (colors)
3. can convey color information to brain
4. 1-3 cones per ganglion cell to brain
5. primarily concentrated in - cones
- opsin
- transmembrane protein in the membrane of pigmented discs of rods and cones
- retinal
- light absorbing molecule that changes shape when struck by a photon of light
- precursor to retinal (eat your carrots!!!!!!)
- vitamin A
- non-activated form of retinal, prior to absorption of photon energy; has a "kinked" double bond
- 11-cis isomer of retinal
- activated form of retinal, after struck by photon of light; double bond straightens out
- all trans isomer of retinal
- visual pigment in rods; in membranes of pigmented discs of outer segment
- rhodopsin
- breakdown of rhodopsin after the absorption of light
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bleaching of pigment
11-cis retinal + scotopsin ~
rhodopsin + light ~ all-trans retinal + scotopsin - causes HYPERPOLARIZATION of rod
- all-trans retinal
- all-trans retinal
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a. Na+ channels (open in dark) are closed
b. rod is hyperpolarized (increased negativity)
c. Ca++ channels in synapse close
d. less neurotransmitter released by the rod - 3 distinct pigments in cones are sensitive to 3 different parts of visible spectrum
- photopsins
- maximum sensitivity at 455 nm
- blue cones
- maximum sensitivity at 530 nm
- green cones
- maximum sensitivity at 625 nm
- red cones
- differential activation of each of the three different cones
- different colors
- inherit gene for one of the photon proteins that is deficient (mainly male), most common are red and green mutations
- color blindness
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a. rhodopsin in rods is quickly bleached out
b. sensitivity to shallow light disappears
c. rods are inhibited by other retinal cells
d. cones are activated to take over (5 mins.)
e. consensual pupil reflex - constriction -
Light and Dark Adaptation of Rhodopsin
1. light adaptation - very dark ~ very bright -
a. cones are gradually cease to be stimulated
b. "bleached out" rods can produce rhodopsin
c. rods eventually take over in the dim light
d. pupillary dilation - pupils increase size - dark adaptation - very bright ~ very dark
- deficiency in function of rods during dim-light situations
- nyctalopia (night blindness)
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nyctalopia (night blindness) - deficiency in function of rods during dim-light situations
general cause - vitamin A deficiency
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The Visual Pathway: Photoreceptors to Occipital Cortex
photoreceptors (rods & cones) ->
bipolar cells ->
ganglion cells (axons = optic nerve) -> - RETINA path
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The Visual Pathway: Photoreceptors to Occipital Cortex
optic nerves (from each eye retina)->
optic chiasma (medial fibers X over)->
optic tracts (opposite visual field)-> - AXON PATH
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The Visual Pathway: Photoreceptors to Occipital Cortex
lateral geniculate body of thalamus -> - THALAMUS
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The Visual Pathway: Photoreceptors to Occipital Cortex
optic radiation (fibers to cortex) - AXON PATH
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The Visual Pathway: Photoreceptors to Occipital Cortex
occipital lobe - primary visual cortex - CEREBRAL CORTEX
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other brain areas that receive visual information:
for control of extrinsic eye muscles - superior colliculi
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other brain areas that receive visual information:
mediate pupillary light reflexes - pretectal nuclei
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other brain areas that receive visual information:
circadian rhythm - suprachiasmatic nucleus of hypothalamus
- two eyes have overlapping regions of the visual field, so that the same point is seen from slightly different angles
- binocular vision
- a result of binocular vision in which person can perceive relative distances based on information gathered in both eyes
- depth perception