Transduction Flashcards
Define phototransduction.
The conversion of light energy to an electrochemical response by photoreceptors
How is phototransduction initiated?
Visual pigments in photoreceptors (cones & rods) in the retina absorb photons of light.
Compare and contrast rods and cones.
R / C
- no color vision (1 VP) / color vision.(3 VP)
- high sensitivity (dim) / low sensitivity (bright)
- low acuity (multi-rod: 1 gang) / high acuity (1 rod: 1 gang)
- more numerous / less numerous
- mostly in peripheral retina / mostly in central retina
How do photoreceptors translate light into electrical impulses?
Retinal, a light-absorbing molecule, combines with proteins called opsins to form 4 types of visual pigment.
How are signals transmitted in the retina when it is dark?
- cGMP-gated channels open, allowing NA+ + CA2+ influx. Photoreceptor depolarizes.
- Voltage-gated CA2+ channels in synaptic terminals open.
- Neurotransmitter is continuously released.
- Neurotransmitter creates IPSPs in bipolar cell. Hyperpolarization.
- Hyperpolarization closes voltage-gated CA2+ channels, inhibiting neurotransmitter release.
- No EPSPs occur in ganglion cell.
- No action potential occurs.
How is visual pigment regenerated?
- _All-*trans*-retinal detaches_ from opsin.
- PDE reconverts it to _11-*cis* retinal_.
- Retinal returns to outer segment of photoreceptor.
- _11-*cis* retinal_ recombines with opsin** to form **rhodopsin.
List and describe the 4 types of opsin.
- Rod opsin - responds to wavelengths around 500nm
- Blue cone opsin - responds around 420nm
- Green cone opsin - 530nm
- Red cone opsin - 560nm
True or False.
When we move from darkness to bright light, retinal sensitivity is lost, but visual acuity is gained.
True.
True or False.
If a person is color blind, you can expect to see an absence of green or red cones in their foveae.
True
Define and describe color blindness.
- Congenital, x-linked condition.
- Absence of one or more cone types.
- More common in males (8 -10%).
- Red/green is most common - seen as same.
Why is night vision fuzzy and indistinct?
Visual acuity is dependent on cones in the fovea, but the intensity of light is too low to activate them.
How does the brain perceive color?
Cones come in three types (red, blue and green); and each is sensitive to different wavelengths of light.
How do photoreceptors differ from other neurons.
Most neurons depolarize when stimulated; photoreceptors hyperpolarize in response to light stimuli.
Cones are sensitive to only three or four wavelengths of light. Why can we see so many colors?
Color perception is achieved by activation of different combinations of cone types.
Why does color vision have a higher resolution than night or dim vision.
Each cone synapses with one bipolar cell and one ganglion cell; but up to 100 rods can converge on a single ganglion cell.
