Phototransduction Flashcards
(25 cards)
What must happen in order to see an image
The pattern of the object must fall of the visual receptors. The amount of light entering the cell must be regulated (too much will bleach the signals). The energy from the waves of photons must be transduced into electrical signals. The brain must receive and interpret the signals.
What is the pathway of signal transmission of light signals.
Photoreceptors transduce light signals into electrical currents. Horizontal cells receive input from photoreceptors and project it to other photoreceptors and bipolar cells. amacrine cells receives input from bipolar cells and projects to ganglion cells. bipolar cells and other amacrine cells.
Distribution of cone cells and rod cells?
Cone cells are concentrated in the fovea whereas rods and dispersed across the retina
4 main parts of photoreceptors
outer segment
inner segment
cell body
synaptic terminal
What is photoreceptors resting membrane potential?
Depolarised- more positive than most neurones
only -40 whereas most neurones are -70
Effects of light on the membrane potential
Light causes the membrane to hyperpolarise (becomes more -ve)- change in membrane potential but DOES NOT create an AP
what cells create an AP in the eye
ganglion cells
Why are photoreceptors resting membrane potential more positive
because of the dark current- A cGMP gated Na channel is open in the dark and closes in the light. When open Na and Ca flow in (remember Na and Ca are positively charged)
Why does the membrane become more negative in response to light
The Na channel closes in response to light so the plasma conc of K is greater than that of sodium so the membrane becomes more negative.
What is rhodopsin
Retinal and opsin.
Retinal- Vit A derivative
Opsin- G protein coupled receptor
Why does the cGMP receptor on photoreceptors close in response to light?
Light converts rhodopsin (opsin + 11-cis-retinal) to all trans retinal which causes a change in conformation of the G protein coupled receptor opsin which initiates a cascade.
Decrease in cGMP causes the cGMP gated Na channel to close to the Na influx decreases and causes hyperpolarisaiton
Main principles of the Dark current
1) opens in the dark and closes in light
2) opens in the presence of cGMP
3) causes Na influx
4) keep the resting memb potential more positive than most neurones
What is visual acuity and what are the main factors that determine it?
able to distinguish between 2 points:
photoreceptor spacing and refractive power
what time of day are rods/ cones used?
rods in dark lighting and cones in daylight
What produces more acuity rods or cones and why?
Cones have more acuity because a few cones contact a single ganglion cell whereas lots and lots of rods send their signals to a single ganglion so more blurry vision
Are rods or cones more sensitive and why?
Rods are more sensitive bc many rods innervate a single ganglion cell so they can pull all the activation together to trigger an AP -more unity. whereas as only a few cone cells innervate each ganglion in the dark ganglion cells cannot generate an AP from just a few cone cells.
What portion of the electromagnetic spectrum is activated by photoreceptors?
Only visible light: 750nm to 350nm
What are the 4 types of photoreceptor
3 cones and 1 rod
3 cones: red(long wave) green and blue (short wave)
Main differences between rods and cones?
Colour: rods don’t produce colour but cones do
Position: rods are in the peripheral retina whereas cones are concentrated in the fovea
visual sensitivity: rods have high sensitivity whereas cones have low
working together: rods have high convergence whereas cones are low
visual acuity: cones have high visual acuity whereas rods have low
Function of the LGN
LGN: lateral genicular nucleus
receives sensory info from the optic tracts and sends optic radiations to the visual cortex
where is the visual cortex
occipital lobe
What makes up the right optic tract?
Right temporal fibres and left nasal fibres
Where does information enter the primary visual cortex
Information enters the striate cortex of the visual cortex (the layered part of the visual cortex) and it synapses in layer 4- it is still segmented in ocular dominance columns at this point :monocular.
What is amblyopia
variety of visual disturbances where there is no problem with the eye but one eyes vision is better than the others- can be caused by strabismus if not corrected in infancy
