Phototransduction

Question 1

What must happen in order to see an image

Answer 1

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.

Question 2

What is the pathway of signal transmission of light signals.

Answer 2

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.

Question 3

Distribution of cone cells and rod cells?

Answer 3

Cone cells are concentrated in the fovea whereas rods and dispersed across the retina

Question 4

4 main parts of photoreceptors

Answer 4

outer segment
inner segment
cell body
synaptic terminal

Question 5

What is photoreceptors resting membrane potential?

Answer 5

Depolarised- more positive than most neurones

only -40 whereas most neurones are -70

Question 6

Effects of light on the membrane potential

Answer 6

Light causes the membrane to hyperpolarise (becomes more -ve)- change in membrane potential but DOES NOT create an AP

Question 7

what cells create an AP in the eye

Answer 7

ganglion cells

Question 8

Why are photoreceptors resting membrane potential more positive

Answer 8

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)

Question 9

Why does the membrane become more negative in response to light

Answer 9

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.

Question 10

What is rhodopsin

Answer 10

Retinal and opsin.
Retinal- Vit A derivative
Opsin- G protein coupled receptor

Question 11

Why does the cGMP receptor on photoreceptors close in response to light?

Answer 11

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

Question 12

Main principles of the Dark current

Answer 12

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

Question 13

What is visual acuity and what are the main factors that determine it?

Answer 13

able to distinguish between 2 points:

photoreceptor spacing and refractive power

Question 14

what time of day are rods/ cones used?

Answer 14

rods in dark lighting and cones in daylight

Question 15

What produces more acuity rods or cones and why?

Answer 15

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

Question 16

Are rods or cones more sensitive and why?

Answer 16

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.

Question 17

What portion of the electromagnetic spectrum is activated by photoreceptors?

Answer 17

Only visible light: 750nm to 350nm

Question 18

What are the 4 types of photoreceptor

Answer 18

3 cones and 1 rod

3 cones: red(long wave) green and blue (short wave)

Question 19

Main differences between rods and cones?

Answer 19

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

Question 20

Function of the LGN

Answer 20

LGN: lateral genicular nucleus

receives sensory info from the optic tracts and sends optic radiations to the visual cortex

Question 21

where is the visual cortex

Answer 21

occipital lobe

Question 22

What makes up the right optic tract?

Answer 22

Right temporal fibres and left nasal fibres

Question 23

Where does information enter the primary visual cortex

Answer 23

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.

Question 24

What is amblyopia

Answer 24

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

Question 25

Management of strabismus

Answer 25

surgical - but must wait until bby is old enough. in the mean time should close the good eye for a few hours each day so the brain has to rely on signals from other eye.