C15 6-9

Question 1

Rods? know

Answer 1

100 mil per retina, rhodopsin (photosensitive pigment), vision in shades of gray, high sensitivity, low acuity, night (scotopic) vision, much convergence in retinal pathways, more numerous in periphery

Question 2

Cones? know

Answer 2

3-10 mil per retina, photopsin (photosensitive pigment), color vision, low sensitivity, high acuity, day (photopic) vision, little convergence in retinal pathways, concentrated in fovea

Question 3

8 ways to compare rods/cones

Answer 3

Number/pigment/color/sensitivity/acuity/time of day/convergence/location

Question 4

Phototransduction definition

Answer 4

Book: The process by which light energy is converted into a graded receptor potential.

Handout: The process by which photoreceptors become activated when the photopigments within the discs of the cells are altered by light entering the eye.
The photoreceptor responds to light stimulation by changing its RMP resulting in a change in the neurotransmitter it releases.

Question 5

What is a photoreceptor?

Answer 5

Modified neuron, resembles tall epithelial cell turned upside down with their tips immersed in the pigmented layer of the retina. Tips are the receptive region (outer segment)

Question 6

Photopigments?

Answer 6

Photosensitive compounds in the rods and cones. Made of opsin (protein) and retinal (light absorbing molecule, aldehyde of vit A1.)

Question 7

Forms of retinal?

Answer 7

11-cis-retinal - when bound to opsin, retinal has a bent or kinked shape.

All-trans-retinal - when the pigment is struck by light and absorbs a photon, it straightens out.

Question 8

Synthesis, bleaching and regeneration of rhodopsin

Answer 8

1. PIGMENT SYNTHESIS 11-cis-retinol, derived from vit A is combined with opsin to form rhodopsin.
2. PIGMENT BLEACHING light absorption by rhodopsin triggers a rapid series of steps in which retinol changes shape (11-cis to all-trans) and eventually releases from opsin
3. PIGMENT REGENERATION Enzymes slowly convert all-trans retinal to its 11-cis form in cells of the pigmented layer (req ATP).

Steps 1+3 in the dark, 2 in light.

Question 9

Bleaching of the pigment

Answer 9

The breakdown of rhodopsin to retinal and opsin

Question 10

How does the bleaching reaction relate to dark/light adaptation?

Answer 10

Rhodopsin bleaches/breaks down in light adaptation and becomes non-functional. In dark adaptation, rhodopsin again accumulates and sensitivity increases allowing for sight in the dark.

Question 11

Light adaptation

Answer 11

dark into light

Both rods and cones are strongly stimulated and large amounts of the visual pigments break down almost instantaneously producing a flood of signals that account for the glare.

Rod system turns off, 60sec the cones are desensitized enough to work

5-10 min the cones continually improve

Lose sensitivity/gain acuity

Question 12

Dark adaptation

Answer 12

Move from well-lit area to dark.

We can’t see because: our cones stop functioning in low-intensity light, and the bright light bleached our rod pigments and the rods are still turned off.

In dark, rhodopsin accumulates, transducin return to outer segment.

20-30 min for rhodopsin to regenerate.