Lecture 6 Study Guide

Question 1

the process of visual light transduction

define transduction

where does it begin?

Answer 1

Once you get through to the retina, you can begin the process of transduction – changing light energy into neural
signals.

• begins when
photons of light enter the outer segment of the photoreceptor.

• The photons go into one of the discs containing thousands of
visual pigment molecules.

• The visual pigment
molecule is a long protein
strand called opsin.

• Only one location on the protein strand is reactive to light – the retinal.

• Each visual pigment
molecule has only one
retinal.

Question 2

transduction becomes isomerization

Answer 2

If a single photon of light is absorbed by a retinal molecule, the
molecule changes shape (and later separates)

This one triggering event leads to an enzyme
cascade – thousands of chemical reactions leading to a change in
receptor activity

Question 3

the functional differences between rods and cones (e.g. How many are there and where are they concentrated?).

Answer 3

– Distribution on retina
• Fovea consists solely of cones (about 50,000 – 110,000).
• Peripheral retina has both rods and cones.
• More rods than cones in periphery.

– There are about 120 million rods and 6 million cones (though,
this may be a high estimate).

Question 4

the dark adaptation process for both rods and cones

Answer 4

• Experiment for cone adaptation
– Test light falls directly on fovea – only stimulates cones.

– Results show that sensitivity increases for three to four minutes and
then levels off.

• Experiment for rod adaptation
– Must use a rod monochromat – someone born with no cone receptors.

– Results show that sensitivity increases for about 20-30 minutes and  then levels off.

Question 5

the Purkinje shift

Answer 5

is the tendency for the peak luminance sensitivity of the human eye to shift toward the blue end of the color spectrum at low illumination levels.

an enhanced sensitivity to shorter wavelengths during dark adaptation when the shift from cone to rod vision occurs.

Question 6

the role of pigment regeneration.

Answer 6

• Retinal and opsin must recombine to respond to light
• Cone pigment regenerates in about 7-10 minutes.
• Rod pigment takes more than 30 minutes.
• Enzymes necessary for pigment regeneration are found in the pigment epithelium.*

• If your retina separates from the pigment epithelium
(detached retina), you lose the ability to regenerate
visual pigment.

Question 7

macular degeneration treatments

Answer 7

If the macula is destroyed, perhaps a healthy portion of the retina can take over its
function?

Possibly – but you would have to magnify the
image for lower resolution outside the fovea.

• Implantable Miniature Telescopic lens
• Telescopic contact lens

Question 8

the rod-cone break

Answer 8

when during experiment for dark adaptation the sensitivity of the rods and cones levels off and then afterwards the cones reach max sensitivity and then the rods reach max sensitivity (more sensitive)

• switch vision from using mostly cones to using mostly rods in darkness

Question 9

Rods vs. Cones

Reasons behind light preferences

Answer 9

Rods

• darkness is ideal for rods because after retinal is isomerized it separates from the visual pigment and darkness allows time for regeneration
• this is why rods are more sensitive to light + retinal only needs 1 photon of light to isomerize
• rods have more convergence too which makes them more sensitive

Cones

• greater acuity because they have no convergence
• one to one ratio (in fovea) allows them to distinguish fine details
• very sensitive to higher intensities of light (BUT requires more units of light to be activated)