Chapter 2 - The Retina

Question 1

Where does the process of “seeing a picture” begin?

Answer 1

At the retina

Question 2

What happens at the retina?

Answer 2

Light is transduced into electrochemical signals

Question 3

Why are there no blood vessels in the fovea?

Answer 3

There are too many light encoding nerves

Question 4

Describe the blind spot

Answer 4

The optic disk contains no photoreceptors (this portion of the visual field is filled in by the brain)

Question 5

What is the retina composed of?

Answer 5

Many types of neurons

• all are transparent
• the only opaque membrane in the retina is the pigment epithelium

Question 6

Describe the anatomy of the retina

Answer 6

It is separated into layers:

1. the nuclear layers
   a) outer nuclear layer
   b) inner nuclear layer
   c) ganglion cell layer
2. the synaptic layers
   a) outer synaptic layer
   b) inner synaptic layer

Question 7

What are the two photoreceptors and what are they each responsible for?

Answer 7

1. rods: scotopic (night vision) - sensitive to luminance
2. cones: photopic vision (colour vision) - requires high illumination and fine visual acuity (detailed vision)
   - both photoreceptors transduce light energy into electrical energy

Question 8

What is the ratio of cones to rods in the retina?

Answer 8

no rods @ the fovea; 90 million total - 140,000/mm^2

more cones @ the fovea; 5 million total - 200,000/mm^2

Question 9

What is the distribution of photoreceptors in the peripheral retina vs. the central retina?

Answer 9

photoreceptors are more spread out in the peripheral retina

Question 10

Describe the anatomy of photoreceptors

Answer 10

1. outer segment
2. inner segment
3. nucleus
4. synaptic terminal

Question 11

Describe the early stages of transduction?

Answer 11

1. visual pigment molecules (photopigments) are made in the inner segment
2. photopigments are then stored in the outer segment (this is where they absorb photons)
3. DNA is stored in the nucleus
4. synaptic terminal is where information is transmitted

Question 12

Describe photopigments

Answer 12

• composed of a protein called opsin (made inside the cell)

- chromophore derived from vitamin A called retinal (obtained from beta carotene we consume)

Question 13

What are the 2 isomers (possible shapes) that the photopigments can take on?

Answer 13

• 11-CIS RETINAL

- ALL-TRANS RETINAL

Question 14

What is photoisomerization?

Answer 14

a change in shape by a photopigment molecule from one isomer to another
- transduction = when light changes the shape of the isomer

Question 15

How are all rods the same?

Answer 15

they have the same shape and contain the same photopigment (rhodopsin)

Question 16

How are cones different?

Answer 16

Cones are separated into 3 subtypes based on their morphology and the photopigments they contain:

1. S cones - s cone opsin (blue light)
2. M cones - m cone opsin (green light)
3. L cones - l cone opsin (red light)
   - each type is maximally sensitive to a different range of wavelengths (colour)

Question 17

What is an operating range?

Answer 17

• gain control mechanism

- adjustment of light sensitivity based on current conditions

Question 18

What are the 3 mechanisms involved in adjusting sensitivity to light?

Answer 18

1. pupils
2. photopigment regeneration
3. internal circuitry of the retina

Question 19

Describe photopigment regeneration

Answer 19

• when a photoreceptor absorbs a photon, the photopigment changes molecular shape and can no longer detect light
• the photopigment must return to its 11-CIS RETINAL shape before it can absorb photons again

Question 20

What happens to photopigment regeneration in dim light?

Answer 20

• little photopigment bleaching, rods and cones will absorb as many photons as possible

Question 21

What happens to photopigment regeneration in bright light?

Answer 21

the number of photons begins to overwhelm the amount of photopigment available
- the photopigment cannot be regenerated quickly enough

Question 22

Describe the 2 step process of light and dark adaptation:

Answer 22

1. sensitivity to light increases for the first 3-4 minutes
   - cones can regenerate their photopigment much faster than rods (about 5 mins) but are not as sensitive to light
2. sensitivity increases again, and levels off after about 25 minutes
   - rods are slower at regenerating their photopigments (about 25 minutes) but are much more sensitive to light
   * * switching from rods to cones is called the rod-cone break**

Question 23

What is photopic range?

Answer 23

range of illumination where cones mechanism is involved in mediating vision

Question 24

What is scotopic range?

Answer 24

range of illumination where rods mechanism is involved in mediating vision

Question 25

What is mesopic range?

Answer 25

range of illumination where rods and cones mechanisms are working together (here, there is no abrupt transition from cone to rod)

Question 26

Describe the vertical pathway of the retina?

Answer 26

• cells are all arranged vertically
• photon enters rod’s outer channel
• activates photopigment (photoisomerization)
• hyperpolarizes the membrane potential of the photoreceptor (no action potential generated, NT release is a graded process)
• glutamate binds to bipolar cell - stimulates bipolar cell
• binds to ganglion cell

Question 27

Describe the horizontal pathway of the retina?

Answer 27

• cells are all arranged horizontally
• initial steps are identical to vertical pathway
• photoreceptor binds with horizontal cell - stimulates horizontal cell
• releases inhibitory GABA
• amacrine cell = secondary horizontal cell

Question 28

What are some important synaptic interactions that occur at the outer synaptic layer?

Answer 28

• the splitting of the visual signal into two separate channels of information flow
  
  • one for detecting objects lighter than background
  • one for detecting objects darker than background

Question 29

What are synaptic interactions necessary for?

Answer 29

to create response patterns that are adjustable

• function within operating range
• contrast related, not absolute response

Question 30

Describe horizontal cells

Answer 30

• run perpendicular to the photoreceptors
• three types
  a) HC I
  b) HC II
  c) HC III
• synapse with nearby photoreceptors
• plays a role in lateral inhibition (antagonistic neural interaction between adjacent regions of the retina - mostly to other photoreceptors and horizontal cells)

Question 31

Describe bipolar cells

Answer 31

• 11 different retinal bipolar cells
  
  • 1 type for rods
  • 10 types for cones
• bipolar cells are divided into ON and OFF types
  
  • ON bipolar cells will respond in the presence of light
  • OFF bipolar cells will respond in the absence of light
• some bipolar cells in the peripheral retina synapse with up to 12-14 cones (diffuse bipolar cells)
• other bipolar cells synapse with a single cone (fovea)
• each cone synapses with two midget bipolar cells in the fovea: ON and OFF bipolar cells

Question 32

Describe amacrine cells

Answer 32

• classified by
  1. dendridic tree size
  2. branching characteristics
• synapse with
  1. bipolar cells
  2. amacrine cells
  3. retinal ganglion cells
• involved in modulation of temporal coding

Question 33

What is myopia?

Answer 33

Nearsightedness

• lens too dense for length of eye
• focus is in front of retina
• corrected with a concave lens

Question 34

What is hyperopia?

Answer 34

Farsightedness

• lens too thin for length of eye
• focus is behind the retina
• corrected with a positive lens

Question 35

What are two diseases that affect the retina?

Answer 35

• Dry macular degeneration

- wet macular degeneration