Lecture 5 - The eye and the retina

Question 1

What are the main components of the eye

Answer 1

• Cornea
• Pupil
• Iris
• Lens
• Vitreous Humor
• Retina
• Optic Nerve
• Optic disk (blind spot)
• Fovea

Question 2

What are the possible refractive errors in vision?

Answer 2

1. Emmetropia
2. Myopia (nearsighted)
3. Hyperopia (farsighted)

Question 3

What causes Myopia?

Answer 3

When the eyeball is two small, or the cornea is too curved, the focal plane is in front of the retina, so light focuses then keeps going, resulting in a blur

Nearsighted is when things that are close are clear and things that are far are blurry

Question 4

What causes Hyperopia?

Answer 4

When the eyeball is too long, or cornea is not curved enough the focal plane lands behind the retina, resulting in a blur

Farsighted is when things that are far are clear, but things that are close are blurry

Question 5

How do images land on the retina?

Answer 5

Upside-down, but our brain better presents the information to us

Question 6

What does n represent in Snell’s law?

Answer 6

n=refractive index

Question 7

What is Snell’s law?

Answer 7

n1sinθ1 = n2sinθ2

Question 8

What is the refractive index of a vacuum?

Answer 8

n = 1.000

Question 9

What is the refractive index of air?

Answer 9

n = 1.003

Question 10

What is the refractive index of water?

Answer 10

n = 1.33

Question 11

What is the refractive index of the cornea?

Answer 11

n = 1.38

Question 12

What is the refractive index of the lens?

Answer 12

n = 1.42

Question 13

What is the refractive index of diamonds?

Answer 13

n = 2.42

Question 14

What does n1/n2 mean?

Answer 14

The ratio of medium densities, going from medium 1 to medium 2

Question 15

What is the link between the value of n and the density of the medium?

Answer 15

The bigger n is, the more dense the material

Question 16

What happens if n1/n2 >1

Answer 16

Light is moving from a more dense medium to a less dense medium

Light bends away from normal

Question 17

What does a smaller n1/n2 mean?

Answer 17

If n1/n2 is smaller, more light is being bent
Corresponds to a bigger n2/n1

Question 18

Between the cornea and the lens, which has the greater focusing power (i.e. bends more light)?

Answer 18

The cornea bends more light since the light is coming from air and not water like in the case of the lens (air is less dense than water)

Question 19

What happens if n1/n2<1?

Answer 19

Light is moving from a less dense medium to a more dense medium

Light bends towards the normal

Question 20

What happens when n1/n2=1?

Answer 20

No refraction occurs, light just continues into the second medium in a straight line

Question 21

What is the link between a curved lens, and light bending?

Answer 21

The more the lens is curved, the more light it bends

Question 22

What is the difference between cornea and lens accommodation?

Answer 22

The cornea has a fixed shape

Lens is adjustable (automatic) –> Presbyopia happens when people get old and their lens hardens and can no longer bend, so they can only see things clearly when they are far from them

Question 23

Why do we say that the retina is inside out?

Answer 23

When light first enters, it must pass through all the layers of the retina to make it to the photoreceptors, and then once light is captured by the photoreceptors and converted to an electric signal, it is sent back to bipolar cells and then ganglion cells (note that their axons form the optic nerve)

Ganglion cells exit through the optic disk towards the thalamus

Question 24

What are the two halves of the retina?

Answer 24

1. Temporal half
2. Nasal half

Question 25

On which half is the blind spot located?

Answer 25

Nasal half

Question 26

Why does the optic disk cause a blind spot?

Answer 26

It has no photoreceptors

Question 27

What are the key components of the retina structure?

Answer 27

1. Pigment epithelium (maintains retina health, and is responsible for photoreceptors being in the back of the retina explaining the inside out configuration) 2. Rods and cones 3. Bipolar cells 4. Amacrine cells 5. Ganglion cells

Question 28

What is the typical configuration of a photoreceptor?

Answer 28

1. Inner segment (where proteins are made) 2. Outer segments (where light detection happens, has discs) 3. Pigment epithelial cell

Question 29

What was the goal of the pulse chase experiment?

Answer 29

To observe the lifetime of a photoreceptor, i.e. the shedding of the photoreceptor disk into the pigment epithelium

Question 30

How was the pulse chase experiment done and what did it show?

Answer 30

There was the use of 3H-Methionine to be incorporated with the new proteins (made in the inner segments), and the lifecycle of the photoreceptors could be observed The pulse chase showed that over time, the newly made protein would move from the inner segments of photoreceptor, to the outer segment and then shed into the pigment epithelium where it was engulfed

Question 31

What are the 4 steps of phagocytosis of photoreceptor into the pigment epithelium?

Answer 31

1. Disk curled 2. Tip became spherical 3. Tip detached from photoreceptor 4. Tip engulfed by pigment epithelium

Question 32

What happens to cell membrane potential when it is dark vs when there is light?

Answer 32

Light hyperpolarizes the cell Dark depolarizes the cell

Question 33

What does cGMP stand for, and what does it do?

Answer 33

Cyclic Guanosine Monophosphate, it is degraded by light and ligand for the ions

Question 34

What is the phototransduction process in the dark?

Answer 34

In the dark cGMP ion channels are opened letting in sodium, causing depolarization in the cell that opens the calcium channels, that later causes the release of neurotransmitters

Question 35

What it the phototransduction process in the light?

Answer 35

In the light cGMP ion channels are closed, as a result sodium cannot get in the cell, and potassium continues to leave through the leak channels, causing a hyperpolarization that does not let calcium in the cell, and does not allow the release of neurotransmitters

Question 36

What is photoisomerization and what two events does it lead to?

Answer 36

Photoisomerization is the process by which the retinal inside the opsin absorbs light, and changes from an 11-cis configuration to an all-trans configuration This change in the shape of the retinal inside opsin causes leads to phototransduction and then when light is on for long enough, adaptation

Question 37

Describe the phototransduction process

Answer 37

This is the process that helps explain why light hyperpolarizes the cell 1. Light enters changing the configuration of retinal in ospin 2. This activates the enzyme transducin 3. Transducin activate PDE (phosopdiesterate) 4. PDE degrades cGMP causing hyperpolarization

Question 38

Describe the process of photoadaptation

Answer 38

Guanylate cyclase produces cGMP Ca++ inhibits the production of guanylate cyclase 1. Light enters changing the configuration of retinal in opsin 2. This activates transducin 3. This activate PDE 4. However, since when light is left on for a long time there is no Ca++ (due to constant hyperpolarization), and so there is production of Guanylate cyclase 5. This cause the opening of the cGMP ion channels letting in Na+ and then Ca++ depolarizing the cell

Question 39

How do the transduction loop and adaptation loop go together?

Answer 39

TRANSDUCTION LOOP: Light enters --> PDE activated --> cGMP degraded ---> Na+ and Ca++ cannot enter ---> hyperpolarization ADAPTATION LOOP: From: Na+ and Ca++ cannot enter --> Guanylate cyclase increases --> cGMP levels increase --> Na+ and Ca++ enter cell --> depolarization

Question 40

When looking at a black bulb against a blank canvas for a long time, after, why do we see a similar light shape in a completely white canvas?

Answer 40

The photoreceptors for the white parts have adapted whereas the part with the black bulb have not adapted and when we are back looking at just a white canvas, there is hyperpolarization for the photoreceptors that were looking at the black bulb so the brain just infers that the spot with hyperpolarization must be where the light is

Question 41

Brain infers colors from activation of three different cone types, what are they?

Answer 41

1. Short wavelength cone 2. Medium wavelength cone 3. Long wavelength cone

Question 42

Which wavelength cone is constantly the least distributed?

Answer 42

Short wavelength cone (blue)

Question 43

What technique was used by scientists to observe the distribution of the 3 cone types?

Answer 43

Distribution does vary from person to person, but the technique they used to observe it followed three steps: 1. Flash 550 nm; red and green absorb it, blue reflects it (observe blue's response) 2. Bleach 650 nm to eliminate red function, then flash 550 nm (observe green and blue' response) 3. Bleach 470 nm to eliminate green function, then flash 550 nm (observe blue and red's response)

Question 44

What are three reasons that rods are more sensitive than the cones?

Answer 44

1. Rods are longer (more disks and so more molecules that can have photopigments) 2. Rods converge more onto a single bipolar cell (they need very little light to trigger a response in the brain making them sensitive to low lights) 3. Rods have greater amplification: with just 1 photon of light absorbed, it actives 800 transducin enzymes, activates 800 PDE, degrades 4,800 cGMP and closes 200 Na+ channels...for the same results in cones, 100 photons of light are required

Question 45

What are the retinal location of rods and cones?

Answer 45

Rods are eccentric (90 million rods), Cones are central (4.5 million cones)

Question 46

What is the main trait of off-center ganglion cells

Answer 46

The fire more APs in the center of receptive field when light is off in the center (since light hyperpolarizes cell)

Question 47

What are two reasons that Cones have better spatial acuity than rods?

Answer 47

1. They are more concentrated in the fovea which has no overlaying of axons and blood vessels 2. They have less convergence (more precise)

Question 48

What is the main trait of on-center ganglion cells

Answer 48

They fire more APs in the center of receptive field when light is on in the center

Question 49

Explain the reason behind the AP firing patterns in off-center ganglion cell

Answer 49

1. They have normal ionotropic glutamate receptor (AMPA R) 2. When light causes hyperpolarization, glutamate is not released 3. Ionotropic glutamate receptor's Na+ channels close 4. Off-center bipolar cells hyperpolarize, releasing less glutamate onto ganglion cells 5. Ganglion cells releases less APs

Question 50

Explain the reason behind the AP firing patterns in on-center ganglion cell

Answer 50

1. They have metabotropic glutamate receptors (mGluR6) 2. When light causes hyperpolarization, glutamate is not released 3. mGluR6 actually open when no glutamate is released causing Na+ to enter 4. On-center bipolar cells therefore depolarize, causing glutamate to release onto ganglion cells 5. Ganglion cells fire APs

Question 51

What are the steps of Biolistic transfection

Answer 51

1. Loas DNA into gold particles 2. Place cold particles into tubing 3. Rotate the tubing 4. Cut tubing into cartridges 5. Load cartridges in gene gun 6. Shoot DNA in neuron hoping that in lands in nucleus and neuron can begin making the protein that the shot DNA codes for

Question 52

What is the purpose of Biolistic transfection?

Answer 52

To see what retinal ganglion cells look like

Question 53

How did investigators use Biolistic transfection?

Answer 53

They shot mouse ganglion with two fluorescent protein: 1. td-Tomato (red) --> fills cytoplasm making it visible in orange 2. PSD95-YFP --> protein found at synapse marked in green to help identify synaptic sites Both proteins were made in the neuron (DNA shot in neuron coded for them)

Question 54

What was the use of a dot finder in biolistic transfection?

Answer 54

Dot-finder was a computer system that set threshold for what should be considered a synapse (puncta) and would color it in blue