5: Eye & Retina

Question 1

What is Emmetropia?

Answer 1

-Normal Vision

Question 2

What is Myopia?

Answer 2

• Nearsighted
• Light focuses in front of the retina
• Eye ball is too long or cornea is too curved
• Difficult to see distances

Question 3

What is Hyperopia?

Answer 3

• Farsighted
• Light focuses behind the retina
• Eye ball is too short or cornea
• Difficult to see close objects

Question 4

What is Presbiopia?

Answer 4

• Comes w age

- Both near & farsighted

Question 5

What is the Function of Pigment Epithelial Cells?

Answer 5

• Keep rods & cones healthy
• Located @ very back of retina
• Constantly replenish disks that are taken up at the back of the eye through phagocytosis
• -Crucial for the health of PRs

Question 6

What is Photoisomerization?

Answer 6

• A light initiated process of change from one isomer to another
• In disks of PRs, there’s a protein called opsin
• -Opsin contains retinal
• Light hits retinal & causes photoisomerization & changes shape

Question 7

What is Phototransduction?

Answer 7

-Light is converted into electrical signals that travel from the retina down the optic nerve

• Retinals new isomer, rhodopsin, activates transducin (enzyme)
• Transducin activates PDE (phosphodiesterase)
• PDE degrades cGMP
• cGMP can’t bind to ion channels to open them therefore those ion channels close
• Closed ion channels can’t let in Na & Ca
• = Hyperpolarization b/c K is still constantly leaving the cell

Question 8

What is Photoadaptation?

Answer 8

• Guanylate cyclase enzyme produces cGMP
• Ca normally inhibits guanylate cyclase
• In light, there is less Ca & guanylate cyclase activity rises
• More cGMP is produced
• Ion channels that had been closed by the light open again b/c guanylate cyclase produces cGMP which can bind to it’s receptors & open ion channels to let in Na & Ca, causing depolarization

Question 9

What are the 3 Luminance Values?

Answer 9

Scotopic

• Very dark, raw vision, only rods
• Starlight

Mesotopic

• Cones & Rods are active
• Better vision
• Moonlight

Photopic

• Rods are saturated
• Too much light, just cones
• TOO much light = bleached

Question 10

What is Eccentricity?

Answer 10

-The distance from the fovea

Question 11

Explain the structure/ function of the fovea.

Answer 11

• Lots of densely packed cones
• -Move away from fovea = density of cones falls off dramatically & density of rods increases
• All blood vessels are angled away from the fovea
• -No light is reflected into fovea
• -Nothing for the light to hit as it enters towards the fovea

Question 12

Why are Rods more Sensitive to Dim Light?

Answer 12

1. They are longer & contain more photopigment than cones do
2. They have greater amplification: close more Na channels in response to the same amount of light
3. Many rods converge onto the same bipolar cell
   - Lots of these bipolar cells converge onto the same ganglion cell
   - -By contrast, there’s much less convergence in the cone system

Question 13

Why do Cones have better Spatial Resolution?

Answer 13

1. They are densely packed at the fovea, which lacks overlying axons & blood vessels, reducing light scattering
2. Cones have much less convergence

Question 14

Describe an Off-center Bipolar Cell.

Answer 14

-Hyper/ depol in the absence/ presence of glutamate

Ie.

• Light causes cone to hyperpol
• Less gluta is released onto bipolar cell
• Causes ionotropic, AMPA, glutamate Na channel to close
• Causes hyperpolarization of the bipolar cell & ganglion cell

Question 15

Describe an On-center Bipolar Cell.

Answer 15

• Cone is hyperpolarizes & doesn’t release much glutamate
• Glutamate doesn’t bind to weird metabotropic glutamate receptor
• mGluR6 can’t release enzymes to close an Na chan that’s usually open
• Channel stays open in the absence of glutamate
• Causes depolarization of the bipolar cell and depolarization of the ganglion cell