Lecture 30: Visual System I, Retina Transduction

Question 1

What is the equation for focal length? Power of lens?

Answer 1

1/focal length = 1/di + 1/do

1/focal length (m) = something diopters

Question 2

How does lens have to shift or contract when dealing with near and far sight?

Answer 2

Far vision, eye is least spherical

Near vision, eye is most spherical

Question 3

What is responsible for contracting or relaxing the lens?

Answer 3

Zonules of Zinn (Zonular fibers)

Question 4

When you need to accommodate, what do zonule fibers do? When looking at something from afar?

Answer 4

They relax (making lens more spherical)
They contract, making lens more flat

Question 5

How does light travel through the eye?

Answer 5

Light goes through cornea
Anterior chamber
Lens
Vitreous humor
Hits the photoreceptor of the retina first (which is the layer closest to the BRAIN)
-once the photoreceptor is activated, it sends signals to either an on-center or off-center bipolar cell
-then bipolar sends shit to off-center or on-center ganglion cell
-ganglion cell sends shit to optic nerve

Question 6

What is the macula?

Answer 6

An oval-shaped highly pigmented yellow spot near the center of the retina of human eye
Function: specialized for acuity vision
Contains the fovea
-central vision

Question 7

What is the fovea?

Answer 7

It is the center most part of the macula.
Is responsible for our central sharpest vision
Lacks inner nuclear and ganglion cell layers
Contain ONLY cones

Question 8

What are the two main structures that refract light?

Answer 8

i. cornea (does bulk of refraction)

ii. lens (is the only one that can be adjusted)

Question 9

Rhodopsin

Answer 9

protein that receives the photons and are excited

Question 10

What are the three layers of cells in the retina?

Answer 10

i. Outer nuclear layer (ONL)
-Photoreceptors: cell bodies of rods and cones
-closest to brain
ii. Inner nuclear layer (INL)
-bipolar cells
-horizontal cells
-amacrine cells
iii. Ganglion cell layer (GCL)
-innermost layer of the eye (closest to vitreous humor)
Significance: ganglion cells are only cells in the retina that produce action potentials
-send axons to optic nerve

Question 11

What is the point of the choroid (choroid plexus) that are closest to the brain?

Answer 11

They absorb light that did not get absorbed in photoreceptors…sharpens acuity and prevents reflection

Question 12

What are the two types of photoreceptors? Are they technically axons?

Answer 12

Rods and Cones
-they are not axons because they don’t carry action potential
What is the function of a rod?
They are very sensitive to light (in that only one photon is needed to activate a rod cell)
-therefore they can function well at night (when there is not a light present)
-not color vision because only has one type of light-sensitive pigment
No rods in fovea so you cant see shit at night…
Rod used for starlight vision

Question 13

What is the function of a cone?

Answer 13

Responsible for color vision

Not very responsive to light (so need to use them during the day to get enough light to stimulate these fuckers)

Question 14

Are rods and cones distributed uniformly in the retina?

Answer 14

No! Most cones in fovea and more rods in periphery

Cat

Question 15

How is acuity preserved in the central retina?

Answer 15

Because a cone in fovea goes to a nucleus in the occipital lobe via two unique and monogamous pathways (two bipolars for each cone)

Question 16

How does photon activate a signal (in a rod?)

Answer 16

Photon stimulates rhodopsin (GPCR)  GTP  GMP  cGMP-gated channel  channel opens and Na flows in

• In the dark, cGMP is at high concentration leave the sodium channel open and allow the rods to do its thing and GLUTAMATE is continually released
• paradoxically light inhibits this activation because rods are not used during the day…so light hyperpolarizes rods…actually makes sense

Question 17

What does light do to a rod?

Answer 17

Causes hydrolysis of cGMP permitting these channels to close, leading to hyperpolarization and decreased glutamate release of Off-cells

Question 18

What are the two types of biopolar cells?

Answer 18

Off bipolars = depolarize with glutamate in DARK
-carries information about light decrements
On bipolars = depolarize with glutamate in LIGHT
-carries info about light increments

Question 19

How many bipolar cells does one photoreceptor have?

Answer 19

One on-postsynaptic bipolar and one off-postsynaptic bipolar

When one turns on, the other turns off (either light or dark)

Question 20

How does brain perceive contrast?

Answer 20

Cones send signals to the brain of how bright something is COMPARED to its surroundings, not how much absolute light it is receiving
-this is known as CONTRAST

Question 21

Mechanism of contrast in vision?

Answer 21

One center pathway (the signal that the photon directly elicits
And a circular surrounding pathway
-surrounding photoreceptors are INHIBITED by horizontal cells around the center pathway
-Remember that horizontal cells are GABAergic so always inhibitory

Question 22

What are the two types of ganglion cells we have to know?

Answer 22

P-cells (midget)

M-cells (parasol)

Question 23

What is the function of P-ganglion cells?

Answer 23

P-ganglion cells (midget cells) receive from SINGLE cone bipolars (small Receptive Fields, RF) and convert the input to firing rates.
Project to the LGN
Lamina 3 and 6
High spatial resolution

Question 24

What is the function of M-ganglion cells?

Answer 24

M-ganglion cells (parasol cells…shits OPPOSITE) receive MANY bipolars (large receptive fields, RFs) and are especially responsive to motion
Project to LGN
Lamina 1 and 2

Question 25

How are optical images represented in the cortex?

Answer 25

Optical images are represented by the firing rates of population of ganglion cells. A single ganglion cell can tell the brain remarkably little. Population coding