04-05. Neurons (in the eye)

Question 1

Sequence for neuronal communication

Answer 1

1. membrane permeability changes
2. Sodium flows in
3. Inside more positive (depolarization)
4. Critical Value (~ –45 mv) reached and action potential occurs.
5. Potassium flows out (repolarization)
6. Action potential travels down the axon and stimulates synaptic vesicles
7. Synaptic vesicles release NTs into the synapse that influence the permeability of the next neuron.
   –> makes it more/less likely to have
   action potential

Question 2

3 important facts about action potentials??

Answer 2

1. ALL-OR-NONE
2. REFRACTORY PERIOD
3. BACKGROUND firing rate

Question 3

All or nothing means?

Answer 3

• action potential or there isn’t
• one size only, no bigger or smaller
• to communicate bigger stimulus, they change the RATE at which they have action potentials

Question 4

Refractory period?

Answer 4

• period where another Act. P can’t occur
• limits the rate at which they can have Act. Ps

Question 5

Background firing?

Answer 5

• Act. Ps all on their own, without any input or stimulus
• you know if it receives an inhibitory input, because it will drop below its normal rate

Question 6

3 membranes in the eye?

Answer 6

outer: sclera
–> tough protective cover
mid: choroid*
–> blood vessels
inner: retina

Question 7

Function of ciliary muscles/zonule fibers?

Answer 7

• they ADJUST (change the shape of) the LENS so you can focus on things at diff. distances
• they attach to the choroid

Question 8

Optic axis? Fovea?

Answer 8

Imaginary line of sight that is your direct focus, which runs through the lens and directly to the fovea (most sensitive part of the retina)

Question 9

5 types of cells in retina??

Answer 9

• photoreceptors
• horizontal cells
• bipolar cells
• amacrine cells
• retinal ganglion cell

Question 10

Why are the cells in the order they are in the retina? (even tho it seems a lil backwards)

Answer 10

• photoreceptors (PR) –> only cells sensitive to light
• PRs need to be attached to the pigment epithelium to keep functioning (not see-thru)
• so, PR are at the back, next to PE, and the rest of the cells are in front of them

Question 11

What happens to the photopigment molecules?

Answer 11

• process of photoisomerization –> molecule changes shape
• releases NT
• Act. P in bipolar/horizontal cell
• photopigment regeneration –> puts the molecule back in its original shape
  –> this is why PRs need pig. epith.

Question 12

What is the blind spot?

Answer 12

its the bundle of ganglion cells that form the optic nerve, where it exits the eye back towards the brain

Question 13

Why don’t we notice blind spot? If we isolate it, what do we see?

Answer 13

• we will see the background of whatever’s near it continued
• the blind spot is background, its far from the fovea
• we fill in the gaps
• our eyes are frequently moving

Question 14

7 differences between rods and cones?

Answer 14

1. shape
2. number
3. spatial distribution
4. adaptation to darkness
5. spectral sensitivity
6. light sensitivity
7. spatial acuity

Question 15

1. shape ?

Answer 15

rods - straight
cones - cone / triangle ish

Question 16

2. number ?

Answer 16

rods - 100M /eye –> more
cones - 5M /eye –> less

Question 17

3. spatial distribution ?

Answer 17

(density in specific places on retina)

cones - highly dense in the fovea, not anywhere else
rods - none in fovea, highly dense a little ways away from the fovea, overall more dense than cones

Question 18

4. adaptation to darkness ?

Answer 18

process of adjusting retinal sensitivity (changing operating range)

adapt at diff. rates

cones:
- adapt really fast (≈4 - 8 min max)
–> ≈8 min is the rod-cone break
- much LESS sensitive in low light
- abs. thresh. is much higher (need more stim)

rods:
- adapt slower ( ≈25 - 30 min)
- much MORE sensitive in low light
- abs. thresh. and max visual sensitivity are much lower (need less stim)

Question 19

Spectral sensitivity def?

Answer 19

sensitivity to diff. wavelengths (colors)

(degree to which a photopigment molecule absorbs light of diff. wavelengths)

Question 20

Photopigment def? types?

Answer 20

molecule (within photoreceptors) with the ability to absorb light and start transduction of light into neural signals

1 type for rods, 3 types in cones (each cone has diff. photopigment)

Question 21

5. spectral sensitivity ?

Answer 21

rods:
- low (only 1 type)
- overall shorter than cones

cones:
- high (3 types – R/B/G)
- short, medium, and long

Question 22

Purkinje shift?

Answer 22

There’s an overall difference in spect. sensitivity of rods and cones
(rods generally shorter)

Rods are much much more sensitive in the dark

So, when you’re dark adapted, something that’s blue will look brighter than something red

Red colors appear to fade faster than blue

Question 23

Convergence def?

Answer 23

Ratio in retinal circuits where…

many photoreceptors :: 1 ganglion cell

Question 24

Spatial summation def?

Answer 24

• Involves convergence
• all PRs in a circuit are from a specific AREA on the retina, and they “ADD UP” to affect the ganglion cell’s response
• works like a funnel

Question 25

6. Sensitivity to light ?

Answer 25

• rods are more sensitive to light because their convergence ratio is higher
• dim light stimulates each rod, which all “adds up” to make the ganglion cell respond
• cones don’t add anything up, so they need more stimulus (brighter light) to produce a response

Question 26

7. spatial acuity ?

Answer 26

rods:
- high convergence = low acuity

cones:
- low convergence = high acuity

Question 27

Why not cones everywhere?

Answer 27

• blind in the dark
• too much sensory info
  –> (rods periphery)
• cortical magnification around fovea
  –> it would take up too much space