l18: photoreceptors

Question 1

dynamic ranges of photoreceptors

Answer 1

encounter a range of light intensities: absolute darkness to sunlight is 10 order change so must be able to encode, have different types of receptors (rods/cones) that operate at different dynamic ranges

Question 2

rods vs cones (basic)

Answer 2

rods- night vision, very sensitive, saturate in light, poor acuity, no color
cones- day time and color, not very sensitive, never saturate, good acuity

Question 3

organization of retina

Answer 3

rods, cones
bipolar cells relay photoreceptors to ganglion
horizontal cells (outer plexiform) connect photoreceptor to bipolar
amacrine cells (inner plexiform) connect bipolar to ganglion
ganglion cell –> optic nerve

Question 4

photoreceptorphotoreceptor

Answer 4

outer segment: have disks that contain photo pigments (rodopsins or conopsins)
inner segment: cell body
synaptic terminal: release glutamate

Question 5

what happens to a photoreceptor when light flash

Answer 5

hyperpolarize when stimulated which decreases glutamate release, takes awhile to decay
- more depolarized at rest to accomodate (-40 not -70)

Question 6

photoreceptor recording

Answer 6

outward current because cell hyperpolarizes

Question 7

rod and cone kinetics

Answer 7

cones are much faster than rods due to channel types and sensitivity of opsins

Question 8

cGMP

Answer 8

rod in dark: high levels of cGMP
- gates channel and opens it to pass Na and Ca, depolarizes cell and also have leak channels
light on: reduction of cGMP which closes channels and reduces inward current = increasing outward current and hyperpolarizing

Question 9

phototransduction in rod

Answer 9

light causes conformational change from cis to trans retinal inside opsin protein
change allows g protein to bind and activate (transducin)
when activated exchanges GTP for GDP and binds to phosphodiesterase

Question 10

retinoid cycle

Answer 10

photoreceptors stick into pigement, purpose is to change cis to trans

Question 11

PDE regulatory molecule

Answer 11

can hydrolyze cGMP and close channels, reducing inward current and hyperpolarizing

Question 12

signal amplification in photoreceptors

Answer 12

1 rhodopsin molecule absorbs 1 photon - 500 transducin molecules - 500 photodiasterase molecules - 10^5 cGMP - 250 Na channels - rod cell membrane hyperpolarized by 1 mV

Question 13

arresting photoreceptor responses

Answer 13

1. rhodopisn kinase- phosphorylates activated rhodopsin
   - makes rhodopsin less available to transducin when it absorbs photons
2. arrestin- binds phosphorylated rhodopsin “arresting” the photoresponse
   - competes with transducin for site on rhodopsin molecule, blocks cascade and keeps channels open

Question 14

internal Ca

Answer 14

more Ca = hyperpolarize
less Ca = depolarize
- internal Ca controls sensitivity of photoreceptors to light

Question 15

increasing light levels (less Ca)

Answer 15

1. increased RK activity (terminates residual R activity) –> depolarization
2. increased GC activity (higher cGMP) –> strong depolarization
3. cGMP channels have higher affinity for cGMP –> stronger depolarization

Question 16

responses of ganglion cells during light adaptation

Answer 16

shift in response of photoreceptor: expands dynamic range

if didnt do this you owuld be saturated at non-bright stimulus

Question 17

trichromatic theory

Answer 17

know how it is detected in retina (different cones) but unsure after
red/green channel: L-M, blue/yellow channel: S - (L+M)
rod: L+M+S (achromatic)

Question 18

chromatic system

Answer 18

red/green (long and medium wave-)
yellow/blue (long and medium wave + SW -)
red cones adapt over time and become less sensitive (lwoer Ca bc brighter) so when red is turned off green sensitivity overpowers

Question 19

color opponant surround receptive fields

Answer 19

horizontal cell receives input from opposite color
pure centre ganglion cell: only green
mixed center ganglion cell: 2 reds and 1 green

Question 20

distribution of photoreceptors in human retina

Answer 20

5 million cones
concentration in fovea about 1.5mm
most acute is foveola = 0.4mm