Visual System II

Question 1

How is light transduced= generally

Answer 1

Light transformed to electrical signal = transduction in rods and cones
Light hyperpolarizes photoreceptor buy closing cGMP gated cation Channel

Question 2

How is light transduced= describe whole pathway

Answer 2

Rod outer segment = in one disk
Light —> activates gpcr rhodopsin = changes conformation —> activates transducin G protein —> activated cGMP phosphodiesterase which degraded cGMP - second messenger inside cells —> cGMP breaks down = causes hyper polarization-40–> -60

Question 3

describe photoreceptor in dark

Answer 3

Photoreceptor filled with cGMP molecules = high concentration cGMP in dark
Ion channels on external surface photoreceptor make pore when open - permeable to sodium ions = channel opens and sodium flows in = depolarizers photoreceptors

Question 4

what happens when single rhodopsin activated

Answer 4

Single rhodopsin activation = large amount cGMP degraded
Lots of amplification
One photon causes hyperpolarization because of amplification in a second messenger pathway

Question 5

what does light to do photoreceptor

Answer 5

In dark = photoreceptor depolarized
Then since light degrades cGMP= ion channels close to photoreceptor hyperpolarizes = more neg, neg 40 to neg60

Question 6

what connects photoreceptor to rgc

Answer 6

bipolar cells

Question 7

usually what type of receptor is glutamate for

Answer 7

usually excitatory - Ampa repcetros

Question 8

describe role of bipolar cells = in cone in dark

Answer 8

In dark = depolarized and releasing nt —> glutamate = inhibitory so hyperpolarizes bipolar cells bc receptors at bipolar cells are gpcrs (metabotropic = activates second messenger cascade) —> so not releasing glutamate on ganglion cells so less aps

Question 9

describe role of bipolar cells = in cone in light

Answer 9

Cone hyperpolarized = so decrease amount of glutamate release = less inhibition, constant inhibition relieved (gpcr!) so bipolar cells depolarize and release excitatory glutamate (no ap tho) = activates ampa so then ganglion cell fires many aps—> optic nerve —> brain

Question 10

How is intensity of light conveyed

Answer 10

Transformed into frequency of aps in rgcs

Question 11

describe process of how low intensity light conveyed

Answer 11

Smaller hyperpolarization of cone
Less depolarization of bipolar cells
Less glutamate = fewer aps from rgcs

Question 12

describe process of how high intensity light conveyed

Answer 12

Lots of hyperpol of cone
Large depoalrization of bipolar cells
Many aps bc lots glutamate on rgcs

Question 13

do cones and bipolar cells and horizontal cells fire aps

Answer 13

NOOOOOOOOOOO
Their membrane potential changes continually

Question 14

What connects cones in surrounding region to central photoceptro

Answer 14

Cones in surround region connected to central photoreceptor through inhibitory horizontal cells

Question 15

describe center vs surround cones = in dark

Answer 15

In dark = release glutamate on horizontal cell —> depolarizers horizontal cell —> release nt on presynaptic terminal of centre cone = inhibitory = inhibitory = inhibits amount of glutamate release from central cone
Horizontal cells inhibit
So central cone not releasing as much glutamate as could be

Question 16

describe center vs surround cones = light shinning on surround

Answer 16

Light shining on surround cones = because of inhibitor connections will have opposite effect
Light hyperpolarizes surround cone = reduces glutamate = less inhibitory not, so
Hyperpolarizes horizontal cell
So terminal releases more glutamate on central cone = more inhibition of bipolar cells usually
Ends up inhibiting ganglion cell

Question 17

light shine central cone

Answer 17

increasing firing ganglion cell

Question 18

light singe surrounding cone

Answer 18

bc of inhibitory connections = inhibit ganglion cell

Question 19

what happens when light shines on central photoreceptors

Answer 19

activates ganglion cell = increase firing

Question 20

when light shines on surrounding photoreceptors what happens

Answer 20

inhibits ganglion cell

Question 21

Rfs of rgcs

Answer 21

on center off surround = reflect convergent input from multiple photoreceptors
Also have more complex rf than photoreceptor = bc of integration of info from multiple types of photoreceptors - central or surround cones
(Also off center on surround…)

Question 22

Describe on centre rgcs

Answer 22

Connected to on center bipolar cell - long extension
Many aps = light in centre
Inhibition if light in surround
Light all over = baseline firing

Question 23

describe lateral inhibition

Answer 23

The on center off surround and off center on surround = lateral inhibition
Visual system interested in constants and edges
Sensitive to differnces in illumination

Question 24

What Happens when photoreceptor makes excitatory connections with bipolar cells

Answer 24

= ganglion cells are off center on surround
Excitatory synapse = depolarized, diff type bipolar cell = hyperpolarized by glutamate-flips sign
Short extension

Question 25

describe off centre rgcs

Answer 25

Light in centre inhibits Light in surround excites Light all over= baseline

Question 26

what does each cone form in fovea

Answer 26

Each cone forms center of on centered and an off centered pathway = connected to both on and off centre bipolar cells (One cone = center and also surround for other ganglion cells)

Question 27

describe what cells in fovea are sensitive to and how this arises in context of bipolar cells

Answer 27

If Sudden chnage in illumination —> increase = increase firing on centre ganglion cell, decrease illumination = increase firing off center ganglion cell So sensitive to increase or decrease in illumination - broadens range over which retina can sense changes in illumination In fovea = high visual acuity, rfs small, high density cones in ganglion cells

Question 28

what happens outside fovea

Answer 28

More photorepctors feed into single ganglion cell so rfs lager Like 3 centre cones, 5 inhibitory surround cones Less precision/acuity and rfs get more sensitive to light bc gather input from more cones and photorepctros

Question 29

why are peripheral ganglion cells more sensitive to light

Answer 29

Also in periphery = get input from rods