Module D-06

Question 1

the 2 types of photoreceptors

Answer 1

Rods and cones

Question 2

Describe the parts of the photoreceptor cells

Answer 2

1) outer segments are oriented towards the retinal pigment epithelium (RPE)
2) inner segments towards the interior of the eye ball

Question 3

Where is the visual pigment for phototransduction found?

Answer 3

In the outer segment

Question 4

What are rod spherules and cone pedicles?

Answer 4

the inner segments form synapses transmitting the visual information on to the subsequent cells of the retina

Question 5

Which cells do the photoreceptors synapse onto?

Answer 5

bipolar cells and horizontal cells

Question 6

Which neurotransmitter is released by the photoreceptors?

Answer 6

Glutamate

Question 7

Under what conditions is glutamate released from the photoreceptors?

Answer 7

Under Dark but reduced in light

Question 8

What conditions do rods let us see?

Answer 8

highly sensitive to light and enable us to see under low intensity light conditions

Question 9

What conditions do cones let us see?

Answer 9

less sensitive to light

work best at higher light intensities

Question 10

In which of the photoreceptors is signal amplification developed?

Answer 10

rods

Question 11

which of the photoreceptors has higher temporal resolution?

Answer 11

Cones

Question 12

how many types of Cones and rods are there?

Answer 12

3 types of cones

1 type of rods

Question 13

In which of the photoreceptors is convergence higher?

Answer 13

Rods

Question 14

Fovea only contains ________

Answer 14

Cones

Question 15

Why is there no central vision in dim light conditions?

Answer 15

because the fovea only contains cones no rods

Question 16

Describe the process of Disk Shedding

Answer 16

• stacks of disks containing visual pigment molecules in the outer segments of
  photoreceptors are constantly renewed.
• New disks are added at the base of the
  outer segment, while old disks are displaced up the outer segment and are
  pinched off at the tips.

Question 17

What happens to the discarded discs ?

Answer 17

Phagocytosed by the retinal pigment epithelium (RPE) cells

Question 18

What is the visual pigment of rods?

Answer 18

Rhodopsin, found on the disk membrane

Question 19

What are the two components of Rhodopsin?

Answer 19

1) Opsin, a protein which is synthesized
in the photoreceptor(cones have different
types of opsins).
2) Retinal, a chromophore, is the light absorbing compound or the visual pigment (of both rods on cones).

Question 20

What is Retinal derived from?

Answer 20

Vitamin A which is synthesized from Beta carotene in our food

Question 21

Describe structure of Opsin molecule

Answer 21

A seven membrane spanning domains.
Its amino terminal is located in the disk interior,
its carboxy terminal in the cytoplasm of the
photoreceptor

Question 22

Describe the structure of Retinal

Answer 22

covalently attached to one of the amino acids of the seventh membrane spanning region of the protein

Question 23

What differs Photoreceptors from other sensory receptors?

Answer 23

depolarized during darkness, i.e. in the absence of their adequate “stimulus”

Question 24

What is the name of the G protein involved in phototransduction?

Answer 24

Transducin

Question 25

Describe the state of the photoreceptor during darkness , ie, the dark current?

Answer 25

1)the visual pigment, which is coupled to a G protein is in its inactive state 2)the G protein does not activate the enzyme cGMP phosphodiesterase. 3) As a consequence, there is plenty of cytoplasmic cGMP (cyclic guanosine 3’5’ monophosphate) available, which keeps the cGMP gated channels in the photoreceptor membrane open. 4) This allows a continuous inward current of sodium ions, 5)photoreceptors stay depolarized in the dark. release their neurotransmitter (glutamate)

Question 26

Describe the Phototransduction Process

Answer 26

1) Light is absorbed by the visual pigment 2) causes a conformational change of the retinal molecule from its inactive 11-cis isomer to its active all-trans isomer. 3) Visual pigment, the enzyme cGMP phosphodiesterase is activated via the G protein. 4) The enzyme activity causes a breakdown of cytoplasmic second messenger molecule cGMP, metabolizing it to 5’ GMP. 5) Drop in cGMP concentration in the cytoplasm causes closing of cGMP gated channels 6)Photoreceptors are hyperpolarized 7) Reduce (or terminate) the release of their neurotransmitter

Question 27

What is the range of the visible part of the light spectrum?

Answer 27

400-700 nm

Question 28

What are the 3 cone types?

Answer 28

1) S (short wavelength sensitive) cones- “blue” 2) M (medium wavelength sensitive) cones- “green” cones 3)L (long wavelength sensitive) cones- red

Question 29

What is the maximum sensitivity of S cones?

Answer 29

430 nm

Question 30

What is the maximum sensitivity of M cones?

Answer 30

530 nm

Question 31

What is the maximum sensitivity of L cones?

Answer 31

560 nm

Question 32

What is Retinitis Pigmentosa?

Answer 32

``` genetically determined (autosomal dominant, autosomal recessive, and X linked recessive) degenerative diseases, in which rods preferentially degenerate ```

Question 33

What are the symptoms of Retinitis Pigmentosa

Answer 33

- starts with night blindness, then loss of peripheral vision (tunnel vision) then blindness

Question 34

What causes photoreceptor degeneration in retinitis pigmentosa?

Answer 34

Accumulation of pigment Reduced phagocytosis by RPE cells during the process of disk shedding

Question 35

What is Nyctalopia?

Answer 35

Night Blindness

Question 36

What causes night blindness?

Answer 36

Vitamin A deficiency

Question 37

Which is the most common type of color blindness?

Answer 37

Red -green color blindness (X linked recessive)

Question 38

What are the two types of red-green color-blindness?

Answer 38

Protanopia: L cone (“red cone”) absent, 1.3% of males Deuteranopia: M cone (“green cone”) absent, 1.2% of males

Question 39

What are the 5 cell types in the retina?

Answer 39

1) Photoreceptor 2) Horizontal cell 3) Bipolar cell 4) Amacrine cell 5) Ganglion cell

Question 40

In what layer of the retina do photoreceptors synapse with bipolar cells?

Answer 40

Outer plexiform layer

Question 41

In what layer of the retina do Bipolar cells synapse with retinal ganglion cells?

Answer 41

Inner plexiform layer

Question 42

In what layer of the retina are Horizontal cells found?

Answer 42

Inner plexiform layer

Question 43

What type of cell are the horizontal cells?

Answer 43

inhibitory interneurons involved in the center / surround organization of the receptive fields of bipolar cells

Question 44

In what layer of Amacrine cells found?

Answer 44

the inner plexiform layer

Question 45

What type of potentials are produced in the cell types of the retins

Answer 45

Only graded potentials , no APs, except retinal ganglion cells which move out of the retina and produce APs

Question 46

where do the Retinal ganglion cells synapse ?

Answer 46

lateral geniculate nucleus (LGN) of the diencephalon

Question 47

What are the 2 types of retinal bipolar cells?

Answer 47

1) ON bipolar cells- depolarized when light is on | 2) OFF bipolar cells- depol. when light is off

Question 48

Glutamate receptors on the postsynaptic membrane of OFF bipolar cells are _______ receptors

Answer 48

ionotropic

Question 49

Effect of glutamate on OFF bipolar cells

Answer 49

an excitatory transmitter, depolarized during | darkness

Question 50

Glutamate receptors on the postsynaptic membrane of ON bipolar cells are ____________

Answer 50

metabotropic type

Question 51

Effect of glutamate on ON bipolar cells

Answer 51

an inhibitory transmitter, depolarized during | light

Question 52

Describe how photoreceptors and horizontal cells participate in opposite effect of a stimulus in the area surrounding a receptive field

Answer 52

When shining light into the surround area of the receptive field of the ON center bipolar cell the photoreceptors in the periphery form excitatory (sign conserving) synapses on adjacent horizontal cells. These horizontal cells in turn form inhibitory (sign converting) synapses in the indirect wiring pathway

Question 53

Describe signal transduction on an ON center ganglion cell when light is on its receptive field

Answer 53

Photoreceptor hyperpolarizes during light ON => ON center bipolar cell is depolarized => excitatory (sign conserving) synapse with ON center ganglion cell => generate AP, increases AP frequency

Question 54

Describe the effect on an ON center ganglion cell when light is on its Periphery

Answer 54

reduces frequency of APs (APs over time)

Question 55

Describe signal transduction on an OFF center ganglion cell when light is on its receptive field

Answer 55

Photoreceptor hyperpolarizes during light ON => OFF center bipolar cell is depolarized => excitatory (sign conserving) synapse with OFF center ganglion cell => decreases AP frequency

Question 56

Describe the effect on an OFF center ganglion cell when light is on its Periphery

Answer 56

Increases frequency of APs