Photoreceptor cell physiology. The Dark Current Lecture 7

Question 1

Does light hit the photoreceptors first?

Answer 1

No light must diffuse through the retina to reach the photoreceptors.

Question 2

Go over the pathways for photoreceptors

Answer 2

now

Question 3

What is the dark current?

Answer 3

The membrane potential of the photoreceptor cell in the dark.

Question 4

What does the dark current refer to?

Answer 4

The state of depolarisation of the photoreceptor cell in the dark.

Depolarises to a RESTING membrane potential of -40mV

Question 5

What is the state of cGMP in the DARK?

Answer 5

High concentration of cGMP in the dark

Question 6

What does cGMP do in the dark?

Answer 6

It binds to cGMP gated cation (Na,Ca) channels in the OUTER rod segment.

Question 7

Where is cGMP located?

Answer 7

In the cytoplasm of the rod photoreceptors outer portion.

Question 8

When cGMP binds to its cGMP receptors in the membrane of the outer rod segment, what is happening simultaneously?

Answer 8

In the inner rod segment there is a constant efflux of K ions.

Question 9

Despite K efflux, what is the overall state of the photoreceptor cell when cGMP receptors are activated and Na/Ca influx?

Answer 9

The photoreceptor cell is in a state of depolarisation.

Question 10

What establishes the ion gradients?

Answer 10

Na/K ATPase ensures there is a electrochemical gradient.

Question 11

In the dark adapted retina photoreceptor what is essential for function i.e step one.

Answer 11

The presence of a Na/K ATPase to create high external Na and high internal K.

Thus an electrochemical gradient.

Question 12

In the dark adapted retina photoreceptor what ensure function i.e step two

Answer 12

Guanylate cyclase ensures high cytoplasmic cGMP in the DARK.

Question 13

Describe what happens to the photoreceptors in dark conditions:

Answer 13

1. High cGMP
2. cGMP cation channels in the outer segment of the photoreceptor rod. cGMP binds.
3. Na and Ca influx through cGMP gated channels.
   - high Ca at synaptic terminal causes increased glutamate release.
4. Depolarisation
5. K efflux continues in the background.

Question 14

What does the inner segment of the photoreceptor contain?

Answer 14

non-gated selective channels. Thus resulting in K efflux.

Question 15

What is the overall state of depolarisation in the dark adapted photoreceptor considering all its ion channels.

Answer 15

Depolarised!

Resting membrane potential is around -40mV.

Question 16

In the dark what happens to photoreceptors?

Answer 16

They depolarised and released glutamate!

Question 17

In the light what happens to photoreceptors that differs from the dark?

Answer 17

1. Decrease in cGMP (low levels)
2. Na/Ca cGMP channels close.
3. Reduced levels of Na and CA
   - terminal Ca reduced therefore glutamate release stops
4. Hyperpolarisation - continued K efflux.

Question 18

What happens to rods and cones in response to an increment in light?

Answer 18

Both rods and cones hyperpolarise in response to an increment in light.

Question 19

Whats the difference between rods and cones to light sensitivity?

Answer 19

Rods are more sensitive to light than cones and therefore require less light to function than cones.

Question 20

In terms of photons how sensitive are rods?

Answer 20

Rod pigments can respond to 1 photon of light whereas cones require tens of hundreds of photons to become active.

Question 21

Whats the similarities between rods and cones structure?

Answer 21

Rods and cones both have outer+inner segments and synaptic terminals.

Question 22

Whats the differences in structure between rods and cones?

Answer 22

Rods are long and thing.
Cones are shorter and fatter.

Optic discs are invaginated in rods and are pinched off in cones.

Question 23

What are some general differences between rods and cones?

Answer 23

1. 20x more rods than cones.
2. Location: Rods - more in peripheral retina, cones - only cones in fovea.
3. sensitivity - rods 1000x more sensitive to light than cones thus good for low vision conditions
4. in type of vision : Rods = black and white
   cones = colour vision
5. Types. Rods = one type. Cones = 3
6. Recovery time: Rods = slow ,Cones = fast. (rapidly adapt to changes in illumination)

Question 24

What do all photoreceptors contain?

Answer 24

Visual pigment

Question 25

What visual pigment protein do rods contain?

Answer 25

Rhodopsin (like rod)

Pigment also contains a chromofore that is the same for all photoreceptors.

Question 26

What is rhodopsin?

Answer 26

• It is a light sensitive receptor protein involved in visual phototransduction

Question 27

Where is rhodopsin found?

Answer 27

In rods. It is a GPCR therefore has a protein coupled pathway.

Question 28

How sensitive is rhodopsin?

Answer 28

It is extremely sensitive to light and allows for vision in low light conditions.

Question 29

What happens to rhodopsin when it is exposed to light?

Answer 29

When rhodopsin is exposed to light, it photo bleaches. IN humans it is fully regenerated after 30mins. After which rods are more sensitive.

Question 30

What are the properties of the rod photoreceptor that enables it to detect low light?

Answer 30

1. One type of rod, all contain the same visual pigment = (opsin protein) rhodopsin + Retinal (chromophore)
2. Most sensitive to the intensity of blue-green light.
3. Rods contain a lot of visual pigment

Question 31

What conditions do rods allow vision for?

Answer 31

Scotopic (low light) conditions.

Question 32

Under scotopic conditions, which photoreceptors are active?

Answer 32

• Only rods are functioning
• Cones have no light to respond
• We see no colour

Question 33

What are the types of cone photoreceptors?

Answer 33

3 types:

1. L-cone (red-long wave)
2. M-cone (green- medium wave)
3. S-cone (blue- short wave)

Question 34

How do we see a range of colours?

Answer 34

The difference in the signal received from the three cone types allows the brain to perceive a continuous range of colours.

i.e yellow is perceived when the L-cone is stimulated slightly more than the m-cone

Question 35

Do cone photoreceptors have the same visual pigment?

Answer 35

No, they all have the same chromophore = retinal but the different visual pigment (photo opsin) there is red photo opsin, green photo opsin and blue photo opsin.

Question 36

What are the cones most sensitive to?

Answer 36

Colour and brightness of light

Question 37

Compared to rods, cones have less of…

Answer 37

Pigment therefore require more light to function.

Question 38

What sort of vision do cones allow?

Answer 38

Photopic (colour/light condition)

Question 39

What photoreceptors are function in photopic vision?

Answer 39

1. Occurs in daylight conditions
2. Only the cones are functioning
3. Rods bleach out quickly in bright lights
4. Three types of cones cells provide a wide range of colour vision in bright light.

Question 40

What is mesonic vision? and what photoreceptors mediate it?

Answer 40

1. Occurs in moderately low levels of light i.e after sunset
2. Both cones and rods are activated
3. The lower light levels allows the rod to replenish the light sensitive rhodopsin and begin functioning.
4. The cones still have enough light to provide some colour vision

Question 41

What should you consult for exam prep?

Answer 41

Summaries or goals of each lecture

Go over her slide that contains the cellular layers and receptive field. i.e what happens in dark vs light conditions.