Photoreceptors

Question 1

What are the two types of photoreceptors found in the retina? What are their functions?

Answer 1

• Rods: B&W vision, bright and dim light

- Cones: colour vision, only bright light

Question 2

What photochemical pigment is found in rod cells?

Answer 2

Rhodopsin and it is found in the inner segment in the vesicles.

Question 3

What 3 layers make up the retina?

Answer 3

1) the RODS & CONES synapse with…
2) the BIPOLAR nuerone cells with synapse with…
3) the GANGLION NEURONE, whose axons together make the optic nerve.

Question 4

IN THE DARK — What happens to rod cells when in the dark?

Answer 4

Na+ flow into outer segment via non-specific cation channels. Na+ moves down concentration gradient to inner seg. Na+ is actively pumped out.

Question 5

IN THE DARK — What does the influx on Na+ cause to the rod cell in the dark?

Answer 5

Influx produces slight depolarisation of cell. PD across membrane is -40mV (resting PD is -70mV).

Question 6

IN THE DARK — What does the slight depolarisation of the rod cell (IN THE DARK) cause?

Answer 6

Slight depolarisation triggers release of neurotransmitter (glutamate) from rod cells. In dark, rod cells release this continuously. It inhibits bipolar neurone, stopping it from depolarising.

Question 7

IN THE LIGHT — What happens when light energy reaches rhodopsin?

Answer 7

it breaks down into RETINAL and OPSIN - this is BLEACHING.

Question 8

IN THE LIGHT — what does bleaching of rhodopsin cause?

Answer 8

Opsin binds to membrane causing series of reactions which close Na+ cation channels. Influx of Na+ decreases while inner segment continues to pump Na+ out so Na+ build up outside cell. Inside more negative than outside - hyperpolarised.

Question 9

IN THE LIGHT — What happens after the rod cell becomes hyperpolarised?

Answer 9

Release of glutamate neurotransmitter stops. This means no inhibition of bipolar neurone so it depolarises. If change in potential diff. reaches threshold, an action potential is transmitted to brain via optic nerve.

Question 10

So, very basically, how do photoreceptors convert light into an electrical impulse?

Answer 10

1) light enters eye, hits the photoreceptor and is absorbed by rhodopsin.
2) light bleaches it cause chemical change.
3) This triggers a nerve impulse along bipolar neurone.
4) Sends impulse to brain via optic nerve.