Photoreceptors

Question 1

What are photoreceptors?

Answer 1

Receptors in your eye that detect light.

Question 2

What contains photoreceptor cells?

Answer 2

The retina

Question 3

What controls how much light enters through the pupil?

Answer 3

The muscles of the iris.

Question 4

What is the fovea?

Answer 4

An area of the retina where there are lots of photoreceptors.
The fovea recieves the highest intensity of light therefore the cone cells and not rod cells are found there.

Question 5

What does the optic nerve do?

Answer 5

It is a bundle of nerves which carries impulses from the photoreceptor cells to the brain.

Question 6

How do photoreceptors work?

Answer 6

Light enters the eye, hits the photoreceptors and is absorbed by light sensitive pigments.
Light bleaches the pigments, causing a chemical change and altering the membranes permeability to sodium.
A generator potential is created and if it reaches the threshold, a nerve impulse is sent along a bipolar neurone.
Bipolar neurones connect photoreceptors to the optic nerve, which takes impulses to the brain.

Question 7

What are the two types of receptor in the human eye?

Answer 7

Rods and cones

Question 8

Where are rods mainly found?

Answer 8

In the peripheral parts of the retina, where light intensity is lowest.

Question 9

Where are cones mainly found?

Answer 9

Packed together in the fovea.

Question 10

What colours do rods give information in?

Answer 10

Black and white (monochromatic)

Question 11

What colours do cones give information in?

Answer 11

Colour (trichromatic)

Question 12

What are the three types of cones?

Answer 12

Red- sensitive
Green- sensitive
Blue- sensitive

Question 13

How many rods and cones join neurones?

Answer 13

Many rods join one neurone

One cone joins one neurone

Question 14

Rods and cons sensitivity to light

Answer 14

Rods =  high
Cones = low

Question 15

Rods and cons visual acuity

Answer 15

Rods = low
Cones = high

Question 16

What is visual acuity?

Answer 16

The ability to tell apart points that are close together.

Question 17

Many rod cells share a single sensory neurone. Rod cells can therefore respond to light of very low intensity. Why is this?

Answer 17

This is because a certain threshold frequency has to be exceeded before the generator potential is created in the bipolar cells to which they are attached.
This is made more likely as a number of rod cells are attached to a single bipolar cell.

Question 18

How is a generator potential created in a rod cell?

Answer 18

The pigment rhodopsin must be broken down (low intensity light is sufficient to do this).

Question 19

Why do rod cells have low visual acuity?

Answer 19

Many rod cells link to a single bipolar cell.
The light received by rod cells sharing the same neurone will only generate a single impulse regardless of how many neurones are stimulated.
This means that they cannot distinguish between the different sources of light that stimulated them.

Question 20

Why do cone cells only respond to high intensity light?

Answer 20

Each cone cell often has its own sensory neurone.
This means that they cannot combine to meet the threshold frequency required for the generator potential.
Also, the pigment required for the generator potential iodopsin, requires a higher light intensity to be broken down.

Question 21

Why do cone cells have a high visual acuity?

Answer 21

Each cone cell has its own connection to a single bipolar cell, meaning if two adjacent cone cells are stimulated, the brain recieves two different signals which it can distinguish between.

Question 22

Explain the role of the Na+ and K+ channels in producing the membrane resting potential.

Answer 22

More potassium pores than sodium pores
More potassium leave via active transport
Therefore more positive on outside;

Question 23

How does the Na+ gate allow depolarisation of this membrane.

Answer 23

Sodium ions pumped out (to create diffusion gradient)

Sodium gates open to allow sodium ions (to flood) in;

Question 24

Explain how the action potential is generated

Answer 24

Sodium ion channels open;

Allowing rapid influx of sodium ions;

Question 25

Explain how the axon membrane is repolarised

Answer 25

Sodium ion channels close and potassium ion channels open;

Allowing efflux of potassium ions;