I/E: Receptors

Question 1

How is the strength of the stimulus measured?

Why is this?

Answer 1

The frequency of action potentials, as all action potentials are one size.

Question 2

Mechanoreceptors

Answer 2

Receptors that detect mechanical stimuli, eg. pressure and vibrations.

Question 3

Pacinian corpuscles

Answer 3

Pressure receptors in the skin.

Question 4

Describe the structure of a pacinian corpuscle:

Answer 4

• Contains a sensory nerve ending.
• SNE is wrapped in layers of lamellate (connective tissue).

Question 5

What happens when a Pacinian corpuscle is stimulated?

Answer 5

• Lamellae are deformed and press on sensory nerve ending.
• Sensory neurone’s cell membrane stretches.
• Deforms streth-mediated sodium ion channels.
• Channels open and sodium ions diffuse into the cell.
• Creates a generator potential.
• If this reaches threshold, action potential is triggered.

Question 6

Photoreceptors

Answer 6

Light receptors in the eye.

Question 7

Describe how light is detected in the eye

Answer 7

• Light enters eye through pupil (amount is controlled by the retina).
• Light rays focused by the lens onto retina containing photoreceptors, mainly the fovea.
• Nerve impulses are carried from retina to brain by the optic nerve.

Question 8

Fovea

Answer 8

Area of the retina where there are lots of photoreceptors.

Question 9

Where is the blindspot in the eye?

Answer 9

Where the opctic nerve leaves the eye - there aren’t any photoreceptors there, so it’s not sensitive to light.

Question 10

Describe how photoreceptors convert light into an electrical impulse:

Answer 10

• Light is absorbed by light-sensitive optical pigments in photoreceptors.
• Bleaches the pigment, causes a chemical change and alters the membrane permeability to sodium ions.
• Generator potential is created and nerve impulse is sent along bipolar neurone if this reaches threshold.
• Connects to optic nerve and impulse is carried to the brain.

Question 11

What connects photoreceptors to the optic nerve?

Answer 11

Bipolar neurones

Question 12

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

Answer 12

Rods and cones

Question 13

Where are rods found?

Where are cones found?

Answer 13

Rods are mainly found in the peripheral parts of the retina.

Cones are mainly found in the fovea, tightly packed.

Question 14

What type of vision do rods give?

Answer 14

Monochromatic vision = only give information in black and white.

Question 15

What type of vision do cones give?

Answer 15

Trichromatic vision = give information in colour.

Question 16

What are the three types of cones?

What happens when they’re stimulated in different proportions?

Answer 16

Red-sensitive

Green-sensitive

Blue-sensitive

You see different colours when they are stimulated in different proportions.

Question 17

Describe the sensitivity of rods and cones:

Why is this?

Answer 17

Rods = very sensitive to light (work well in dim light).

Because many rods join one neurone, so many weak generator potentials combine to reach threshold.

Cones = less sensitive than rods (work best in bright light).

Because only one cone joins one neurone, so it takes more light to reach threshold.

Question 18

Describe the visual acuity of rods and cones:

Answer 18

Rods = low visual acuity

Because many rods join the same neurone, which means light from two close points can’t be told apart.

Cones = high visual acuity

Because cones are close together and only joins one neurone, so when light hits two cones, two action potentials go to the brain and are distinguishable.