3.4.2 Receptors

Question 1

Receptors only detect …

Answer 1

one particular stimulus

Question 2

Describe a receptor in its resting state (not being stimulated)

Answer 2

There’s a difference in charge between inside and outside of cell

Question 3

What is the difference in charge between inside and outside of receptor cell generated by and what does this mean?

Answer 3

• Difference is generated by ion pumps and ion channels
• Means there’s a potential difference (voltage) across the membrane

Question 4

What is meant by resting potential?

Answer 4

Potential difference when cell is at rest

Question 5

Describe how a generator potential is created

Answer 5

1. When stimulus is detected, cell membrane is excited and becomes more permeable
2. Allows more ions to move in & out = alerting potential difference
3. Change in p.d. due stimulus = generator potential

Question 6

Explain how a bigger stimulus results in a bigger generator potential being produced

Answer 6

Bigger stimulus excites membrane more = bigger movement of ions and bigger change in p.d.

Question 7

State when an action potential is generated

Answer 7

If generator potential is big enough i.e. reaches threshold level, it’ll trigger an action potential

Question 8

Describe the size of action potentials

Answer 8

Action potentials are all same size

Question 9

How is the strength of a stimulus measured?

Answer 9

By frequency of action potentials

Question 10

State what SOPI stands for

Answer 10

Sodium Out Potassium In

Question 11

What type of receptors are Pacinian corpuscles?

Answer 11

Mechanoreceptors

(Detect mechanical stimuli e.g. pressure, vibrations)

Question 12

Pacinian corpuscles the contain end of a _____ ____

Answer 12

Pacinian corpuscles contain the end of a sensory neurone

(Called sensory nerve ending)

Question 13

What is the sensory nerve ending wrapped in?

Answer 13

Lamellae

Question 14

Describe what happens when a Pacinian corpuscle is stimulated

Answer 14

1. When the Pacinian corpuscle is stimulated the lamellae are deformed and press on the sensory nerve ending
   
   • = the sensory neurone’s cell membrane to stretch
2. This deforms (stretch-mediated) sodium ion channels
3. This opens the channels = greater pressure & allows for Na⁺ to diffuse into the cell creating a generator potential

Question 15

Where are photoreceptors found?

Answer 15

Retina

Question 16

What is the fovea?

Answer 16

An area of retina where there’s loads of photoreceptors

Question 17

Nerve impulses from photoreceptor cells are carried from retina to brain by ___ ___ (bundle of neurones)

Answer 17

Optic nerve

Question 18

Where is the blind spot located?

Answer 18

Where optic nerve leaves eye

(aren’t any photoreceptor cells)

Question 19

What do photoreceptors do?

Answer 19

Convert light into electrical impulse

Question 20

What happens when light enters the eye?

Answer 20

It hits photoreceptors and is absorbed by light-sensitive pigments

Question 21

Light _____ pigments

Answer 21

bleaches

Question 22

What happens when light bleaches pigments?

Answer 22

1. Causes chemical change and alters membrane permeability to sodium ions
2. Generator potential is created & if it reaches threshold = nerve impulse is sent along bipolar neurone

Question 23

What do bipolar neurones do?

Answer 23

Connect photoreceptors to optic nerve, which takes impulses to brain

Question 24

Name the 2 types of photoreceptor the human eye has

Answer 24

• Rods
• Cones

Question 25

Where are rods found?

Answer 25

Found in peripheral parts of retina

Question 26

Where are cones found?

Answer 26

Cones packed together in fovea

Question 27

Why do rods and cones give information in different colours?

Answer 27

Rods and cones contain different optical pigments making them sensitive to different wavelengths of light

Question 28

Rods give information in ____

Answer 28

B&W (monochromatic vision)

Question 29

Cones give information in ____

Answer 29

Colour (trichromatic vision)

Question 30

Name the 3 types of cones (each containing a different pigment)

Answer 30

1. Red-sensitive
2. Green-sensitive
3. Blue-sensitive

When stimulated in different proportions = see different colours

Question 31

State which one is more sensitive to light: rods or cones?

Answer 31

Rods

Question 32

Explain why rods are very sensitive to light (work well in dim light)

Answer 32

• ∵ many rods join one neurone = so many weak generator potentials combine to reach threshold and trigger action potential
• & rhodopsin can be broken down easily (pigment needs to be broken down in order to create an action potential)

Question 33

Explain why cones are less sensitive than rods (work best in bright light)

Answer 33

• ∵ one cone joins one neurone
• Takes more light to reach threshold and trigger action potential
• & requires lots of light intensity to breakdown iodopsin

Question 34

Which one gives a higher visual acuity (ability to tell apart points
that are close together): rods or cones?

Answer 34

Cones

Question 35

Explain why cones give a high visual acuity

Answer 35

• ∵ cones are close together and one cone joins one neurone
• When light from 2 points hits 2 cones, 2 action potentials (one from each cone) go to the brain
• Can distinguish 2 points that are close together as 2 separate points

Question 36

Explain why rods give a low visual acuity

Answer 36

• Give low visual acuity ∵ many rods join same neurone
• Which means light from 2 points close together can’t be told apart

Question 37

The membrane potential at Q was the same whether medium or heavy pressure was applied to the finger tip. Explain why.

Answer 37

• Threshold has been reached
• (Threshold or above) causes all or nothing principle

Question 38

Explain why it takes time for the rod cells to recover their sensitivity to light after moving into darkness. (2)

Answer 38

• rhodopsin bleached / broken down by light
• time for resynthesis