3.6.1.2 Receptors

Question 1

Why are receptors described as specific?

Answer 1

Receptors only detect one particular stimulus.

Question 2

In the human eye, which type of photoreceptor is more sensitive to light?

Answer 2

Rod cells/photoreceptors

Question 3

Explain how a generator potential is created when a Pacinian corpuscle is stimulated.
[3 marks]

Answer 3

• When a Pacinian corpuscle is stimulated, the lamellae are deformed and press on the sensory nerve ending.
• This causes the sensory neurone’s cell membrane to stretch and the deformation of stretch-mediated sodium ions.
• The sodium ion channels open and sodium ions diffuse into the cell creating the generator potential.

Question 4

Explain how the human eye can provide high visual acuity.

[3 marks]

Answer 4

• In the retina/fovea, cones are close together and each cone joins one bipolar neurone.
• When light from two points hits two cones, action potentials from each cone go to the brain.
• This means you can distinguish two points that are close together as two separate points.

Question 5

For a particular receptor cell, an action potential is triggered when the generator potential reaches - 60 mV.
What name is given to the value at which an action potential will be triggered?

Answer 5

Threshold level

Question 6

Suggest how a person’s perception of touch might be affected by drugs that block stretch-mediated sodium ion channels in cell membranes.

Answer 6

• Pressure from touch would normally deform the stretch-mediated sodium ion channels in Pacinian corpuscles.
• However, by blocking sodium ion channels the drug would stop sodium ions from diffusing into the cell and generating an action potential.
• This would mean the person wouldn’t be able to perceive that they were being touched.

Question 7

Explain how a generator potential is produced.

Answer 7

• When a stimulus is detected, the cell membrane is excited and becomes more permeable,…
• …allowing more ions to move in and out of the cell.
• This alters the potential difference across the cell membrane and therefore produces a generator potential.

Question 8

What type of stimulus does a Pacinian corpuscle respond to?

Answer 8

Mechanical

Question 9

Describe the structure of a Pacinian corpuscle.

Answer 9

• A Pacinian corpuscle contains the end of a sensory neurone.
• The sensory nerve ending is wrapped in layers of connective tissue called lamellae.

Question 10

Explain how the presence of a stimulus triggers an action potential in a Pacinian corpuscle.

Answer 10

• When a Pacinian corpuscle is stimulated the lamellae are deformed and press on the sensory nerve ending.
• This causes deformation of stretch-mediated sodium ion channels in the sensory neurone’s cell membrane.
• The sodium ion channels open and sodium ions diffuse into the cell, creating a generator potential.
• If the generator potential reaches the threshold, it triggers an action potential.

Question 11

Explain why cones give a higher visual acuity than rods.

Answer 11

• Cones are close together and each cone joins one bipolar neurone.
• So when light from two points that are close together hits two cones, an action potential from each cone goes to the brain.
• This means that the light can be distinguished as coming from two separate points.
• This doesn’t happen in rods because many rods join the same bipolar neurone, which means light from two points close together can’t be told apart.

Question 12

Other than visual acuity, give three differences between rods and cones.

Answer 12

Any three from:

• Rods are found mainly in the peripheral parts of the retina and cones are mainly found packed together in the fovea.
• Rods only give information in black and white but cones give information in colour.
• Rods are very sensitive to light but cones are less sensitive.
• Many rods join one bipolar neurone, but only one cone joins one bipolar neurone.