6.1.2 - Receptors

Question 1

A Pacinian corpuscle detects pressure changes in the skin. Describe the structure of a Pacinian corpuscle. (3)

Answer 1

• Found at the end of a sensory neurone
• (This part of the sensory neurone is) covered in layers of membrane
• (The layers of tissue are) separated by gel which contains sodium/Na+ ions
• The membrane layers contain stretch mediated sodium ion channels

Question 2

Describe how the stretch-mediated sodium ion channels facilitate the establishment of a generator potential when pressure is applied to the Pacinian corpuscle. (3)

Answer 2

• Pressure causes deformation of the Pacinian corpuscle / stretch-mediated sodium ion channels are stretched
• Therefore sodium ions can diffuse/flow into the neurone/axon
• (This influx of sodium ions) causes depolarisation of the membrane (leading to a generator potential)

Question 3

Explain why the membrane potential was identical despite different intensities of pressure being applied. (2)

Answer 3

• The threshold value was met/exceeded
• Of the all or nothing principle
• The threshold potential triggers the opening of voltage-gated ion channels
• Causing a generator potential to be established (leading to an action potential)

Question 4

Describe how the summation of rod cells provides an advantage to nocturnal animals. (2)

Answer 4

• Stimulation of multiple rod cells which synapse with a single bipolar cell provides a combined generator potential
• (This combined generator potential means) the threshold level is achieved to establish an action potential in the bipolar cell
• This enables organisms to see in much dimmer light / have better night time vision

Question 4

Suggest why the fovea provides vision with higher acuity than the rest of the retina. (2)

Answer 4

• The fovea contains only cone cells
• Each cone cell/receptor in the fovea is connected to a single neurone whereas rod cells found in the rest of the retina share a neurone
• There is no summation of receptor cells in the fovea whereas in the rest of the retina, summation of rod cells occurs

Question 5

Explain how the sensory neurone maintains a resting potential when no pressure is applied. (2)

Answer 5

• The cell membrane of the sensory neurone is more permeable to potassium ions and less permeable to sodium ions
• Sodium ions are actively transported/pumped out (of the neurone) while potassium ions are transported/pumped in

Question 6

Explain the mechanism by which the application of pressure to the Pacinian corpuscle produces the changes in membrane potential. (3)

Answer 6

• The applied pressure causes the membrane/lamellae to become stretched/deformed
• Which causes the sodium ion channels in the membrane to open and sodium ions to move in
• A greater/larger pressure causes more channels open / more sodium ions to enter

Question 7

Explain how a lack of functional myelin can result in slower responses to stimuli. (2)

Answer 7

• There is less/no saltatory conduction/ the action potential/impulse is unable o ‘jump’ from node to node
• So more depolarisation is required over the length/area of the membranes

Question 8

The fovea of the eye of an eagle has a high density of cones. An eagle focuses the image of its prey onto the fovea. Explain how the fovea enables an eagle to see its prey in detail. Do not refer to colour vision in your answer. (3)

Answer 8

• High (visual) acuity
• (Each) cone is connected to a single neurone
  ‘accept no retinal convergence’
  ‘accept ‘bipolar/nerve cell’ for neurone’
• (Cones send) separate (sets of) impulses to brain

Question 9

The retina of an owl has a high density of rod cells. Explain how this enables an owl to hunt its prey at night. Do not refer to rhodopsin in your answer. (3)

Answer 9

• High (visual) sensitivity
  ‘accept retinal convergence’
• Several rods connected to a single neurone
  ‘accept bipolar/nerve cell for neurone’
• Enough (neuro)transmitter to reach/overcome threshold
  /Spatial summation to reach/overcome threshold

Question 10

Explain what causes vision using the fovea…
- to be in colour
- to have high visual acuity (2)

Answer 10

• (Three) different types of (cone) cells /to different wavelengths / different frequencies / different colours
• Impulses along separate neurone from each receptor cell / each receptor cell connects to separate neurone