[3.6.1.2] Receptors

Question 1

Describe the basic structure of a Pacinian Corpuscle

Answer 1

Question 2

Describe how a generator potential is established in a Pacinian corpuscle.

Answer 2

1. Mechanical stimulus e.g. pressure deforms lamellae and stretch-mediated sodium ion (Na⁺) channels.
2. So Na⁺ channels in membrane open and Na⁺ diffuse into sensory neurone.
   
   • Greater pressure causes more Na⁺ channels to open and more Na⁺ to enter.
3. This causes depolarisation, leading to a generator potential.
   
   • If a generator potential reaches threshold, it triggers an action potential.

Question 3

Explain what the Pacinian corpuscle illustrates.

Answer 3

• Receptors respond only to specific stimuli.
  
  • Pacinian corpuscle only responds to mechanical pressure.
• Stimulation of a receptor leads to the establishment of a generator potential.
  
  • When threshold is reached, action potential sent (all-or-nothing principle).

Question 4

Explain the difference in sensitivity to light for rods & cones in the retina.

Answer 4

Rods are more sensitive to light

• Several rods connected to a single neurone.
• Spatial summation to reach / overcome threshold (as enough neurotransmitter released) to generate an action potential.

Cones are less sensitive to light

• Each cone connected to a single neurone.
• No spatial summation.

Question 5

Explain the difference in visual acuity for rods & cones in the retina.

Answer 5

Rods give lower visual acuity

• Several rods connected to a single neurone.
• So several rods send a single set of impulses to brain (so can’t distinguish between separate sources of light).

Cones give higher visual acuity

• Each cone connected to a single neurone.
• Cones send separate (sets of) impulses to brain (so can distinguish distinguish between 2 separate sources of light)

Question 6

Explain the differences in sensitivity to colour for rods & cones in the retina.

Answer 6

Rods allow monochromatic vision

• 1 type of rod / 1 pigment.

Cones allow colour vision

• 3 types of cones - red-, green- and blue-sensitive.
• With different optical pigments -> absorb different wavelengths.
• Stimulating different combinations of cones gives range of colour perception.