6.1.2 Receptors

• Describe the basic structure of a Pacinian corpuscle

  Lamellae (layers of connective tissue)

  Stretch mediated sodium ion channel (closed)

  Sensory neurone ending

  Gel

  Sensory neurone axon

  Myelin sheath (Schwann cells)
• Describe how a generator potential is established in a Pacinian corpuscle

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

  • ● 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)
• Explain the differences in sensitivity to light for rods & cones in the retina

  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
• Explain the differences in visual acuity for rods & cones in the retina

  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 between 2 separate sources of light)
• Explain the differences in sensitivity to colour for rods & cones in the retina

  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