6.1.2 Receptors

• structure of pacinian corpuscle:

  A) lamellae

  B) sensory neurone ending

  C) sensory neurone axon

  D) myelin sheath (schwann cells)

  E) stretch mediated sodium ion channel

  F) gel

  
• 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

  ○ Greater pressure causes more Na+ channels to open and more Na+ to enter

  3. This causes depolarisation, leading to a generator potential

  ○ 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
• rod/cone cells

  A) cone cells

  B) rod cells

  
• explain how a resting potential of -70mV is maintained

  • membrane more permeable to potassium ions
  • membrane less permeable to sodium ions
  • sodium ions actively transported out and potassium ions actively transported in
• explain how applying pressure to the Pacinian corpuscle produces changes in membrane potential

  • pressure causes membrane to become deformed
  • sodium ion channels open and sodium ions move in
  • greater pressure causes more channels to open and more sodium to diffuse in
• membrane potential was the same whether medium or heavy pressure was applied to fingertip, explain why

  • threshold has been reached
  • so maximum response has been carried out
  • all or nothing principle