Receptors

• Features of all sensory receptors

  • Receptors respond only to specific stimuli
  • Stimulation of a receptor leads to the establishment of a generator potential
• Pacinian corpuscle

  • Single nerve fibre surrounded by layers of connective tissue which are separated by viscous gel & contained by a capsule
  • Stretch-mediated Na+ channels on plasma membrane
  • Capillary runs along the base layer of tissue
• Stimulus Pacinian corpuscle responds to

  • Pressure deforms the membrane, causing stretch-mediated Na+ ion channels to open (& Na+ moves in)
  • If influx of Na+ raises the membrane to threshold potential, a generator potential is produced
  • Depolarization leads to generator potential
  • Action potential moves along the sensory neuron
• How resting potential of -70mV is maintained in sensory neurons when no pressure is applied

  1. Membrane more permeable to potassium ions & less to sodium ions; so maintains negative charge inside cell

  2. Sodium ions actively transported / pumped out

  3. Potassium ions actively transported in - via sodium-potassium pump
• Types of photoreceptor cell in the retina

  • Rod cells
  • Cone cells
• Rod cells

  • Evenly distributed around periphery but not in central fovea
  • Sensitive to light intensity
• Cone cells

  • Located mainly central fovea
  • No photoreceptors at blind spot
  • Sensitive to different wavelengths of visible light
• How high density of rod cells lead to seeing at night

  • High visual sensitivity
  • Several rods connected to a single bipolar neuron
  • Enough neurotransmitter to reach threshold OR spatial summation to reach threshold
• Staring at purple square

  Saw green afterimage