3.6.1.2 Receptors

• • Thermoreceptor - detect temperature changes
  • Chemoreceptor - detect specific chemicals
  • Baroreceptor - detect blood pressure
  • Photoreceptor - detect light (wavelength or intensity)
• Pacinian corpuscles are found in the skin and joints and detect pressure
• The Pacinian corpuscle:

  • When stimulated the stretch-mediated sodium ion channel proteins are deformed and open in the membrane, sodium ions can diffuse into the neurone causing depolarisation
  • If the threshold potential difference is exceeded a generator potential will be produced
  • The intensity of the stimulus is proportional to the frequency of the action potentials received by the CNS
  • The jelly like substance provides resistance to the pressure, so will spring back to its original position once the pressure is removed and the proteins will close
• Rod cells in the retina:

  • Produce black and white images
  • Sensitive to low light intensities
  • Contain a light sensitive pigment called rhodopsin
  • Groups of rods synapse with one bipolar neurone - more sensitive than cones
  • High visual activity
  • None at the fovea, distributed around the rest of the retina
• Visual sensitivity:

  • The ability to detect low light intensity
  • Many rods synapse to one bipolar neurone (spatial summation)
  • Each rod generates with a generator potential with a weak stimulation
  • Spatial summation leads to an action potential in bipolar neurone and then in the sensory neurone of the optic nerve
• Synaptic connections in the eye:

  • Iodopsin/rhodopsin absorb light leading to a generator potential in the cone/rod
  • At each synapse, enough neurotransmitter must be released to cause sodium ion channel proteins to open and cause depolarisation to send an AP to the brain
• Cone cells in the retina:

  • Produce colour images
  • Sensitive to high light intensities
  • Green, Blue and Red sensitive cones - each is stimulated by different wavelengths of light
  • Contains a lights sensitive pigment called iodopsin
  • 1 synapses with 1 bipolar neurone
  • High visual activity
  • Highly concentrated at the fovea, few elsewhere
• Trichromatic colour vision:

  • Colour vision is based on the three primary colours of light - red, green and blue
  • The three types of cones absorb light of particular wavelengths in varying proportions to give a perception of more than three colours
• Visual activity:

  • Ability to see a well resolved image
  • Individual cones close together = high visual activity, because each cone synapses with 1 bipolar neurone (then 1 sensory neurone)
  • The brain must receive separate action potentials in separate sensory neurones via the optic nerve to perceive two separate points
  • Only in high light intensities
  • Must have two unstimulated cones/groups of rods between 2 stimulated ones to perceive 2 separate points
• Pacinian corpuscle