RECEPTORS

• What photoreceptors are in the eye?

  rods and cones
• what are photoreceptors?

  • detect light
  • found in retina
  • fovea contains most photoreceptors
  • iris controls amount of light that enters
• BLIND SPOT= Where optic nerve leaves eye and contains no photoreceptors
• BARORECEPTORS

  • detect pressure
  • found in aorta and corteriod arteries
  • stimulated by blood pressure]
• CHEMORECEPTORS

  • Monitor co2 lvl, 02 lvl and ph
  • found in aorta, carterioid arteries and medulla
• How do photoreceptors work?

  • light enters, hits photoreceptors and is absorbed by light sensitive pigments
  • light bleaches pigments causing a chemical change which alters Na+ ion permeability causing them to enter
  • Generator potential created and if reaches threshold an impulse is sent along bipolar neurone
  • these connect the photoreceptors to optic nerve
• Describe how stimulation of a Pacinian corpuscle produces a generator potential.

  • contain stretch mediated channels in membrane
  • when pressure applied, lamella deform and press on sensory nerve ending
  • This causes membrane to stretch causing Na+ channels to open
  • Na+ diffuse into cell causing depolarisation leading to a generator potential
• what is meant by the all-or-nothing principle.

   An action potential is only generated/produced when threshold is reached
• no new generator potential could be produced. What is this time period called?

  refractory period
• in response to touch, nerve impulses can be transmitted at speeds of 76.2 m s–1

  Suggest three reasons why, in this investigation, the estimated speed of student A’s impulse transmission was less than 76.2 m s–1

  • time taken for Na+ channels to open
  • time taken for muscle contraction
  • student may have been distracted
  • synaptic transmission
• The retina of an owl has a high density of rod cells.

  Explain how this enables an owl to hunt its prey at night.

  don't refer to rhodopsin (basically don't talk about colour vision)

  • Rod cells have high visual sensitivity
  • several rods connect to 1 neurone
  • spacial summation to reach/overcome threshold
• When the image is focused on the fovea, the person sees the object in colour.Explain why.

  fovea contain high amount of cone cells

  cone cells detect colour vision

  3 types of cone cells as sensitive to 3 diff colours/ light wavelengths (blue, green , red)
• mechanoreceptors detect pressure
• chemoreceptors detect chemicsld
•  Explain why vision using the fovea has high visual acuity.

  contain cones

  each cone is connected to a seperate neurone

  rods in other areas share a single neurone
•  Explain why vision using other parts of the retina has high sensitivity to light.

  contains rod cells

  rod cell pigment (rhodopsin) is very sensitive to light

  many rods join 1 neurone so require meany weak depolarisations to combine to reach threshold for action potential
•  Explain why it takes time for the rod cells to recover their sensitivity to light after moving into darkness.

  rhodopsin has been bleached so needs time to resynthesise
• rod cell pigment


  rhodopsin
• cone cell pigment


  iodopsin
• Describe how the number and distribution of rods and cones across the retina would differ in a nocturnal mammal from the number and distribution in a human. Explain your answer.

  nocturnal mammal would contain more rods and fewer cones

  rods have a higher sensitivity to light

  rhodopsin is bleached at lower light levels
• Explain what causes vision using the fovea to be in colour;

  contains 3 types of different cone cells which are sensitive to different wavelengths of light
• explain what causes fovea to have high visual acuity

  contains cones

  each cone cell (receptor) connects to a separate neurone

  this means impulses are sent down separate neurones
• he blink reflex is caused by stimulation of receptors in the eye or eyelid.

  Suggest two types of stimuli to which these receptors might respond.

  pressure

  light

  touch

  temperature

  smell
• The blink reflex can be stopped by drugs which prevent the opening of sodium ion channel proteins in the axons of motor neurones.

  Suggest how these drugs affect the passage of nerve impulses along the axons.

  would be no Na+ ion influx to cause depolarisation
• Channel proteins on presynaptic neurones are involved in reflex responses.

  Explain how.

  allows calcium ions in at end of presynaptic neurone causing release of neurotransmitter