Eyes

• What is a receptor?


  Cell or protein on cell membrane which detects stimulus
• How do receptors work?


  Specific to one stimulus

  Convert energy of stimulus to electrical energy
• What is the generator potential?

  Change in potential of receptor due to stimulus
• When will an action potential be stimualted from a receptor?


  The stimulus is big enough

  The membrane is excited enough

  Generator potential above threshold level
• If a stimulus is weak why is an action potential not produced?

  Membrane of receptor not excited enough

  Generator potential not above threshold level
• What is a Pacinian corpuscle?


  Receptors in the skin that only respond to pressure and vibrations

  Example of how receptors are specific
• How do Pacinian corpuscles trigger action potentials ?


  When stimulated e.g. by pressure

  Lamella deformed and press on sensory neurone ending

  Stretch-mediates sodium ion channels open

  Na+ diffuse into cell

  Generator potential above threshold created
• What are photoreceptors?


  Receptors in eye that detect light
• Describe the structure of the Pacinian corpuscle

  End of sensory neurone (sensory nerve ending)

  Wrapped in layers of connective tissues (lamellae)
• How do photoreceptors work?

  Light enters eye and hits photoreceptors

  Light absorbed by optical pigments

  This alters membrane permeability to Na+

  Generator potential

  If threshold reached, impulse sent alone bipolar neurone
• What do the bipolar neurones connect?

  Photoreceptors to optic nerve
• What does the optic nerve do?

  Send impulses to the brain
• What are rod cells?


  Receptors with a high sensitivity to light

  Found in peripheral parts of retina
• What is the visual acuity of rods and cones?

  Rods is low

  Cones is high
• What colour is the information given by rods and cones?

  Rods is black and white

  Cones is in colour
• How do rods have a high sensitivity to light?

  Many rods join on bipolar neurone

  Many weaker generator potentials combine to reach threshold level and trigger action potential
• Why is it important that each cone cell is only joined to one bipolar neurone?

  Gives high visual acuity

  Can distinguish between two points that are close together
• What is visual acuity?

  The ability to distinguish between points that are close together
• What optical pigments do cone cells contain?

  Blue sensitive

  Green sensitive

  Red sensitive
• How can you see in different colours?

  Optical pigments in cone cells stimulated to different degrees
• Why are rod cells important?


  Allow us to see in dim light
• Why do rods have a low visual acuity?

  Many rod cells connected to same bipolar neurone

  Can't distinguish between 2 points close together
• Why are cone cells less sensitive than rod cells?

  Each cone cell joins one bipolar neurone

  Larger stimulus (i.e. more light) needed to create generator potential above threshold