Stimuli & response

• receptors detect external and internal stimuli.

  • receptor cells respond to stimuli by converting energy.
• Pacinian corpuscle:

  • a pressure receptor located in the dermis
  • it is a sensory neurone with a non-mylinated ending located inside a capsule formed of several layers of connective tissue.
• All or nothing Principle:

  1. when pressure is applied lamellae deform and press into the sensory neurone
  2. this opens the stretch mediated sodium ion channels.
  3. sodium ions diffuse into the sensory neurone cell.
  4. causing depolarisation & a generator potential.
  5. if a specific threshold is reached the generator potential will cause an action potential.
  6. by opening voltage gated sodium ion channels.
• In resting state / no pressure applied, stretch mediated sodium ion channels are too narrow to allow sodium ions through.
• In resting state, sodium ions are constantly pumped out of the sensory neurone allowing facilitated diffusion down a concentration gradient.

  When stretch mediated sodium ion channels are opened.
• what are the different types of cell in the eye?

  rods and cones
• The pigments in rods and cones are broken down by light.

  • if enough pigment is broken down a generator potential is caused.
  • This sends an impulse via the optic nerve to the brain.
• which eye cells is responsible for seeing colour?

  cone cells (cone = colour)
• what pigment is in cones?

  iodopsin
• what pigment is in rods?

  rhodopsin
• Rods:

  • many rods synapse with a single bipolar cell.
  • This allows spatial summation (stimulation from many rods combined)
  • It is more likely the generator potential will reach a threshold and generate an action potential to send impulses to the brain.
  • Giving greater sensitivity to light.
• at the fovea only cones are present
• Cones:

  • each cone synapses with a single bipolar cell.
  • so the brain receives impulses from each cone giving a greater resolution.
• convergence means that information from several rods is combined into one signal to the brain.

  • lowering resolution
• rods:

  • found throughout the retina but not the fovea.
  • allow vision in low light.
  • contain rhodopsin which is rapidly broken down in bright light and takes time to regenerate.
  • rhodopsin can be broken down in low light levels.
• cones:

  • located mainly at the fovea.
  • allow vision in bright light.
  • contain iodopsin which is less easily broken down than rhodopsin.
  • so cones only produce a generator potential in bright light.
• what are the three types on come cell?

  red

  blue

  green
• There are three types of cone cell, each with a different type of iodopsin that absorbs different wavelengths of light.
• a tropism is a response of a plant to a directional stimulus.

  • plants respond to stimuli to regulate their growth
• a positive tropism is growth towards the stimulus
• a negative tropism is growth away from the stimulus
• phototropism is growth in response to light.

  • shoots are positively phototrophic
  • roots are negatively phototrophic
• gravitropism is growth in respinse to gravity

  • shoots are negatively gravitrophic
  • roots are positively gravitrophic
• • plants respond to stimuli using specific growth factors
  • growth factors are produced in the growing regions of a plant
  • growth factors called auxins stimulate growth of shoots by cell elongation.
  • high concentrations of auxins inhibits root growth
• IAA is an auxin produced in the tips of shoots.
• phototropism:

  1. IAA moves towards the more shaded parts of shoots & roots causing elongation of these cells.
  2. so this area bends towards the light in shoots.
  3. in roots IAA concentration increases on shaded side, inhibiting growth of the lower side.
  4. so roots bend down away from the light.
• gravitropism:

  1. IAA moves to the underside of the shoots & roots.
  2. causing elongation in the underside of the shoots making them bend upwards.
  3. causing inhibited growth on the underside of the roots making them bend down.
• taxes is when an organism moves in response to a directional stimulus e.g light
• kinesis is when an organism moves in response to a non-directional stimulus. e.g humidity
• rods give low visual acuity because many rods join the same neurone, meaning light from two points close together cannot be told apart.
• cones give high visual acuity because one cone joins with one neurone, so when light from different points hits different cones, separate action potentials are generated.

  • This means two seperate points can be distinguished easily.