biology Stimuli and responses

Stimuli and responses


  • Response by plants to directional stimuli.
  • Growth towards is positive. Growth away is negative.
  • Response is stimulated by plant growth regulators called auxins.
  • Auxins stimulate cells in the stem/roots to grow faster.
  • When one site is stimulated the stem/roots bends.
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  • A simple response that some mobile organisms use to keep themselves in a favourable environment.
  • Is a directional response to the stimulus.
  • The organism doesn't depend on chance to get into a favourable environment.
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  • Simple response that some mobile organisms use to keep themselves in a favourable position in their environment.
  • Non-directional response to a change in the environment.
  • The rate of random movement increases.
  • If the organism finds itself in a more favourable environment, it stops moving.
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Simple reflex arc

Stimulus > receptor > sensory neurone > CNS > motor neurone > effector > response.

  • The response doesn't need to be thought out.
  • The response is always the same.
  • This type of control is very important for protecting the organism from damage.
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Autonomic control of heart rate

  • Heartbeat is controlled by the autonomic nervous system.
  • The cardioregulatory centre is found in the medulla (brain stem).
  • There are two parts - acceleratory centre and inhibitory centre.
  • Acceleratory sends impulses along the sympatheic nerve to the SAN.
  • Noradrenaline is released and heart rate increases.
  • Inhibitory sends inpulses along the vagus (parasympatheic) nerve.
  • Acetyl choline is release and heart rate decreases.
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Control of blood pressure

If blood pressure is too low:

  • Cartoid sinus (baroreceptor) is less stretched.
  • Impulses sent at a lower frequency.
  • Acceleratory centre is stimulated.
  • Impulses sent along sympathetic nerve.
  • SAN increases heart rate and blood pressure.

If blood pressure is too high:

  • Cartoid sinus (baroreceptor) is more stretched.
  • Impulses sent at a higher frequency.
  • Inhibitory centre is stimulated.
  • Impulses sent along vagus nerve.
  • SAN slows heart rate and decreases blood pressure.  
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Control of blood pH/ Carbon dioxide

Rise in CO2/ fall in pH:

  • Chemoreceptors send impulses to ventilation centre (VC) and cardioregulatory centre (CR).
  • VC increases rate and depth of breathing.
  • CR sends impulses along sympathetic nerve, speeding up heart beat and increasing blood pressure.
  • CO2/ pH returns to normal levels.

Fall in CO2/ rise in pH:

  • Chemoreceptors send impulses to VC and CR.
  • VC reduces rate and depth of breathing.
  • CR sends impulses along vagus nerve, decreasing heart rate and blood pressure.
  • CO2 / pH returns to normal levels.  
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Pacinian Corpuscles

  • detects changes in pressure on the skin and around joints and tendons.
  • Contains two protein channels - a sodium ion pump and a channel protein that allows sodium ions into the cell by facilitated diffusion.
  • The channel protein is normally closed.
  • When pressure is applied the channel proteins are stretched and change shape, causing them to open.
  • Sodium ions flood in along their gradient causing an influx of positive charges.
  • This creates an electrical potential difference.
  • The difference initiates an electrical impulse.
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Rods & cones

  • Are light sensitive cells found in the retina.
  • Transduce light energy into electrical impulses.
  • Rods are sensitive to low light levels and give us night vision (no colour).
  • Cones need high levels of light to work and give day vision (colour).
  • Both synapse with bipolar cells.
  • Several rods connect to one bipolar cell.
  • Cones connect to an individual bipolar cell.
  • Rods are found all over the retina except to fovea and blind spot.
  • Cones are concentrated in the fovea.
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Visual acuity

  • The ability to distinguish to (close together) spots of light seperately.
  • During the day cones give high visual acuity.
  • As cones are connected to their own bipolar cell, when they are stimulated they send their individual nerve impulses to the brain.
  • In order to see two points of light, they must fall onto two seperate cones with a gap of at least one cone between them.
  • Light rays falling on rods that are close together result in nerve impulses going to the same ganglion cell, so they can't be percieved seperately.
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Sensitivity to low light levels

  • Rod cells give better sensitivity to dim light.
  • Because many rods are connected to one bipolar cell, if several rods are stimulated their generator potentials combine.
  • This means there is a big enough generator potential to produce a nerve impulse.
  • This is called summation because the effects are added together.
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