biology 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.

• 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.

• 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.

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.

• 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.

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.  

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.  

• 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.

• 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.

• 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.

• 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.