Stimuli and response

• What is a stimulus?

  A detectable change in the internal or external environment of an organism that leads to a response in the organism.
• What detects a stimulus?

  Receptors detect a stimuli and are specific to one type of stimulus.
• Receptor


  Detects stimuli specific to one type of stimulus.
• Co-ordinator


  Formulates a suitable response to a stimulus.
• Effector


  Produces a response
• What are the 2 different means of communication in organisms?

  • Endocrine system - hormones, slower process, found in plants and animals
  • Nervous system - more rapid process found in animals
• Stimulus -> Receptor -> Coordinator -> Effector -> Response
• 4 main examples of responses:

  1. Taxis
  2. Kinesis
  3. Tropism
  4. Reflex
• Taxes


  A simple response whose direction is determined by the direction of the stimulus
• Positive taxis


  Moves towards the stimulus
• Negative taxis

  Moves away from the stimulus
• Examples of taxes

  • Earthworms move away from light (negative phototaxis)
  • Single celled algae move towards light (positive phototaxis)
• What is Kinesis?


  A response where an organism does not move towards or away from a stimulus, instead it changes the speed at which it moves and the rate at which it changes direction.
• Example of kinesis

  Woodlice - lose water in dry conditions

  Dry - move more rapidly and change direction more often

  Damp - move less rapidly and change direction less often
• Tropisms


  The direct stimuli which contributes to plant survival (such as light, nutrients and gravity) are responded to by positive or negative tropisms. Tropisms get their names based on the stimulus they refer to.
• What are 3 types of tropisms displayed by plants?

  1. Phototropism
  2. Gravitropism
  3. Hydrotropism
• What does IAA stand for?

  Indoleacetic acid (the main auxin in plants)
• How does IAA bring about phototropism in plants?

  • Cells in the tip of the shoot produce IAA which is transported down the shoot
  • IAA is initially transported evenly around all regions
  • Light causes movement of IAA from the light side to the shaded side of the shoot.
• How does IAA affect ROOT growth in response to light?

  • High concentrations of IAA inhibits cell elongation in roots
  • So cell elongation is greater on the light side that the dark side so roots bend away from light
  • Roots are negatively phototropic
• How does IAA affect root growth in response to gravity?

  • Gravity influences the movement of IAA from upperside to lower side of the root.
  • IAA inhibits the elongation of root cells
  • Greater concentration on lower side of roots cause elongation on the upper root side which cause the roots to bend downward due to the force of gravity.
• Explain how the movement of IAA in shoots helps a plant survive.

  • IAA causes plant roots to grow into the soil which allows plants to absorb essential nitrates and phosphates
  • IAA causes plant shoots to grow towards light which allows fro maximum photosynthesis
• Reflex arc


  The pathway of neurons involved in a reflex is known as a reflex arc

  
• Central nervous system (CNS) is made up of the brain and spinal chord.
• The peripheral nervous system is divided into:

  1. Sensory neurons - detect a stimulus and conduct action potentials through the nerve and dorsal root of the spinal cord
  2. Motor neurons - conduct action potentials through the ventral root and spinal nerve to an effector organ.
• The motor nervous system can be further subdivided:

  1. Voluntary nervous system - the part of the nervous system associated with the voluntary control of body movement via skeletal muscles
  2. Autonomic nervous system - in control of automatic involuntary functions, important in homeostasis
• Spinal cord

  A column of nervous tissue that runs along the back and lies inside the vertebral column for protection. Emerging at intervals along the spinal cord are pairs of nerves.

  
• Reflex actions

  An automatic and rapid response to a stimulus which minimises damage to the body from potential harm e.g. touching something sharp or hot.
• Label the diagram below to show the neurons involved in a reflex arc

  A) Stimulus

  B) Receptor

  C) Sensory neuron

  D) Intermediate neurone

  E) Spinal cord

  F) Motor neuron

  G) Effector

  H) Response

  
• Why are reflex arcs so important?

  • Involuntary, meaning you don't need to use your brain to carry out complex responses resulting in overload
  • Sometimes responses are overridden by the brain along inhibitory nerve fibres so you dont drop something such as an expensive plate.
• Pacinian corpuscle


  A sensory receptor on the skin that responds to changes in mechanical stimuli such as pressure and vibration:

  • Specific to a single type of stimulus
  • Produces a generator potential by acting as a transducer
• Why are sensory receptors known as transducers?

  They take one form of energy and convert it to electrical energy
• Pacinian corpuscle structure

  
• Generator potential

  
• Receptor


  A group of specialised cells that can generate an electrical impulse in a sensory neuron.
• What are the 2 main types of receptors found on the retina of the eye?

  • Rod Cells - Sensitive to low light levels
  • Cone Cells - Sensitive to abundant light levels (can identify colours)
• Why are Rod and Cone cells known as transducers?


  They convert light energy into electrical signals of a nerve impulse.
• What pigments do Rod and Cone cells contain?

  • Rod cells - rhodopsin
  • Cone cells - iodopsin
• • The breakdown of optical pigments results in a generator potential being produced
  • The pigments within the receptors are broken down by different conditions
  • Rhodopsin within rods breaks down in dim light
  • Iodopsin within cones breaks down in bright light only
• Sensitivity refers to the amount of light required to stimulate the receptor.
• Rods are very sensitive to even low light intensities

  • They allow humans to distinguish between light and dark objects when light is very dim.
  • They do not allow humans to see in colour.