stimuli and response

• nervous system carry out functions dependent upon continuous input of information from inside the body and from the environment.
• inputs are detected by sensory receptors and relayed by effectors
• transfer from receptor to effector is done by the nervous system
• a stimulus is a change in external or internal environment of an organism that leads to a response in the organism.
• receptors detect a stimulus and are specific to the type of stimulus they detect information from inside the body or from external environment
• coordinator formulates response to a stimulus from a molecular level, the brain or spinal cord (CNS) or a simple collection of cells
• effectors produce a response from a muscle or gland.
• nervous communication is fast, unconscious and only in humans and animals
• hormonal communication is slower and found in both animals and plants
• 1. stimulus
  2. receptor
  3. coordinator
  4. effector
  5. responce
• movement is the main response action. it enhances chance of survival. the response is built in and automated reflex.
• taxis is a simple response where direction is determined by direction of stimulus and is a fast responce
• positive taxis is directional response moving towards the stimulus
• negative taxis is directional response moving away from the stimulus
• examples of taxis

  • positive phototaxis photosynthetic algae moving towards light
  • positive chemotaxis bacteria move towards area of glucose
  • negative phototaxis earthworms move away from light
• kinesis is response where rate at which movement changes. slow and non directional response to stimulus
• in kinesis the more unfavourable the conditions the faster the organism moves and changes direction more
• in kinesis the more favourable the conditions the less rapidly the organism moves and less changes of direction.
• woodlice have more rapid movement and more change of direction in dry areas
• woodlice have less change of direction and move slower in damp areas
• kinesis increases organisms chance of survival by having them move to stay in favourable conditions or move to favourable conditions
• maggots do both taxis and kinesis.

  taxis is moving towards chemicals emitted by dead rotting matter.

  kinesis is during temperature extremes.
• tropism is response to direct stimuli that contribute to plant survival. they can be negative or positive tropisms.
• hydrotropism = water
• gravitropism = gravity
• phototropism = light
• plant shoots and roots exhibit tropism
• roots exhibit positive gravi and hydro tropism but negative phototropism
• plants have no nervous system so respond with hormonal substances called plant growth factors. they are called auxins
• auxins are produced in meristem of plants
• IAA causes root growth to perform negative phototropism. high conc of IAA causes cell elongation in roots. cell elongation is greater on light side than dark so roots grow away from light.
• IAA in plants shoots causes positive phototropism. IAA is initially evenly distributed. light causes movement of IAA from light to shaded area of shoot. IAA concentration builds of shaded side and causes cell elongation on shaded side. shaded side elongates faster than light side so shoot bends towards light.
• IAA causes positive gravitropism in roots. cells in tip of root produce IAA and initially evenly distributed in root. gravity influences movement of IAA to build on lower side of root. greater conc of IAA on lower side than upper. IAA inhibits root elongation in roots so lower side elongates less than upper side. this causes roots to bend towards force of gravity.
• shoots have negative gravitropism. greater concentration of IAA builds on lower side of shoot increasing cell elongation so shoot grows away from gravitational pull.
• IAA increases plant cells walls ability to stretch. this only happens in meristem as the older the cell gets the more rigid the cell wall becomes. meaning the lower down the shoot the ability for cell elongation reduces or stops.
• acid growth hypothesis is that IAA causes hydrogen ions to be activley transported across cytoplasm into cell wall to make plant cell walls more elastic.
• CNS central nervous system is made up of brain and spinal cord
• PNS peripheral nervous system is made up of sensory and motor neurones
• sensory neurones carry signals from CNS to sensory organs (muscles and glands)
• motor neurones carry signals from CNS to control activities of muscle and glands