• Nerve Communication


    • A stimulus is a detectable change in the internal or external environment of an organism that produces a response in the organism
    • The ability for an organism to respond to a stimulus increases its chance of survival. These organisms which survive can therefore carry out natural selection.
    • Stimulus are detected by receptors.
      • Receptors are able to transfer energy of a stimulus into a form of energy that can be processed by the organism and lead to a response
      • The response is carried out by Effectors
    • Animals use their nervous system to communicate between receptors and effectors
    • Each effector and receptor is connected to a coordinator
      • The coordinator connects information from each receptor with the appropriate effector

    Stimulus à Receptor à Coordinator à Effector à Response


    • Taxis is a simple response by direction which is determined by a direction of the stimulus
    • A motile organism will respond directly to the environment changes by moving its entire body either towards a favourable stimulus or away from an unfavourable one
    • Taxes are classified according to whether the movement is:
      • towards the stimulus (positive taxis)
      • away from the stimulus (negative taxis


    • Kinesis is an indirect movement towards/away from a stimulus determined by a more unpleasant environment causing a more rapid change in direction compared to a favourable one where changes in directions are reduced.
    • Kinesis results in an increase in random movements
    • Kinesis aims to return the organism back into a favourable environment.
    • Kinesis is particularly important for less directional stimuli
      • g. Humidity and Temperature do not have a fixed gradient from one extreme to another


    • Tropism is a growth movement of a part of a plant in response to a directional stimulus
    • Plants will grow towards and away from a stimulus
      • Plant shoots grow towards light (positive phototropism)
      • Plant roots grow away from light (negative phototropism) but towards gravity (gravitropism)
      • Plant roots grow towards water (positive hydrotropism)

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