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  1. diffuse When particles spread out from a region of higher concentration to a region of lower concentration.
  2. effector The organ, tissue or cell that produces a response.
  3. hormone Chemical messenger produced in glands and carried by the blood to specific organs in the body.
  4. motor neurone The nerve cell that carries electrical impulses from the CNS to effectors such as muscles or glands.
  5. neuron Another term for a nerve cell that is specialised to transmit electrical impulses.
  6. neurotransmitter Chemical involved in passing nerve impulses from one nerve cell to the next across a synapse.
  7. reflex action Automatic and rapid response to a stimulus.
  8. relay neurone The nerve cell that transmits electrical impulses from sensory neurones to motor neurones.
  9. sensory neurone The nerve cell that transmits electrical impulses from receptors in the sense organs to the CNS.
  10. stimulus Something that sets off a reaction in the nervous system, for example, light, heat, sound, gravity, smell, taste, or temperature. These changes in the environment are detected by receptors in an organism. The plural is stimuli.