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The nervous system

sense organs detect stimuli

  • a stimulus is a change in your environment which you may need to react to
  • you have five different sense organs: eyes, ears, nose, tongue and skin
  • they all contain different receptors. receptors are groups of cells which are sensitive to a stimulus. they change stimulus energy into electrical impulses
  • a stimulus can be light, sound, touch, pressure, chemical or a change in position or temperature
  • the eyes contain light receptors- sensitive to light.
  • the ears contain sound receptors- sensitive to sound. also, balance receptors- sensitive to changes in position
  • the nose contains smell receptors- sensitive to chemical stimuli
  • the tongue contains taste receptors- chemical stimuli
  • the skin is sensitive to touch, pressure, pain and temperature change

the central nervous system coordinates a response

  • the central nervous system is where all the information from the sense organs is sent, and where reflexes and actions are coordinated. The central nervous system consists of the brain and spinal cord only
  • neurones transmit the information very quickly to and from the CNS
  • 'instructions' from the CNS are sent to the effectors
  • sensory neurones- the nerve cells that carry signals as electrical impulses from the receptors in the sense organs to the central nervous system
  • relay neurones- the nerve cells that carry signals from sensory neurones to motor neurones
  • motor neurones- the nerve cells that carry signals from the central nervous system to the effector muscles or glands
  • effectors- muscles and glands are known as effectors-muscles contract in response to a nervous impulse, whereas glands secrete hormones

synapses and reflexes

synapses connect neurones

  • the connection between two neurones is called a synapse
  • the nerve signal is transferred by chemicals which diffuse across the gap
  • these chemicals then set off a new electrical signal in the next neurone

reflexes help prevent injury

  • reflexes are automatic responses to certain stimuli- they can reduce the chances of being injured
  • the passage of information in a reflex is called a reflex arc

the reflex arc

the reflex arc goes through the central nervous system

  • the neurones in reflex arcs go through the spinal cord or through an unconscious part of the brain
  • when a stimulus is detected by receptors, impulses are sent along a sensory neurone to the CNS
  • when the impulses reach a synapse between the sensory neurone and a relay neurone, they trigger chemicals to be released. These chemicals cause impulses to be sent along the relay neurone
  • when the impulses reach a synapse between the relay neurone and a motor neurone, the same thing happens. Chemicals are released and cause impulses to be sent along the motor neurone
  • the impulses then travel along the motor neurone to the effector
  • because you don't have to think about the response it's quicker than normal responses

stimulus, receptor, neurones, effector, response


hormones are chemical messengers sent in the blood

  • hormones are chemicals released directly into the blood


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