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Science

The nervous system

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Reflex actions

When a receptor is stimulated, it sends a signal to the central nervous system, where the brain co-ordinates the response. But sometimes a very quick response is needed, one that does not need the involvement of the brain. This is a reflex action.

Reflex actions are rapid and happen without us thinking. For example, you would pull your hand away from a hot flame without thinking about it. The animation below allows you to step through each stage of the reflex arc.

This is what happens:

  1. receptor detects a stimulus - change in the environment
  2. sensory neurone sends signal to relay neurone
  3. motor neurone sends signal to effector
  4. effector produces a response

The way the iris in our eye adjusts the size of the pupil in response to bright or dim light is also a reflex action.

In bright light:

  • Radial muscles of the iris relax.
  • Circular muscles of the iris contract.
  • Less light enters the eye through the contracted pupil.

In dim light:

  • Radial muscles of the iris contract.
  • Circular muscles of the iris relax.
  • More light enters the eye through the dilated pupil.

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