Clock it with SCIBook a tutor
ResourcesRevision notesFlashcardsMind mapsExam questionsTopic testsPracticalsGCSE Biology coursesAQA Biology (Triple)AQA Combined ScienceEdexcel Biology (Triple)Edexcel Combined ScienceMoreVideosAboutBook a tutor

Reflex arcs

Cells and control · Neurones and reflexes · note 4 of 5

Spec 2.14Download these notes (PDF)7 pages

Reflex arcsSpec 2.14

A reflex is a fast, automatic response to a stimulus that does not involve conscious thought. The pathway it follows is called a reflex arc. Reflexes protect the body from harm.

  1. The stimulus is detected by a receptor (for example, a pain receptor in the skin).
  2. A sensory neurone carries impulses to the CNS.
  3. At a synapse the impulses pass to a relay neurone in the CNS.
  4. At another synapse the impulses pass to a motor neurone.
  5. The motor neurone carries the impulses to an effector, such as a muscle.
  6. The effector produces a response, such as the muscle contracting to pull the hand away.

The impulses do not have to travel to the conscious part of the brain before the response, so the reflex is very fast.

A reflex arc: a stimulus is detected by a receptor in the skin of a finger, a sensory neurone carries impulses into the spinal cord, a relay neurone in the spinal cord passes them on, and a motor neurone carries them out to a muscle (the effector); the two synapses are circled.Tap to enlarge
A reflex arc: receptor, sensory neurone, relay neurone, motor neurone, effector. Impulses cross a synapse between each pair of neurones.