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Reflex actions and the reflex arc

Homeostasis and response · The human nervous system · note 2 of 3

Spec 4.5.2.1Download these notes (PDF)9 pages

Reflex actions and the reflex arcSpec 4.5.2.1

Reflex actions are automatic and rapid. They do not involve the conscious part of the brain. Reflexes are important because they protect the body from harm, for example pulling your hand away from a hot object before you have time to think.

  1. A receptor detects the stimulus and an impulse starts in the sensory neurone.
  2. The sensory neurone carries the impulse to the CNS.
  3. At a synapse, chemicals diffuse across the gap and start an impulse in the relay neurone.
  4. The relay neurone passes the impulse on, through another synapse, to the motor neurone.
  5. The motor neurone carries the impulse to the effector.
  6. The effector (a muscle or gland) produces the response.
Structures in a reflex arc
StructureFunction
ReceptorDetects the stimulus
Sensory neuroneCarries impulses from the receptor to the CNS
Relay neuroneCarries impulses within the CNS, from the sensory neurone to the motor neurone
SynapseA gap between two neurones. A chemical diffuses across the gap and starts an impulse in the next neurone
Motor neuroneCarries impulses from the CNS to the effector
EffectorA muscle (contracts) or a gland (secretes) that produces the response
Reflex arc: a receptor in the skin of a finger touching a hot object, a sensory neurone to the spinal cord, a relay neurone in the spinal cord, a motor neurone to a muscle in the arm (the effector), with a magnified synapse showing a chemical diffusing across the gap.Tap to enlarge
A reflex arc. The impulse goes receptor → sensory neurone → relay neurone → motor neurone → effector, crossing a synapse by chemical diffusion at each junction.