Reflex Arcs (AQA A Level Biology)

Reflex Arcs

• There are three main types of neurone: sensory, relay and motor

  • Sensory neurones carry impulses from receptors to the Central Nervous System (CNS - the brain or spinal cord)

  • Relay (intermediate) neurones are found entirely within the CNS and connect sensory and motor neurones

  • Motor neurones carry impulses from the CNS to effectors (muscles or glands)

• Sensory neurones, relay (intermediate) neurones and motor neurones work together to bring about a response to a stimulus

• A reflex arc is a pathway along which impulses are transmitted from a receptor to an effector without involving ‘conscious’ regions of the brain

• As it does not involve the brain, a reflex response is quicker than any other type of nervous response

• Examples of simple reflex actions that are coordinated by these pathways are:

  • Removing the hand rapidly from a sharp or hot object

  • Blinking

  • Focusing the eye on an object

  • Controlling how much light enters the eye

How sensory neurones, intermediate (relay) neurones and motor neurones work together to carry out a reflex action

• In the example above:

  • A pin (the stimulus) is detected by a pain receptor in the skin

  • The sensory neurone sends electrical impulses to the spinal cord (the coordinator)

  • Electrical impulses are passed on to relay neurone in the spinal cord

  • The relay neurone connects to the motor neurone and passes the impulses on

  • The motor neurone carries the impulses to the muscle in the leg (the effector)

  • The impulses cause the muscle to contract and pull the leg up and away from the sharp object (the response)

The pathway of a reflex arc