The human nervous system: Reflex actions are automatic and rapid; they do not involve the conscious part of the brain.

The idea

A reflex action is a fast, involuntary response that protects the body from harm. A receptor detects a stimulus, such as heat or a sharp object, and generates an electrical impulse. The impulse travels along a sensory neurone to a relay neurone in the spinal cord or an unconscious region of the brain. The relay neurone passes the impulse to a motor neurone, which carries it to an effector, such as a muscle or gland. The muscle contracts or the gland secretes, producing the response. Because the pathway does not pass through the conscious part of the brain, no decision-making delay occurs, so the response is automatic and rapid.

For example: Touching a hot surface triggers a withdrawal reflex that moves the hand away before the pain is consciously felt.

What earns the marks

• A reflex action is automatic and rapid because the impulse pathway does not involve the conscious part of the brain.
• A receptor detects the stimulus and initiates an electrical impulse in a sensory neurone.
• The impulse is passed from the sensory neurone to a relay neurone in the spinal cord or an unconscious region of the brain.
• The relay neurone passes the impulse to a motor neurone, which carries it to an effector such as a muscle or gland.
• The effector produces the response, for example a muscle contracts to withdraw a hand from a hot surface.
• The reflex arc is the receptor–sensory neurone–relay neurone–motor neurone–effector pathway.
• Conscious thought is not required, so the response is faster than a voluntary reaction that involves decision-making in the conscious brain.

Common mistakes and how to put them right

• Stating that reflex actions involve the conscious part of the brain.

  Put it right: The conscious part of the brain is not involved; the relay neurone is in the spinal cord or an unconscious region of the brain.

• Describing the reflex arc in the wrong order, such as motor neurone before sensory neurone.

  Put it right: Use the sequence receptor, sensory neurone, relay neurone, motor neurone, effector.

• Saying that a reflex action is learned or voluntary.

  Put it right: Reflex actions are automatic and involuntary from birth.

• Confusing the roles of sensory and motor neurones.

  Put it right: Sensory neurones carry impulses from receptors to the central nervous system; motor neurones carry impulses from the central nervous system to effectors.

Exam tips

• When asked why a reflex is rapid, link your answer to the absence of conscious processing rather than simply saying it is fast.
• Practise labelling a reflex arc diagram with receptor, sensory neurone, relay neurone, motor neurone and effector.
• Use the term effector rather than only muscle or gland, because effectors include both.
• If a question asks for the pathway, write the components in order and state the direction of impulse travel.