Structures and Functions in Living Organisms · 7 question types
Past paper frequency (2018 to 2024)
This topic accounts for approximately 14% of your exam marks.
Nervous system structure, reflex arcs, and hormones are all commonly examined.
A reflex action is a very fast, automatic, involuntary response to a stimulus. You do not have to think about it; by the time you become aware of the stimulus, your body has already responded. Examples include pulling your hand away from a sharp object, blinking when something flies near your eye, coughing when food goes the wrong way, and the pupil narrowing in bright light.
Reflex actions are useful because they are fast and automatic, which protects the body from damage in dangerous situations.
The neural pathway followed by a reflex action is called a reflex arc. It bypasses the conscious part of the brain to save precious milliseconds. The arc usually goes through the spinal cord rather than the brain.
Tracing the reflex when a person steps on a sharp pin:
The whole thing takes a fraction of a second. A separate, slower signal also goes up to the brain so that you become aware of the pain shortly after the foot has moved.
The reflex arc: stimulus → receptor → sensory neurone → relay neurone (in spinal cord) → motor neurone → effector → response
