Cambridge O Level Biology · Syllabus 5090 · Coordination and Control
Reflex Arc
What is Reflex Arc?
The pathway of neurones along which the impulses of a reflex action travel. The complete sequence is stimulus, receptor, sensory neurone, relay neurone in the central nervous system, motor neurone, effector, response. It is the shortest route between detecting a stimulus and responding to it.
This definition is part of the Coordination and Control chapter in Cambridge O Level Biology.
Common mistakes with Reflex Arc
- A wiring point students invent. Not every pathway contains all three neurones, and the syllabus does not claim it does. What you must be able to describe is the reflex arc, which does contain all three. Do not write that a sensory neurone connects directly to a muscle: sensory neurones end in the CNS, and the instruction to a muscle always leaves along a motor neurone.
Questions students ask about Reflex Arc
What is the order of a reflex arc?
Stimulus, receptor, sensory neurone, relay neurone in the central nervous system, motor neurone, effector, response. The effector is a muscle or a gland, and the impulse never travels backwards along this pathway.
Last reviewed Syllabus 2026-2028