Reflex actions and reflex arcs

Coordination and response · Nervous control · note 2 of 3

Spec 14.1.5, 14.1.6Download all Nervous control notes (PDF)Download all notes (PDF)7 pages

Reflex actions and reflex arcsSpec 14.1.5, 14.1.6

In short

A reflex action is a means of automatically and rapidly integrating and coordinating stimuli with the responses of effectors, which are muscles and glands. A simple reflex arc has five parts in order: receptor, sensory neurone, relay neurone, motor neurone and effector. Reflexes are useful because they are rapid and automatic, so they protect the body from harm.

Reflex action: A means of automatically and rapidly integrating and coordinating stimuli with the responses of effectors (muscles and glands).

A reflex action is automatic and rapid. You do not have to think about it first. Reflexes protect the body from harm, for example pulling your hand away from something hot.

A simple reflex arc is the pathway that the impulses follow. It has five parts: receptor, sensory neurone, relay neurone, motor neurone and effector.

Parts of a simple reflex arc
Part	Function
Receptor	Detects the stimulus and starts an impulse
Sensory neurone	Carries impulses from the receptor to the CNS
Relay neurone	Carries impulses within the CNS from the sensory neurone to the motor neurone
Motor neurone	Carries impulses from the CNS to the effector
Effector	A 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. — A reflex arc. The impulse goes **receptor → sensory neurone → relay neurone → motor neurone → effector**.

Exam tip:

Describe the pathway in order and name each part. If asked why a reflex is useful, use the words ‘rapid’, ‘automatic’ and ‘protects the body from harm’.

Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026

Frequently asked questions

What is a reflex arc?

A reflex arc is the pathway that impulses follow in a reflex action. It has five parts in order: a receptor detects the stimulus, a sensory neurone carries impulses to the CNS, a relay neurone passes them on within the CNS, and a motor neurone carries them to an effector, a muscle or gland, which responds.

Why are reflexes faster than normal responses?

Reflexes are fast because they are automatic, so you do not have to think about the response first. Impulses pass straight from the receptor through the sensory, relay and motor neurones to the effector. This rapid, automatic response protects the body from harm, such as pulling your hand away from something hot.

How does a nerve impulse cross the synapse?

Supplement A nerve impulse crosses a synapse by chemical means, because the electrical impulse cannot cross the gap. The impulse makes vesicles release neurotransmitter molecules into the synaptic gap. These diffuse across the gap and bind with receptor proteins on the next neurone, which stimulates a new impulse in that neurone.

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