Reflex actions and the reflex arc

Homeostasis and response · The human nervous system · note 2 of 8

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Reflex actions and the reflex arcSpec 4.5.2.1

In short

A reflex action is an automatic and rapid response that does not involve the conscious part of the brain. In a reflex arc, an impulse passes from a receptor along a sensory neurone to the CNS, through a relay neurone, then along a motor neurone to an effector. Reflexes protect the body from harm.

Reflex actions are automatic and rapid. They do not involve the conscious part of the brain. Reflexes are important because they protect the body from harm, for example pulling your hand away from a hot object before you have time to think.

A receptor detects the stimulus and an impulse starts in the sensory neurone.
The sensory neurone carries the impulse to the CNS.
At a synapse, chemicals diffuse across the gap and start an impulse in the relay neurone.
The relay neurone passes the impulse on, through another synapse, to the motor neurone.
The motor neurone carries the impulse to the effector.
The effector (a muscle or gland) produces the response.

Structures in a reflex arc
Structure	Function
Receptor	Detects the stimulus
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
Synapse	A gap between two neurones. A chemical diffuses across the gap and starts an impulse in the next 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**, crossing a synapse by chemical diffusion at each junction.

Common mistake:

The reflex arc does not go through the conscious part of the brain, which is why it is fast. Do not write that the brain is never told; just say the response happens before you are aware of it.

Written and checked against the AQA GCSE Biology (8461) specification · Updated October 2026

Frequently asked questions

What is a reflex arc?

A reflex arc is the pathway of an automatic, rapid response. A receptor detects a stimulus, a sensory neurone carries the impulse to the CNS, a relay neurone passes it on, and a motor neurone carries it to an effector, a muscle or gland, which responds. It does not involve the conscious part of the brain.

Why are reflexes faster than normal responses?

Reflexes are faster because the reflex arc does not go through the conscious part of the brain. The impulse passes from the sensory neurone, through a relay neurone, to the motor neurone, so the response happens before you are aware of it. This protects the body, for example pulling your hand away from a hot object.

How does a nerve impulse cross the synapse?

A nerve impulse crosses a synapse using chemicals. A synapse is a gap between two neurones. When an impulse reaches the end of one neurone, chemicals diffuse across the gap and start an impulse in the next neurone. This is how an impulse passes from a sensory neurone to a relay neurone, and on to a motor neurone.

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