6.1.5Organisms Respond to Changes in their Environments (A Level only)

Reflex Arcs — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 8 parts Lesson 7 of 44 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision

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Lesson summary

6.1.5 Reflex Arcs — AQA A Level Biology 7402 notes: Describe the pathway of a simple reflex and explain its protective function.A reflex is a rapid, involuntary response to a specific stimulus.It does not involve conscious decision by the brain.Reflexes are innate and the same st…

Sensory neurone
Relay (intermediate) neurone
Motor neurone
monosynaptic
Exam-style question 1
Exam-style question 2

Section 1 of 8Reflexes

A reflex is a rapid, involuntary response to a specific stimulus.

It does not involve conscious decision by the brain.

Reflexes are innate and the same stimulus always produces the same response.

Because the pathway is short and has few synapses, the response is very fast, which protects the body from damage.

Examples include the withdrawal of a hand from a hot object, the blink reflex of the eye when something approaches, the knee jerk, and the pupil reflex in bright light.

Section 2 of 8The reflex arc

A reflex arc is the pathway of neurones involved in a reflex. The components are:

The reflex arc

Receptor

Receptor detects the stimulus, e.g. a pain or heat receptor in the skin.

Sensory neurone

Sensory neurone carries impulses from the receptor to the central nervous system, entering the spinal cord via the dorsal root.

Relay (intermediate) neurone

Relay (intermediate) neurone in the grey matter of the spinal cord, links the sensory neurone to the motor neurone by synapses.

Motor neurone

Motor neurone carries impulses out of the spinal cord via the ventral root to the effector.

Effector

Effector a muscle (or gland) that responds, e.g. the biceps contracts to flex the arm.

Impulses cross two synapses in the spinal cord: sensory to relay and relay to motor. At each, a neurotransmitter diffuses across the synaptic cleft and binds to receptors on the postsynaptic membrane. Information also passes up the spinal cord to the brain, so you become aware of the stimulus, but the response has already begun.

Section 3 of 8Why reflexes are protective

Reflexes are advantageous because:

They are very fast, as there are few neurones and no time is used in thinking.

They are automatic, so they do not require learning and are effective from birth.

They protect against damage, for example the blink reflex protects the cornea, and withdrawal reflexes prevent burns.

They free the brain to attend to other things, as no conscious attention is needed.

They help in homeostasis, for example the pupil reflex protects the retina from bright light.

Some reflexes can be modified by the brain. For example, you can override a withdrawal reflex when picking up a hot dish you do not want to drop. This happens as the brain sends inhibitory impulses to the motor neurone, which shows that the pathway has connections to higher centres.

Section 4 of 8Example: the pupil reflex and the knee jerk

In bright light, photoreceptors in the retina send impulses along the optic nerve to the brain.

The reflex pathway then sends impulses along parasympathetic neurones to the radial and circular muscles of the iris.

The circular muscles contract and the radial muscles relax, so the pupil constricts and less light enters, protecting the retina.

In dim light, the opposite occurs.

Monosynaptic

The knee jerk is a monosynaptic reflex, with a single synapse between sensory and motor neurone, and is used clinically to test the nervous system.

Reflex	Receptor	Effector	Benefit
Hand withdrawal	Pain receptors	Biceps	Prevents burns
Blink	Touch receptors on cornea	Eyelid muscles	Protects eye
Pupil	Retinal photoreceptors	Iris muscles	Prevents retinal damage

Section 5 of 8Reflexes in practice and in data questions

Questions may ask about the effect of a toxin, drug or injury on a reflex.

Identify the likely site: an anaesthetic blocking sodium channels in sensory neurones will stop the impulse from reaching the spinal cord, so there is no response; a drug that blocks acetylcholine receptors at the neuromuscular junction stops muscle contraction even though impulses arrive.

Data on reaction time show that a reflex takes around 30–50 ms, compared with 200 ms or more for a voluntary response to a visual cue.

Reflex time can be linked to conduction velocity: a myelinated neurone conducts faster by saltatory conduction, and the synaptic delay of about 0.5 ms per synapse adds to the total.

In diseases which demyelinate neurones, such as multiple sclerosis, reflexes slow, which can be used in diagnosis.

You should also be able to draw and label a diagram of a reflex arc in the spinal cord, showing the dorsal root ganglion containing the cell body of the sensory neurone, the grey matter containing the relay neurone, and the ventral root through which the motor neurone leaves. Notice that the cell body of the sensory neurone lies outside the cord, whereas the motor neurone cell body lies inside it.

Section 6 of 8Exam-style questions

Describe the pathway of a spinal reflex from receptor to effector. [4]4 marks
Explain why a reflex is faster than a voluntary response. [2]2 marks

Section 7 of 8Key terms

Term	Meaning
Reflex	Rapid, involuntary and automatic response to a stimulus.
Reflex arc	Pathway of neurones from receptor to effector in a reflex.
Relay neurone	Neurone in the central nervous system linking sensory and motor neurones.
Motor neurone	Neurone carrying impulses from the CNS to an effector.
Sensory neurone	Neurone carrying impulses from a receptor to the CNS.
Monosynaptic	Having a single synapse in the pathway.

Section 8 of 8Exam technique

Saying the brain coordinates a spinal reflex. The spinal cord does.

Forgetting the relay neurone.

Saying the receptor is the effector.

Writing that reflexes are learned.

Last updated: 18 October 2026