ScienceCBSE Class 10 · Control and Coordination

Reflex Action and Reflex Arc

Updated 14 October 2026 · Fact-checked

A reflex action is a sudden, automatic response to a stimulus, made without thinking, such as pulling your hand away from a flame. Its pathway, the reflex arc, runs from a receptor through a sensory neuron to the spinal cord, then through a motor neuron to an effector muscle. Reflex arcs work faster than the brain's thinking.

Line drawing of a reflex arc: afferent nerve from skin to spinal cord, efferent nerve from spinal cord to muscle, with arrows and English la — Follow the arrows: skin (receptor) → afferent nerve → spinal cord → efferent nerve → muscle (effector), with no brain involved in the reflex.
Picture: Pearson Scott Foresman · Public domain · Wikimedia Commons

Understand Reflex Action and Reflex Arc

How the nervous system collects information. Changes in the environment are detected by receptors, the specialised tips of some nerve cells, usually in sense organs: gustatory receptors on the tongue detect taste, olfactory receptors in the nose detect smell, and so on.

The neuron and the impulse. A nerve cell (neuron) has three main parts:

dendrites, where information is picked up; it sets off a chemical reaction that creates an electrical impulse;
the cell body, which the impulse reaches next;
the axon, along which the impulse travels to its end.

At the end of the axon, the electrical impulse causes the release of some chemicals. These chemicals cross a tiny gap, the synapse, and trigger a fresh electrical impulse in a dendrite of the neuron on the other side. A similar junction passes impulses from neurons to muscle cells or glands (the neuromuscular junction). So information travels as an electrical impulse within a neuron, and as a chemical signal across a synapse.

Why reflexes? Suppose you touch a flame. If the brain had to receive the heat signal, think about it, decide and send instructions back, it could take long enough for you to get burnt. Instead, the nerve that detects heat is connected more directly to the nerve that moves the muscles. This quick connection is the reflex arc.

Where is it? On their way to the brain, nerves from every part of the body come together as a bundle in the spinal cord. The reflex arc connection is made in the spinal cord itself, where input and output nerves first meet. The information still travels on to the brain, which is why you feel the pain a moment after you have already moved your hand.

The path of a reflex arc.
1. Receptor in the skin detects the stimulus (heat).
2. Sensory neuron carries the impulse to the spinal cord.
3. Relay neuron in the spinal cord passes it on.
4. Motor neuron carries the impulse from the spinal cord to the muscle.
5. Effector (muscle in the arm) contracts and pulls the hand away.

Reflex arcs probably evolved because the thinking process is not fast enough; many animals have little or none of the complex neuron network needed for thinking. Even in animals with large brains, reflex arcs remain more efficient for quick responses.

Three kinds of action. Voluntary actions (writing, talking, moving a chair) are decided by the fore-brain. Involuntary actions (heartbeat, breathing, salivation, digestion) happen without our control and are run largely by the mid-brain and hind-brain. Reflex actions (pulling back from a flame, change in pupil size in bright light) are sudden responses made through reflex arcs. How muscles finally move: muscle cells have special proteins that change shape and arrangement in response to impulses, making the cells shorter.

Reflex Action and Reflex Arc: formulas and key facts

Reflex arc pathway: $receptor \to sensory neuron \to relay neuron (spinal cord) \to motor neuron \to effector$; The message also goes on to the brain.
Impulse inside a neuron: $dendrite \to cell body \to axon \to axon end$; Electrical impulse.
Across a synapse: $electrical \to chemical \to electrical$; Chemicals released at the axon end cross the gap to the next dendrite.
Types of action: $voluntary (fore-brain); involuntary (mid- and hind-brain); reflex (spinal cord arc)$; Each with a different control centre.

How to solve Reflex Action and Reflex Arc questions

For reflex-action questions, follow the signal and name each part.

1Identify the stimulus and the response (heat → withdrawal of hand; bright light → pupil gets smaller).
2Name the receptor (in the skin, the eye, etc.).
3Trace the signal: sensory neuron → spinal cord (relay neuron) → motor neuron → effector muscle.
4State that the response happens before the brain processes the sensation, and that information still reaches the brain.
5For 'difference' questions, compare by control (thinking or not), centre (brain or spinal cord) and speed.
6For the reflex arc diagram, label receptor, sensory neuron, spinal cord, relay neuron, motor neuron, effector (muscle), with arrows for direction.

Reflex Action and Reflex Arc in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Reflex arc and the role of the brain

Suppose you accidentally touch a hot pressure cooker in the kitchen. Your hand jerks back at once, and only a moment later do you feel the pain and realise what happened.

Trace the path of the signal that caused your hand to move, and explain why you moved before you felt the pain.

Try it first, then show the solutionSolution

Heat receptors in the skin of the fingers detect the stimulus.
A sensory neuron carries an impulse to the spinal cord.
In the spinal cord, a relay neuron passes the impulse straight to a motor neuron.
The motor neuron carries the impulse to the arm muscles (effector), which contract and pull the hand away.
The impulse also goes up the spinal cord to the brain, where the sensation of pain is felt; this route is longer, so the feeling arrives after the movement.

Answer: Receptor → sensory neuron → relay neuron in spinal cord → motor neuron → arm muscle. The reflex arc in the spinal cord acts before the slower brain route makes you aware of the pain.

Memory hook: Spinal cord moves the hand first; the brain feels the burn second.

Tricks to remember Reflex Action and Reflex Arc

Short memory aids. Say each one aloud once, then use it on a question the same day.

RSRME
'Really Smart Rats Move Early': Receptor, Sensory, Relay, Motor, Effector.
Use it when: Order of the reflex arc.
DCA
Dendrite, Cell body, Axon: the order an impulse travels in a neuron.
Use it when: Neuron questions.
Synapse = chemical bridge
Electric in the wire, chemical across the gap.
Use it when: Explaining what happens at a synapse.

Reflex Action and Reflex Arc: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Synapse · 2 marks · Very short answerBoard level

What happens at the synapse between two neurons?

Try it first, then show the solutionSolution

The electrical impulse reaching the end of the axon of one neuron causes the release of certain chemicals (1 mark).
These chemicals diffuse across the gap (synapse) and set up a new electrical impulse in a dendrite of the following neuron (1 mark).

Answer: The impulse is converted into a chemical signal that crosses the gap and starts a new impulse in the next neuron.

Example 2: Reflex action vs walking · 3 marks · Short answerHarder board level

What is the difference between a reflex action and walking? Which part of the nervous system controls each?

Try it first, then show the solutionSolution

A reflex action is sudden and automatic, done without thinking; walking is a voluntary action, decided consciously (1 mark).
A reflex is controlled through a reflex arc in the spinal cord (1 mark).
Walking is initiated by the fore-brain (thinking part), and its precision and balance are maintained by the cerebellum (1 mark).

Answer: Reflex: automatic, spinal cord. Walking: voluntary, fore-brain with the cerebellum for balance and precision.

Example 3: Neuron structure · 3 marks · Short answerHarder board level

Draw a labelled diagram of a neuron. Identify the part (i) where information is acquired, (ii) through which it travels as an electrical impulse, and (iii) where it is converted into a chemical signal.

Try it first, then show the solutionSolution

Diagram labels: dendrites, cell body (with nucleus), axon, nerve ending (1 mark).
(i) Dendrite tips (½ mark); (ii) the axon (½ mark).
(iii) The end of the axon (nerve ending), where chemicals are released into the synapse (1 mark).

Answer: Dendrites acquire, axon conducts, axon ending converts the impulse to a chemical signal.

Example 4: Spinal cord injury · 3 marks · Short answerHOTS / competency

Which signals will be disrupted in a spinal cord injury? Would a person with a spinal cord injury in the lower back still show a knee-jerk-type reflex in the leg if the reflex arc itself is below the injury? Explain.

Try it first, then show the solutionSolution

The spinal cord carries messages between the brain and the body through spinal nerves, so signals from body parts below the injury to the brain (sensations) and from the brain to those parts (voluntary movement) would be disrupted (1½ marks).
A reflex whose arc is formed in the spinal cord below the injury could still occur, because the receptor, sensory neuron, relay in the cord and motor neuron are intact; but the person might not feel the stimulus, since the message cannot reach the brain (1½ marks).

Answer: Sensation and voluntary control below the injury are lost; a reflex arc below the injury can still work, without the person feeling it.

Example 5: Pupil reflex · 3 marks · Short answerHarder board level

A bright torch is shone into your eyes and your pupils become smaller. (a) Is this voluntary, involuntary or reflex? (b) Trace the sequence of events. (c) Why is it useful?

Try it first, then show the solutionSolution

(a) A reflex action (sudden, automatic response to a stimulus) (½ mark).
(b) Light receptors in the eye detect the bright light → a sensory nerve carries the impulse → it is relayed → a motor nerve carries the instruction → muscles of the iris contract to make the pupil smaller (1½ marks).
(c) It quickly reduces the light entering the eye, protecting it without waiting for us to think (1 mark).

Answer: (a) Reflex. (b) Receptor in eye → sensory → relay → motor → iris muscles. (c) Protects the eye quickly.

Where students lose marks in Reflex Action and Reflex Arc

The slips examiners see most often, and what to write instead.

Mistake: Putting the brain in the reflex arc
Students assume every response needs the brain.
Wrong working
Receptor → sensory neuron → brain → motor neuron → muscle.
Right working
Receptor → sensory neuron → spinal cord (relay neuron) → motor neuron → muscle; the brain gets the information afterwards.
Do this: Remember: reflex arcs are formed in the spinal cord.
Usually costs: 1-2 marks.
Mistake: Wrong direction in the neuron
Students reverse dendrite and axon.
Wrong working
The impulse travels from the axon to the dendrite.
Right working
The impulse travels dendrite → cell body → axon.
Do this: Dendrites receive, axons send.
Usually costs: 1 mark.
Mistake: Diagram without arrows or the effector
Students draw neurons but forget labels.
Wrong working
Unlabelled loop from skin to spinal cord.
Right working
All five parts labelled with arrows showing the direction of the impulse.
Do this: Check labels: receptor, sensory neuron, relay neuron, motor neuron, effector, spinal cord.
Usually costs: 1-2 marks in diagram questions.
Mistake: Calling the heartbeat a reflex
Both are 'automatic'.
Wrong working
Heartbeat is a reflex action.
Right working
Heartbeat is an involuntary action controlled by the hind-brain (medulla).
Do this: Reflex = sudden response to a stimulus; involuntary = continuous automatic process.
Usually costs: 1 mark.

Exam tips

Use the terms receptor, sensory neuron, relay neuron, motor neuron and effector in every reflex arc answer.
Mention that information also reaches the brain; examiners look for it.
For the neuron diagram, label dendrites, cell body, nucleus, axon and nerve ending.
In comparisons, give a point-by-point table: control, centre, speed, example.
Avoid saying 'without the nervous system' for reflexes; they use the nervous system but bypass thinking.

Practice questions on Reflex Action and Reflex Arc

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
The gap between two neurons across which a chemical signal passes is called:
(a) axon (b) dendrite (c) synapse (d) impulse
Hint
It is a junction, not a part of the neuron.
Answer
(c) synapse.
Q2 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Reflex arcs are formed in the spinal cord.
Reason (R): The spinal cord is where nerves from all body parts gather before reaching the brain.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Where is the best place for input and output nerves to connect?
Answer
(a). The spinal cord is where input and output nerves first meet.
Q3 · 3 marks · Short answerBoard level
Draw a labelled diagram of a reflex arc for the withdrawal of a hand from a hot object and state why the brain is not directly involved in the response.
Hint
Five parts plus the spinal cord.
Answer
Labels: receptor (in skin), sensory neuron, spinal cord with relay neuron, motor neuron, effector (arm muscle), arrows showing direction. The response must be very fast; thinking in the brain takes longer, so the connection is made directly in the spinal cord, though information still reaches the brain.
Q4 · 4 marks · Case-basedHOTS / competency
Read the passage and answer.
Suppose a student pinches her nose shut and puts some sugar in her mouth, then repeats this without pinching her nose. She finds she tastes the sugar less clearly with her nose blocked, and remembers that food also tastes dull when she has a cold.
(a) Name the receptors for taste and for smell. (1 mark)
(b) Through which part of a neuron does the information from a receptor first enter? (1 mark)
(c) Suggest why blocking the nose changes how food tastes, and what this shows about how the brain builds a sensation. (2 marks)
Hint
The fore-brain has association areas that combine inputs.
Answer
(a) Gustatory receptors (taste), olfactory receptors (smell). (b) The dendritic tip. (c) Flavour depends on smell as well as taste; with the nose blocked, olfactory receptors receive little information, so the brain gets less input. The fore-brain's association areas combine information from different receptors (and memory) to interpret a sensation, so missing one input changes the experience.
Q5 · 2 marks · Very short answerHarder board level
Suppose a student says, 'My heartbeat and my pulling my hand off a hot iron are the same kind of action, because I control neither of them.' Correct her with two points of difference, naming what controls each.
Hint
Both happen without conscious control, but differ in what controls them and when they occur.
Answer
She is right that neither is consciously decided, but they are different kinds of action. (1) The heartbeat is an involuntary action that goes on all the time, controlled by the mid-brain and hind-brain; pulling the hand away is a reflex action, a sudden response that happens only when a particular stimulus (heat) acts (1 mark). (2) The reflex is made through a reflex arc in the spinal cord (receptor, sensory neuron, relay neuron, motor neuron, arm muscle), with the brain told only afterwards (1 mark).
Q6 · 3 marks · Short answerHOTS / competency
A doctor taps just below a patient's kneecap and the leg jerks forward. The patient says she 'did not decide' to move. (a) What type of action is this? (b) Name the effector. (c) If the leg did not move, but the patient felt the tap, which parts of the arc might be damaged? Explain.
Hint
Feeling the tap means the sensory route to the brain works.
Answer
(a) A reflex action. (b) The thigh muscle that straightens the leg. (c) Since she felt the tap, the receptor and sensory neuron are working and the signal reached the brain; the fault may lie in the relay connection in the spinal cord, the motor neuron, or the muscle (effector).

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Where are reflex arcs formed?Show answer
In the spinal cord.
2.What starts an impulse in a dendrite?Show answer
A chemical reaction set off by information at its tip.
3.What crosses the synapse?Show answer
Chemicals released from the axon end.
4.Name the effector in hand withdrawal.Show answer
Muscle in the arm.
5.Why did reflex arcs evolve?Show answer
Thinking is not fast enough for dangerous situations.
6.Receptors for smell are called?Show answer
Olfactory receptors.

Reflex Action and Reflex Arc: frequently asked questions

What is a reflex arc?

The pathway of a reflex action: receptor, sensory neuron, relay neuron in the spinal cord, motor neuron and effector muscle.

Why is a reflex action faster than a voluntary action?

The connection between the input and output nerves is made in the spinal cord, so the signal does not wait for the brain to think and decide.

Is the brain involved in a reflex action?

The response is made through the spinal cord, but the information also reaches the brain, which is why we feel the pain soon after.

What is the difference between reflex and involuntary actions?

Reflex actions are sudden responses to a stimulus through a reflex arc; involuntary actions like heartbeat and breathing go on continuously under the control of the mid-brain and hind-brain.

How does a muscle contract when an impulse reaches it?

Special proteins in muscle cells change their shape and arrangement in response to the impulse, making the cells shorter.