IB Biology SLTopic 3 — Coordinating Body SystemsPaper 1 & 2Core idea~10 min read
Reflex Arcs & Movement Control
Touch something hot and your hand is already moving before you feel anything. You did not decide to move it — the spinal cord made that call and told your brain afterwards. This page is about the shortcut that makes that possible, and about the part of the brain that stops your movements being clumsy.
📘 What you need to know
A reflex is an automatic, rapid response that happens without conscious thought.
A reflex arc is the pathway: receptor → sensory neurone → relay neurone → motor neurone → effector.
In a spinal reflex the relay neurone sits in the grey matter of the spinal cord. The brain is not part of the decision.
You become aware of the stimulus after the response has already happened.
Reflexes reduce damage and therefore aid survival — blinking, coughing, the pupil reflex, the knee jerk.
The motor cortex of the cerebrum starts voluntary movement; the cerebellum coordinates it.
The cerebellum works from feedback sent by proprioceptors in muscles and joints, the eyes and the inner ear.
What makes something a reflex
Three features, and questions usually want all three:
Automatic — you cannot choose to do it or not do it.
Rapid — the pathway is short, so the delay is tiny.
Protective — it minimises damage to the body, which is why it has been kept by natural selection.
Reflex
Stimulus
What it protects you from
Blinking
Object approaching, dust, dryness
Damage to the surface of the eye
Coughing
Food or mucus in the airway
Blocked airways and infection
Pupil reflex
Bright light
Damage to the retina
Knee jerk
Stretch of the tendon below the knee
Falling over — it helps you stay upright
Pain withdrawal
Sharp pin or a hot surface
Burns and cuts to the hand
The knee jerk surprises people because nothing dangerous is happening. Its real job is balance: when you are standing and your knee starts to buckle, the tendon stretches and the muscle instantly pulls back. Doctors just tap it to check the pathway is working.
The pain reflex arc
This is the example the syllabus uses, and the one worth learning line by line.
Only three neurones stand between the pin and the muscle. Every extra synapse would add delay, which is exactly what a protective response cannot afford.
🧩 The pain reflex, step by step
Stimulus — a sharp pin or a hot flame touches the skin of the hand.
Receptor — a pain receptor, called a nociceptor (you may also see it written nocireceptor), detects it.
Sensory neurone — an impulse travels along a sensory neurone into the spinal cord.
Relay neurone — in the grey matter, the impulse is passed straight across to a relay neurone, which synapses with a motor neurone.
Motor neurone — carries the impulse out to the arm.
Effector — the muscle contracts, pulling the hand away. That contraction is the reflex response.
Between each neurone is a synapse, and impulses cross a synapse by the diffusion of a chemical neurotransmitter. Synapses cost time, which is why a short pathway matters so much.
The reflex arc, in order
stimulus → receptor → sensory → relay → motor → effector → response
The brain is told, just not asked. An impulse does travel up the spinal cord to the brain, which is how you become aware of what happened. But it plays no part in coordinating the response, so the withdrawal happens first and the “ouch” arrives second.
A neat way to see this: think about how often you have pulled your hand back from something hot and only then wondered what you touched. That gap between acting and knowing is the reflex arc doing its job.
Controlling movement: who does what
Voluntary movement is a two-part job, and questions love the difference.
The motor cortex, part of the cerebrum, initiates movement. It sends the order to contract.
The cerebellum does not start movement. It coordinates it — making it smooth, accurate and balanced.
Balance is a surprisingly complicated job. It needs information from the eyes, from the semicircular canals in the ears and from many muscles at once, all combined and acted on continuously. The cerebellum also coordinates posture, walking, hand and finger movements, eye movements and speech.
This is a feedback loop, exactly like the ones controlling heart rate and breathing. Once you spot the pattern, control questions get much easier.
Think of reaching for a glass of water. The motor cortex says “reach”. The cerebellum is the one making tiny corrections all the way, so your hand arrives at the glass and not six centimetres past it.
Worked examples
WORKED EXAMPLE
Outline the pathway of a spinal reflex when a person stands on a sharp stone. [3]
DetectionPain receptors in the skin of the foot detect the stimulus and an impulse passes along a sensory neurone to the spinal cord.ProcessingIn the grey matter, a relay neurone passes the impulse to a motor neurone — the brain is not involved.ResponseThe motor neurone carries the impulse to a leg muscle, which contracts and lifts the foot.Receptor → sensory → relay → motor → effectorname every neurone type — missing the relay neurone is the usual lost mark
WORKED EXAMPLE
Explain why a reflex response is faster than a conscious response. [2]
Compare the routeIn a reflex the impulse only travels to the spinal cord and back, not up to the brain and down again.Then say why a shorter route is fasterThe pathway is shorter and there are fewer synapses to cross, and crossing a synapse by diffusion takes time.Shorter pathway + fewer synapses = less delay“fewer synapses” is the mark most students forget
WORKED EXAMPLE
A patient with damage to the cerebellum can still move their arm, but cannot pick up a cup without knocking it over. Explain. [3]
Why movement still happensMovement is initiated by the motor cortex of the cerebrum, which is undamaged.What is missingThe cerebellum normally receives feedback from proprioceptors, eyes and ears and uses it to coordinate the movement.The consequenceWithout those corrections the movement is not smooth or accurate, so the hand overshoots and balance is poor.Initiation is fine; coordination is lostthe whole question turns on “initiates” vs “coordinates”
💡 Exam tip
Learn the reflex arc as a chain and always write it in order. Out-of-order answers score badly even when every word is right.
Say where the relay neurone is: the grey matter of the spinal cord.
If a question says “without conscious thought”, it is telling you the brain does not coordinate the response. Say that back in your answer.
Name the effector and what it does. “A muscle contracts and pulls the hand away” is a better answer than “a muscle”.
For cerebellum questions, use the pair of verbs: the motor cortex initiates, the cerebellum coordinates.
Reflexes exist because they aid survival — that phrase is often worth a mark on its own.
⚠ Common mix-up
Saying the brain is not involved at all. It is informed — that is how you feel the pain. It just does not coordinate the response.
Leaving out the relay neurone. A spinal reflex needs three neurone types, not two.
Calling the receptor an effector. Receptors detect; effectors act. Muscles and glands are effectors.
Thinking the cerebellum starts movement. It does not — the motor cortex does.
Confusing “response” with “stimulus”. The pin is the stimulus; the hand moving away is the response.
Writing that the reflex is faster “because it is automatic”. That is a description, not a reason. The reason is the shorter pathway and fewer synapses.
Up next: Epinephrine & Melatonin — two hormones that show what chemical control can do that nerves cannot.
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