IB Biology HLCoordinating Body SystemsPaper 1 & 2~9 min read
Reflex Arcs & Movement Control
Your hand is already off the hot pan before you feel anything. That is not you reacting quickly — it is your spinal cord making the decision without asking you. The pain arrives a moment later, as a report on something that has already happened.
📚 What you need to know
A reflex response is automatic and rapid, happens without conscious thought, and minimises damage.
A reflex arc is the pathway from receptor to effector that does not involve the conscious regions of the brain.
Three neurones are involved: sensory, relay, motor.
In a pain reflex the receptor is a nociceptor and the relay neurone sits in the grey matter of the spinal cord.
You become aware of a reflex after it has been carried out.
The motor cortex initiates voluntary movement; the cerebellum coordinates and corrects it.
What a reflex is for
Reflexes are automatic and rapid. Because they skip conscious processing, they happen fast enough to minimise damage, and that makes them a survival advantage.
Reflex
What it does
Blinking
Stops the eye surface drying out, and protects it from foreign objects
Coughing
Stops food entering the airways, and clears mucus during infection or allergy
Pupil reflex
Protects the eye from damage by bright light
Knee reflex
Helps keep balance when standing upright
You do become aware of a reflex — just late. The information takes longer to reach the conscious parts of the brain than it takes the reflex arc to complete, so awareness follows the action.
The pain reflex arc
The skin has receptors for pressure, touch and pain. The pain receptor is a nociceptor. Here is the full order, which is the version exams ask for:
The stimulus — a sharp pin or a hot flame — is detected by a nociceptor in the skin of the hand.
An action potential travels along a sensory neurone to the CNS.
The impulse passes to a relay neurone in the spinal cord. Relay neurones are found entirely within the grey matter.
The relay neurone synapses with a motor neurone. Impulses cross the synapse by diffusion of a neurotransmitter.
The motor neurone carries the impulse to an effector — a muscle in the arm.
The muscle contracts and pulls the hand away. This is the reflex response.
Watch the spelling. The IB uses nociceptor (from the Latin nocere, to harm). It is a pain receptor, not a "pain nerve" and not a "noci-neurone".
🧠
Why it is called an arc
An arc goes out and comes back without reaching the top. The signal goes in at the spinal cord and comes straight back out at the spinal cord. It never climbs all the way to the conscious brain before the response happens. If you can picture that curve, you will never write "the brain decides" in a reflex answer.
Movement control: two different jobs
Reflexes are only one kind of movement. Deliberate movement — reaching for a glass, writing, walking — needs two brain regions doing two different things, and exams love to test whether you can tell them apart.
The motor cortex of the cerebrum is responsible for initiating muscle contractions. It starts the movement.
The cerebellum does not initiate movement. Once the movement has begun, it receives feedback signals from the part of the body that is moving and from different sense organs, and sends signals to coordinate and control the movement.
Movements coordinated by the cerebellum include balance, posture, walking, hand and finger movements, eye movements and speech.
Balance is the clearest example of why coordination is a separate job. Staying upright needs the eyes, the semicircular canals in the ears and many muscles to be adjusted together, continuously, in response to what is happening. No single command could do that — it needs a loop.
Worked examples
WE 1
Describe a pain reflex arc
A person stands on a drawing pin and immediately lifts their foot. Describe the pathway of the reflex arc involved. (4 marks)
Step 1: receptor
A nociceptor in the skin of the foot detects the stimulus.
Step 2: sensory neurone
An action potential travels along a sensory neurone to the spinal cord.
Step 3: relay neurone and synapses
In the grey matter of the spinal cord a relay neurone passes the impulse on, crossing synapses by diffusion of a neurotransmitter.
Step 4: motor neurone and effector
A motor neurone carries the impulse to the effector, a leg muscle, which contracts and lifts the foot.
Nociceptor, sensory, relay, motor, effector — without the conscious brainname all three neurone types explicitly. Leaving out "relay" is the single most common way to lose a mark on this question
WE 2
Explain the timing of the pain
A student notices that they pull their hand away from a hot object before they feel any pain. Explain this observation. (3 marks)
Step 1: where the decision is made
The withdrawal is coordinated by the spinal cord alone, through a reflex arc. The conscious brain is not involved in producing the response.
Step 2: why that is faster
The pathway is short — only three neurones and two synapses — so the impulse reaches the effector very quickly.
Step 3: what causes the delay in feeling
Impulses are also sent up to the brain, but that pathway is longer and involves more synapses, so awareness arrives after the hand has already moved.
The reflex completes before the signal reaches the conscious brainsay that impulses do reach the brain. Writing that the brain never finds out is wrong and costs the third mark
WE 3
Distinguish two regions controlling movement
Damage to a patient’s cerebellum leaves them able to reach for a cup but unable to do it smoothly — their hand overshoots and wavers. Explain what this tells you about the roles of the motor cortex and the cerebellum. (3 marks)
Step 1: what still works
The patient can still start the movement, so the motor cortex, which initiates muscle contraction, is intact.
Step 2: what has been lost
The movement is no longer smooth or accurate, so what has been lost is coordination, not initiation.
Step 3: link to the mechanism
The cerebellum normally receives feedback from the moving limb and the sense organs and sends signals to adjust the movement while it is happening. Without that loop the movement cannot be corrected mid-way, so it overshoots.
Motor cortex starts the movement; the cerebellum corrects it as it happensthe word "feedback" is doing the work here. Coordination is impossible without information coming back
💡 Exam tips
Learn the reflex arc as a five-word chain: receptor, sensory, relay, motor, effector. Write it down before you start answering.
Say without involving the conscious regions of the brain, not just "without the brain".
Effector means a muscle or a gland. In a pain reflex it is a muscle.
If asked for the advantage of a reflex, link speed to minimising damage and therefore survival.
Motor cortex = initiates. Cerebellum = coordinates. Never swap them.
⚠ Common mistakes
Writing that the brain sends the signal back down. In a reflex arc the spinal cord does it.
Saying no impulses reach the brain at all. They do — that is how you become aware afterwards.
Missing out the relay neurone. Sensory straight to motor is not the pain reflex arc.
Placing relay neurones in white matter. They are entirely within grey matter.
Saying the cerebellum initiates movement. It coordinates movement that has already started.
Describing the response as "the hand moves". Name the effector and say the muscle contracts.
Up next: Epinephrine & Melatonin — two hormones with very different jobs. One prepares you for a fight in seconds, the other quietly keeps your body on a twenty-four hour clock.
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