OCR GCSE Biology A: Nervous system, coordinated responses and reflex arcs (J247)

This study guide is aligned to the OCR GCSE (9-1) Gateway Science Biology A (J247) specification, Version 4.0 (August 2026), for first assessment in 2018. It covers statements B3.1a, B3.1b and B3.1c in topic B3.1, Coordination and control: the nervous system. None of these three statements is marked as Higher Tier only, so all of it applies to both tiers. The eye, eye defects and the brain (B3.1d to B3.1h) are taught separately and are not repeated here.

Work through this guide first, then use the revision notes for quick recall and the practice questions to test yourself. Course links: OCR GCSE Biology hub, printable checklist and the free 10-minute diagnostics. For earlier ideas on cells and systems, see cell level systems.

What this unit covers

Statement	What you must be able to do
B3.1a	Describe the structure of the nervous system: central nervous system, sensory, motor and relay neurones, sensory receptors, synapse and effectors. Know the structure of a sensory neurone and of a motor neurone in detail.
B3.1b	Explain how the components of the nervous system produce a coordinated response, using four properties of the system (listed below).
B3.1c	Explain how the structure of a reflex arc is related to its function.

The specification also lists data skills for this unit: estimating, and extracting and interpreting data from graphs, charts and tables. The reaction time practical gives you plenty of both.

B3.1a: Structure of the nervous system

The nervous system carries information around the body as electrical impulses. A human is a multicellular organism, so cells (neurones) form a tissue, and the tissues make up organs (brain, spinal cord) in an organ system.

The parts

• Central nervous system (CNS): the brain and spinal cord. It receives information, decides what to do and sends instructions out.
• Sensory receptors: cells or structures that detect a change (a stimulus) and start an impulse. Different receptors detect different stimuli: light, sound, pressure, temperature, chemicals.
• Sensory neurones: carry impulses from the receptors to the CNS.
• Relay neurones: short neurones inside the CNS that connect sensory neurones to motor neurones.
• Motor neurones: carry impulses from the CNS to the effectors.
• Effectors: muscles and glands. Muscles respond by contracting. Glands respond by secreting a substance.
• Synapse: the junction between two neurones, where there is a tiny gap.

Structure of a motor neurone

A motor neurone has these features, each linked to its job:

Feature	What it does
Cell body at the CNS end, with a nucleus and many short branches (dendrites)	Receives impulses from other neurones and controls the cell’s activities.
Long axon	Carries the impulse over long distances, for example from the spinal cord to a leg muscle.
Myelin sheath (fatty insulating layer) around the axon	Insulates the axon and speeds up the impulse.
Branched endings at the effector	Pass the signal on to the muscle or gland.

Structure of a sensory neurone

A sensory neurone is also long and also has a myelin sheath, but it is arranged differently. The cell body sits off to one side, in a short branch away from the main route of the impulse. A long fibre runs from the receptor towards the cell body and on into the CNS, where the neurone ends close to a relay neurone.

Both types are elongated and insulated because the pathway may be a metre long and speed matters.

How a synapse passes the signal on

The impulse cannot jump the gap directly. Instead:

1. The impulse arrives at the end of the first neurone.
2. The end of that neurone releases a chemical (a transmitter substance) into the gap.
3. The chemical diffuses across the gap, which is very narrow, so this is quick.
4. It binds to receptor molecules on the next neurone and sets off a new impulse there.

Receptors for the chemical are only on the receiving side, so the signal passes in one direction only. The chemical step also means each synapse adds a small delay.

Worked example 1: Name and explain

A student says: “The sensory neurone and the motor neurone do the same job.” Correct this.

The sensory neurone carries impulses from a receptor to the CNS. The motor neurone carries impulses from the CNS to an effector. They both carry impulses along an axon, but they carry them in opposite directions relative to the CNS. A relay neurone connects the two inside the CNS.

B3.1b: A coordinated response

A coordinated response means that the right effectors act at the right time, in a way that suits the stimulus. The specification gives four reasons the nervous system can do this:

1. It reaches every part of the body, so any receptor can be linked to any effector.
2. It has many links: neurones connect to many other neurones through synapses, so one stimulus can lead to several responses and several stimuli can be combined.
3. It has different sensory receptors, so it can collect information about many different changes at once.
4. It is able to coordinate responses, because the CNS receives all the information and sends out instructions.

The pathway for any response is:

stimulus -> receptor -> sensory neurone -> CNS (coordinator) -> motor neurone -> effector -> response

Worked example 2: Catching a ball

Explain how the nervous system lets you catch a ball thrown to you.

• Light receptors in your eyes detect the ball moving, and other receptors in your muscles and joints detect the position of your arm. These are different receptors giving different information.
• Sensory neurones carry impulses from all of them to the CNS.
• The brain combines the information, works out where the ball will be and sends impulses along motor neurones.
• The muscles of the shoulder, arm and hand (effectors) contract in the right order, so the hand closes as the ball arrives.

Notice that the answer names the structures in the order the impulse travels, and it uses the words “receptor”, “CNS”, “motor neurone” and “effector”.

Worked example 3: What happens if part of the pathway fails?

A person has damage to the motor neurone that supplies the muscle in their forearm. They can feel the muscle being pinched. Explain this.

The sensory neurones and receptors are working, so impulses still reach the CNS and the person feels the pinch. The motor neurone is damaged, so impulses cannot reach the effector, and the muscle cannot contract. Each component has to work for the whole response to happen.

B3.1c: The reflex arc

A reflex is a fast, automatic response to a stimulus. You do not decide to do it, and it does not need conscious thought. Examples are pulling your hand away from something hot, blinking when something touches your eye and the pupil of the eye narrowing in bright light.

The reflex arc is the pathway:

receptor -> sensory neurone -> relay neurone (in the CNS) -> motor neurone -> effector

Structure linked to function

Structural feature	How it helps the reflex
Only three neurones in the basic arc	A short pathway, so the impulse reaches the effector quickly.
Relay neurone connects sensory directly to motor neurone	The response can be started without waiting for a decision by the conscious part of the brain, so it is automatic.
Few synapses	Each synapse adds a delay, so fewer synapses means a faster response.
Long, insulated axons (myelin)	The impulse travels quickly along each neurone.
Receptor at one end and effector at the other	Detects the danger and acts on it, which protects the body from harm.

Information also passes on to the brain, which is why you usually feel pain a moment after your hand has already moved. The reflex does not wait for it.

Worked example 4: The withdrawal reflex

Explain how your hand is pulled away from a hot pan.

1. Temperature receptors in the skin detect the heat.
2. A sensory neurone carries impulses to the spinal cord.
3. At a synapse, the sensory neurone passes the signal to a relay neurone in the spinal cord.
4. At a second synapse, the relay neurone passes the signal to a motor neurone.
5. The motor neurone carries impulses to a muscle in the arm (the effector).
6. The muscle contracts and the hand moves away.

The brain does not have to decide first, so the response is faster than a deliberate movement and the damage is limited.

Reaction time

Reaction time is the time between a stimulus and the response. A voluntary response, such as pressing a button when a light comes on, passes through the conscious part of the brain, so it is slower than a reflex.

Ruler drop investigation

A helper holds a 30 cm ruler vertically. You hold your thumb and finger either side of the zero mark, without touching it. The helper drops the ruler without warning. You catch it and the distance it fell, d, is read at the top of your finger.

The ruler falls freely, so the reaction time is found from t = √(2d/g), where d is in metres and g is 9.8 m/s². This is an estimate, because it assumes that the ruler falls freely and the reading is exact.

Worked example 5: From distance to time

Five drops gave these distances in cm: 13, 16, 14, 15, 12. Calculate the mean reaction time.

• Mean distance = (13 + 16 + 14 + 15 + 12) / 5 = 70 / 5 = 14.0 cm = 0.140 m
• t = √(2 × 0.140 / 9.8) = √0.02857 = 0.169 s

The mean reaction time is 0.17 s (to 2 significant figures).

Planning a fair test

You might compare reaction times with and without a factor such as practice or a caffeinated drink.

• Change one variable only (the factor under test).
• Keep others the same: same hand, same ruler, same starting position, same person dropping it, same time of day.
• Repeat several times and calculate a mean, so that random error has less effect.
• Look for anomalies, such as a drop where the person was distracted, and say how you treat them.
• The helper should release the ruler at random moments, so that you cannot predict the drop.

Worked example 6: Interpreting results

Five tests of the same person gave reaction times of 0.21, 0.19, 0.23, 0.20 and 0.22 s before a caffeinated drink and 0.18, 0.17, 0.19, 0.16 and 0.20 s after. Describe the effect.

Mean before = 1.05 / 5 = 0.21 s. Mean after = 0.90 / 5 = 0.18 s. The mean reaction time fell by 0.03 s, which is a decrease of 0.03 / 0.21 × 100 = 14%.

The range before (0.19 to 0.23 s) and after (0.16 to 0.20 s) overlap only slightly, which supports a real change. One person tested once cannot show that caffeine is the cause, because practice could also shorten reaction times. A control group, or testing in a different order, would help.

Common errors

• Writing that the impulse “jumps” across a synapse. It is a chemical that crosses the gap.
• Saying a reflex does not involve the brain at all. The decision is made without the conscious part of the brain, but the CNS (here the spinal cord) is still the coordinator.
• Mixing up the direction: sensory neurones go to the CNS, motor neurones come away from it.
• Naming the effector as “the hand” or “the arm”. Name the muscle or gland.
• Using nerve and neurone as if they were the same. A neurone is one cell.
• Forgetting to convert cm to m before using t = √(2d/g).

Official syllabus

OCR, GCSE (9-1) Gateway Science Biology A (J247) specification, Version 4.0 (August 2026), first assessment 2018. Statements B3.1a to B3.1c.