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

These revision notes are aligned to the OCR GCSE (9-1) Gateway Science Biology A (J247) specification, Version 4.0 (August 2026), for first assessment in 2018. They cover B3.1a, B3.1b and B3.1c. No statement here is marked Higher Tier only. For full explanations, read the study guide, then test yourself with the practice questions.

Course pages: OCR GCSE Biology hub, printable checklist and the free 10-minute diagnostics to find weak spots.

Key terms

Term	Meaning
Stimulus	A change that is detected.
Receptor	Detects a stimulus and starts an impulse.
Sensory neurone	Carries impulses from receptor to CNS.
Relay neurone	Short neurone in the CNS linking sensory to motor.
Motor neurone	Carries impulses from CNS to effector.
CNS	Brain and spinal cord.
Effector	A muscle (contracts) or a gland (secretes).
Synapse	Junction between two neurones; a narrow gap crossed by a chemical.
Reflex	Fast, automatic response that does not need conscious thought.
Reaction time	Time from stimulus to response.

B3.1a: Structure

Pathway to learn

stimulus -> receptor -> sensory neurone -> CNS -> motor neurone -> effector -> response

Neurone structure and function

Feature	Link to function
Long axon	Carries impulses over long distances.
Myelin sheath	Insulates and speeds up the impulse.
Cell body with nucleus	Controls the cell; in a motor neurone it is at the CNS end with many dendrites.
Sensory neurone: cell body off to one side	Impulse passes from receptor, past the cell body, on to the CNS.
Branched end of a motor neurone	Passes the signal on to the effector.

Synapse in four steps

1. Impulse reaches the end of neurone 1.
2. Transmitter chemical is released into the gap.
3. It diffuses across the gap.
4. It binds to receptors on neurone 2, which starts a new impulse.

One direction only, because the receptors are on the receiving side. Each synapse adds a small delay.

B3.1b: Coordinated response

The nervous system can produce a coordinated response because it:

• reaches every part of the body;
• has many links (connections between neurones);
• has different sensory receptors;
• is able to coordinate responses (the CNS combines information and sends instructions).

Method for an “explain how” answer

1. Name the stimulus and the receptor that detects it.
2. Say the impulse travels along a sensory neurone to the CNS.
3. Say the CNS coordinates, using any relevant information from other receptors.
4. Say impulses pass along motor neurones.
5. Name the effector (a named muscle or gland) and state the response.

Small worked reminder

A cyclist hears a horn and moves to the left. Sound receptors in the ear detect the stimulus. Sensory neurones carry impulses to the CNS, which combines this with information from the eyes about the road. Motor neurones carry impulses to the arm and leg muscles (effectors), which contract and steer the bike.

Receptors and effectors: matching pairs

Stimulus	Receptor detects	Effector	Response
Sharp object under the foot	Pressure and pain in skin	Leg muscle	Foot lifts
Bright light	Light in the eye	Muscle of the iris	Pupil gets smaller
Smell of food	Chemicals in the nose	Salivary gland	Saliva is secreted
Dust on the cornea	Touch at the surface of the eye	Eyelid muscle	Blink

Receptors differ because each type is sensitive to one kind of stimulus. That is why many different receptors can be linked to the same CNS.

Small worked reminder: damage to the pathway

Which component is damaged if a person feels a touch on the arm but cannot move it away? The sensory neurone and receptor work, because the touch is felt. The fault is in the motor neurone or the muscle (effector). If instead the person cannot feel the touch and no reflex happens, the fault is in the receptor or sensory neurone.

B3.1c: Reflex arcs

Voluntary and reflex responses compared

	Reflex	Voluntary
Speed	Very fast	Slower
Conscious decision	No	Yes
Pathway	Receptor, sensory, relay, motor, effector	Includes conscious part of the brain
Number of synapses	Few	More
Purpose	Protection, e.g. withdrawal, blinking, pupil size	Deliberate actions

Structure and function

• Few neurones and few synapses: short, quick pathway.
• Relay neurone links sensory and motor neurones directly: no conscious decision is needed.
• Myelinated axons: fast conduction.
• Gives protection from harm before pain is felt.

Worked reminder: withdrawal reflex

Heat receptor in skin, sensory neurone to spinal cord, synapse, relay neurone, synapse, motor neurone, arm muscle contracts, hand moves away.

Estimating with a reflex pathway

Estimation is a listed skill. Use rounded values, and say that the answer is approximate.

A reflex path is about 0.8 m long. Impulses travel at about 60 m/s. Estimate the time spent travelling along the neurones. Time = distance / speed = 0.8 / 60 = 0.013 s, so about 0.01 s (10 ms). The real response also includes delays at the synapses, so the total is a little longer. Each added synapse adds to the delay, which is why a longer pathway through the brain gives a longer response.

Why the arcs differ in length

A reflex of the leg is processed in the spinal cord, near the receptors and effectors, so the impulse does not travel up to the brain and back. A voluntary action must reach the conscious part of the brain, so the pathway is longer and has more synapses.

Reaction time

• Reaction time (s) = time between stimulus and response.
• Ruler drop: t = √(2d/g), with d in m and g = 9.8 m/s².
• Convert cm to m by dividing by 100.

Worked reminder

A ruler is caught after falling 22 cm. d = 0.22 m, so t = √(2 × 0.22 / 9.8) = √0.04490 = 0.212 s, or 0.21 s.

Reading a results table

Trial	Distance (cm)	Reaction time (s)
1	22	0.21
2	19	0.20
3	17	0.19
4	15	0.17
5	14	0.17

Reaction time falls with each trial, from 0.21 s to 0.17 s. A sensible explanation is practice, because the person learns to watch the ruler. A fair comparison with a different factor would need the practice effect removed, for example by including some practice drops first.

Fair test checklist

• Change one factor; keep hand, ruler, start position and dropper the same.
• Drop the ruler without warning at random times.
• Repeat and take a mean; identify anomalies.
• Do not claim a cause from one person or a small sample.

Must-know distinctions

• Sensory (to the CNS) and motor (from the CNS).
• Neurone (a cell) and nerve (a bundle of neurones; know the word, use neurone in answers about structure).
• Receptor (detects) and effector (responds).
• Reflex (automatic) and voluntary (conscious).
• Synapse (gap crossed by a chemical) and the axon (carries an electrical impulse).

Quick self-test

1. What two organs make up the central nervous system?
2. Name the three types of neurone in a basic reflex arc, in order.
3. What is the function of the myelin sheath?
4. Explain why impulses pass across a synapse in one direction only.
5. Name the two types of effector.
6. Give the four features that let the nervous system produce a coordinated response.
7. Why is a reflex faster than a voluntary response?
8. A ruler falls 18 cm before being caught. Calculate the reaction time in seconds to 2 significant figures.
9. A person has a damaged sensory neurone in the leg. Predict what happens when the foot touches something sharp.
10. A student’s five reaction times are 0.20, 0.22, 0.19, 0.21 and 0.18 s. Calculate the mean.

Answers

1. Brain and spinal cord.
2. Sensory neurone, relay neurone, motor neurone.
3. It insulates the axon and speeds up the impulse.
4. The transmitter chemical is released from the end of one neurone only, and the receptors for it are on the next neurone only.
5. Muscles and glands.
6. It reaches every part of the body, has many links, has different sensory receptors and can coordinate responses.
7. It has a short pathway with few neurones and synapses, and does not wait for a conscious decision.
8. d = 0.18 m. t = √(2 × 0.18 / 9.8) = √0.03673 = 0.1917, so 0.19 s.
9. No impulse reaches the CNS from the foot, so there is no reflex withdrawal and no sensation of pain. The motor neurones are fine, so the muscles could still move if the brain gave a command.
10. Total = 1.00 s; mean = 0.20 s.

Where marks are usually lost

• Writing “nerve” or “nerves” when the question asks about a specific neurone type.
• Describing the impulse as jumping across the synapse rather than naming a chemical crossing the gap.
• Naming the whole limb as the effector instead of a muscle.
• Leaving out the CNS or the relay neurone in a reflex arc.
• Saying the reflex is fast “because it does not use the brain” with no reference to fewer neurones and synapses.
• Forgetting to convert centimetres to metres in the reaction time equation.
• Giving a mean to too many figures, or including an anomaly without comment.
• Writing a conclusion about cause from one person’s data.

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.