The human nervous system Structure and function (incl. RP7)

SPEC 4.5.2.1 • PAPER 2 • RP7 • HIGHER

The Nervous System

Structure, function, the reflex arc, and Required Practical 7 — reaction time. The spec's full 4.5.2.1 in one lesson.

Why This Lesson Wins Marks

"Describe the path of a nerve impulse" questions appear in nearly every AQA Paper 2 series. They're typically 4 or 6 marks, level-marked, and the mark scheme demands the full named pathway — every neurone, every synapse, in order. Skip one named structure and you cap below full marks.

3

Named Neurone Types

5

Stages In The Pathway

1

Required Practical (RP7)

What's Coming

The pathway, the three neurones, the reflex arc, synapses, and the RP7 ruler-drop method — everything in spec 4.5.2.1 in one go.

By The End You'll Be Able To…

1. Explain

Explain how the structure of the nervous system is adapted to its function — reacting to surroundings and coordinating behaviour.

2. Identify

Identify the five stages of the pathway and the three named neurones in a reflex arc.

3. Describe

Describe how a synapse transmits signals between neurones, and why reflex actions are automatic and rapid.

4. Apply (RP7)

Describe the ruler-drop method for measuring reaction time, identify the variables, and explain how to calculate the result.

What The Nervous System Does

Spec Definition

The nervous system enables humans to react to their surroundings and to coordinate their behaviour. Information from receptors passes along neurones as electrical impulses to the central nervous system (CNS).

The CNS

The CNS is the brain and spinal cord. It coordinates the response of effectors.

The Effectors

Effectors are muscles contracting or glands secreting hormones. Spec wording — memorise both.

Link To 5.01

This is one of the two systems you met in Lesson 5.01 — the nervous response system. It's fast, electrical, and short-lasting. The other (endocrine / chemical) is covered in Lesson 5.05.

The Five-Stage Pathway

The spec draws this exact sequence under 4.5.2.1 — memorise the order. AQA mark schemes accept both "coordinator" (the spec word here) and "coordination centre" (from 4.5.1) for the middle stage.

Information Travel

Between receptor and CNS, and between CNS and effector, the information is carried by neurones as electrical impulses. The next slide names the three neurone types.

Three Named Neurone Types

Three neurones carry impulses between receptor and effector. Use the exact named terms — "nerve cells" or "sensory cell" doesn't score on AQA mark schemes. The spec wants you to explain how the structure of each neurone is adapted to its function.

1. Sensory

Sensory neurone

Carries impulses from receptors to the CNS. Long cell — runs from a sense organ (e.g. skin) all the way to the spinal cord.

2. Relay

Relay neurone

Found inside the CNS (especially the spinal cord). Short — connects sensory to motor neurones across the spinal cord.

3. Motor

Motor neurone

Carries impulses from the CNS to effectors. Long — runs from the CNS out to a muscle or gland.

Neurone vs Nerve

A neurone is a single specialised cell. A nerve is a bundle of many neurones wrapped together. Mark schemes treat these as different things — using "nerve" when the question asks about cells loses the mark.

The Reflex Arc

The spec names the full reflex arc: receptor → sensory neurone → synapse → relay neurone → synapse → motor neurone → effector. The relay neurone sits in the spinal cord — the impulse never reaches the conscious part of the brain.

Spinal Cord — NOT Brain

The relay neurone is in the spinal cord. Mark schemes consistently say "in spinal cord / CNS — ignore brain" for reflex questions. Writing "the brain decides to pull the hand away" loses the mark.

Synapses — The Gap Between Neurones

A synapse is a tiny gap between two neurones. The electrical impulse can't jump the gap — so it triggers a chemical to be released that carries the signal across.

Mark-Scheme Wording

At a synapse, a chemical (a neurotransmitter) is released from the end of one neurone, crosses the gap by diffusion, and triggers a new electrical impulse in the next neurone.

Accepted Mark-Scheme Words

"chemical" (minimum)

"neurotransmitter" (better)

"by diffusion" (for the crossing mechanism)

Words That Don't Score (at the synapse)

"messages cross the gap" (ignored — must be "chemical")

"signals jump" (incorrect)

"electrical impulse crosses" (the impulse stops)

Note On "Messages"

The word "message" is fine when describing what travels along a neurone (e.g. "an electrical message passes along the neurone") — AQA mark schemes have accepted this. It's only at the synapse that "message" fails, because the question is asking what specifically crosses the gap — a chemical, not a message.

Why Synapses Slow Things Down

Diffusion across a synapse takes longer than electrical conduction along a neurone. That's why responses involving multiple synapses (reflex arcs have two) are still measurable in milliseconds, not microseconds — and why RP7 reaction times are typically 0.1–0.5 seconds.

Why Reflexes Are Different

Most actions involve the conscious brain. Reflexes don't — and that's what makes them fast enough to protect you.

Spec Definition — Reflex Action

Reflex actions are automatic and rapid; they do not involve the conscious part of the brain.

FeatureReflex actionVoluntary action

Conscious thought?NoYes

PathwayThrough spinal cord onlyThrough the brain

SpeedVery fast (~0.2 s)Slower

ExamplesWithdrawing hand from heat, blinking, pupil reflex, knee jerkWriting, walking, talking

Why It Matters

Reflexes are important for survival. Skipping the brain saves time — by the time you'd consciously realise your hand was on a hot object, the burn would already be worse. The reflex acts before you're aware.

Worked Example — Reflex Pathway

Original 4-mark question in the style AQA uses for reflex-arc descriptions. Use the PEE framework to build a chain answer that hits every named structure.

"A person touches a hot pan and pulls their hand away. Describe the pathway of the nerve impulse from stimulus to response. Name the neurones and structures involved." — Practice question · 4 marks · AO1 · Spec 4.5.2.1

P

Point: Heat (the stimulus) is detected by temperature receptors in the skin, which generate an electrical impulse.

E

Evidence: The impulse passes along a sensory neurone to the spinal cord (CNS). At a synapse, a chemical crosses the gap by diffusion to a relay neurone, then across another synapse to a motor neurone.

E

Explain: The motor neurone carries the impulse to the muscle (effector), which contracts to pull the hand away — the response. This happens without involving the conscious brain, which is why it's so fast.

Mark Scheme Match

✓ Receptor named (temperature receptor in skin — note: AQA mark schemes ignore "pain") · ✓ Sensory → relay → motor neurones in order · ✓ Synapse named + "chemical crosses gap" · ✓ Effector named (muscle) + response described. 4/4.

Practice Question

(a) Name the three types of neurone involved in a reflex arc.3 MARKS

(b) Describe what happens at a synapse.2 MARKS

(c) Reflex actions are described as automatic and rapid. Explain why reflex actions are faster than voluntary actions.2 MARKS

(a)

Sensory neurone · relay neurone · motor neurone. (1 mark each = 3 marks)

(b)

A chemical (neurotransmitter) is released into the gap between neurones (1 mark); it diffuses across the gap and triggers a new impulse in the next neurone (1 mark).

(c)

Reflex actions do not involve the conscious part of the brain(1 mark); the impulse only travels through the spinal cord, so the pathway is shorter / there are fewer neurones, so the response happens more quickly (1 mark).

⚠️

Mark-loss check: For (a) "sensory cell" / "nerves" don't score — must be "neurone". For (b) "the impulse jumps the gap" is wrong — the impulse stops; a chemical crosses. For (c) "fewer neurones" alone is borderline — pair it with "no conscious brain involvement".

RP7 — Reaction Time

The spec asks: plan and carry out an investigation into the effect of a factor on human reaction time. AQA examines this as both a practical method question and a data-interpretation question.

Reaction Time

The time taken between a stimulus and the response. Typical human reaction times are 0.2 to 0.5 seconds.

Factors Commonly Investigated

Caffeine — does drinking caffeine reduce reaction time?

Practice — does repeating the test improve performance?

Distraction — does background noise or visual distraction slow reactions?

Time of day — are people slower in the morning vs afternoon?

Why Reaction Time, Not Reflex

A reflex is a specific automatic action through the spinal cord. Reaction time in RP7 also involves the brain — the catcher sees the ruler drop (visual stimulus) and consciously decides to grip. It's a voluntary action with a measurable delay.

RP7 — The Ruler-Drop Method

The standard AQA method. Memorise the sequence — examiner reports flag students writing vague descriptions ("hold a ruler, drop it") that miss the named details.

Step-by-Step

Person A sits with their elbow resting on the edge of a table, hand extended over the edge, thumb and forefinger open ready to catch.

Person B holds a 30 cm ruler vertically so that the 0 cm mark is level with Person A's thumb.

Person B drops the ruler without warning.

Person A catches the ruler as quickly as possible between thumb and forefinger.

Record the distance the ruler fell (read at the catch point).

Repeat at least 3 times and calculate the mean distance.

Then change the factor being investigated (e.g. introduce a distraction) and repeat.

Distance, Not Time (Initially)

You measure distance — the ruler falls further the slower the reaction. To convert to time, use a conversion table or the equation d = ½gt² (provided where needed). The shorter the distance, the faster the reaction.

A More Precise Alternative — Digital Timer

AQA mark schemes credit "using a digital timer / computer-based reaction test" as an improvement to the ruler-drop method. Reason: the resolution of a digital timer (e.g. 0.01 s) is higher than reading distance off a ruler. Worth knowing for "suggest one improvement to this method" questions.

RP7 — Variables and Controls

AQA's most common RP7 question is "suggest two ways to improve the validity" — and the mark scheme answers are all control variables. The trap: depending on the scenario, AQA wants different types of controls. Know both categories.

Independent

The factor tested (caffeine, distraction, practice, etc.)

Dependent

Distance ruler falls (cm) → converted to reaction time (s)

Controls

Two categories — see below

Procedural Controls (the method)

Same starting position (0 cm at thumb)

Same hand used to catch

Same arm position (lower arm resting on table)

No force on the ruler — let it drop, don't throw it

No warning before the drop — predicting it pre-triggers the catcher

Same type / weight of ruler

Subject Controls (the person)

Same age

Same sex

Same BMI / similar fitness

Same amount of sleep the night before

Same time of day for the test

Time interval between intervention (e.g. drinking coffee) and test

Which Controls AQA Wants Depends On The Question

If the Q is "improve the validity of the method" → procedural controls. If the Q is comparing groups of people (e.g. "does caffeine affect reaction time?") → subject controls. Recent mark schemes (Jun 2023, Jun 2024) credit both depending on what's being asked.

Reliability

Take at least 3 repeats, ignore anomalies, and calculate the mean distance. Reaction time is naturally variable, so means are essential.

Three Examiner Alerts

Patterns drawn from AQA mark schemes and examiner reports on nervous-system questions. Fix these now and protect easy marks.

Alert 1 — Synapse Wording

The mark scheme phrasing for synapses is: "chemical / neurotransmitter crosses the gap by diffusion". Don't write "the impulse crosses" (it doesn't — it stops). "Messages cross the gap" is also rejected at synapses — must be a chemical. (Note: "electrical message along a neurone" is fine — the rejection is synapse-specific.)

Alert 2 — Spinal Cord, Not Brain

Reflex arcs run through the spinal cord. Examiner reports flag students who write "the brain decides" or "the brain sends a signal" — both lose marks. Mark schemes say "in spinal cord / CNS — ignore brain".

Alert 3 — Generic RP7 Improvements Score Zero

"Be more careful" and "repeat more times" are too vague. AQA's RP7 mark schemes credit named controls — either procedural (same starting position, same hand, no force, no warning) or subject (same age, sex, sleep, time between intervention and test), depending on the scenario. Each named control with a brief reason = a mark.

Common Errors

Phrasings that lose marks vs phrasings that win them. Memorise the right column.

✗ Wrong

"The nerve carries the impulse to the brain."

"Nerve" = bundle. The cell is a "neurone". And reflexes go to spinal cord, not brain.

✓ Correct

"The sensory neurone carries the impulse to the spinal cord (CNS)."

Named neurone type + correct CNS location.

✗ Wrong

"The impulse jumps the synapse."

The electrical impulse stops at the synapse.

✓ Correct

"A chemical (neurotransmitter) crosses the synapse by diffusion."

Mark-scheme wording — three creditable terms.

✗ Wrong

"The brain decides to pull the hand away."

Reflexes don't involve the conscious brain.

✓ Correct

"The reflex pathway runs through the spinal cord without involving the conscious brain."

Captures the defining feature of a reflex.

✗ Wrong

"Be more careful when dropping the ruler." (RP7 improvement)

Vague — doesn't address validity.

✓ Correct

"Always start with the 0 cm mark level with the catcher's thumb so the falling distance is comparable between trials."

Named control + reason it improves validity.

Three Takeaways

Everything in this lesson collapses into three things to carry into the exam.

1. The Named Pathway

Stimulus → receptor → sensory neurone → synapse → relay neurone (in spinal cord) → synapse → motor neurone → effector → response. Use named terms; use "neurone" not "nerve"; use "spinal cord" not "brain" for reflexes.

2. The Synapse Sentence

"A chemical (neurotransmitter) is released, crosses the gap by diffusion, and triggers a new impulse in the next neurone." Three creditable terms in one sentence.

3. RP7 — Controls Win The Marks

Ruler-drop method: distance fallen → reaction time. For "improve validity" questions, name controls + reasons — either procedural (same starting position, same hand, no force, no warning) or subject (same age, sex, sleep, time of day) depending on the question. Generic "be careful" = 0 marks.

You're Set For…

Any reflex-arc description (up to 6 marks) · synapse explanations (1–2 marks) · RP7 method, variables, and improvement questions · the foundation for Lessons 5.03–5.04 (brain & eye, both biology only).

Summary

The Pathway (spec wording)

stimulus → receptor → coordinator (CNS) → effector → response

Sensory neurone — receptors → CNS

Relay neurone — inside CNS (spinal cord)

Motor neurone — CNS → effectors (muscles / glands)

Reflex Arc — Spec 4.5.2.1

Receptor → sensory neurone → synapse → relay neurone (spinal cord) → synapse → motor neurone → effector

Automatic and rapid ; do not involve the conscious part of the brain

Important for survival — e.g. pulling away from heat, blinking

Synapses

A gap between two neurones

A chemical (neurotransmitter) crosses the gap by diffusion

Triggers a new electrical impulse in the next neurone

RP7 — Reaction Time

Method: ruler-drop test — measure distance fallen → convert to time (or use a more precise digital timer)

IV: the factor (caffeine, practice, distraction); DV: distance / time

Procedural CVs: same starting position, same hand, no force, no warning, same ruler

Subject CVs: same age, sex, sleep, time of day, time interval since intervention

Reliability: ≥3 repeats, calculate mean, ignore anomalies

Exam Tip

For reflex-arc questions, write the pathway as a chain using the named structures in order. Every named structure missing = a mark lost. Practise writing it out from memory until you can do all 7 stages (receptor → sensory → synapse → relay → synapse → motor → effector) without thinking.