AQA A-Level Biology: Nervous Coordination (7402)

Nervous coordination is the topic where A-Level Biology stops being descriptive and becomes mechanistic. Section 3.6 (first half) of AQA 7402 demands that students reason about transmembrane voltages, ionic conductances, neurotransmitter diffusion times and sliding-filament cross-bridge cycling — and do so in language precise enough to defend against an A-Level mark scheme. This course completes the AQA 3.6 first-half coverage that, in the Phase 1 catalogue, was previously incomplete. The Phase 2 build adds four new lessons (receptors, reflex arcs, plant tropisms and the RP10 practical) that close the gap between the platform and the specification.

This course sits as course 6 of the 11 in the LearningBro AQA A-Level Biology learning path. It follows directly on from energy transfers — because every action potential, every synaptic vesicle release event and every sarcomere shortening is powered by ATP from oxidative phosphorylation. It feeds directly into homeostasis, which develops the endocrine counterpart to neural signalling. The earlier path courses — biological molecules, cells, exchange and transport and genetic information — supply the membrane biology and protein structure on which this course depends.

This guide walks through all eight lessons of the Nervous Coordination course — neurone anatomy, the action potential, synaptic transmission, sliding-filament muscle contraction, receptors and transduction, reflex arcs and CNS organisation, plant tropisms and the RP10 muscle-fatigue practical — and links each into the wider AQA Biology programme.

Guide Overview

The course breaks Section 3.6 (first half) into eight lessons. Lesson 1, neurone structure, establishes the morphology of motor, sensory and relay neurones together with the role of myelination and saltatory conduction. Lesson 2, action potential and resting potential, develops the ionic basis of the membrane potential from the Hodgkin-Huxley framework. Lesson 3, synaptic transmission and neurotransmitters, handles vesicle release, postsynaptic receptor binding and synaptic integration. Lesson 4, sliding-filament muscle contraction, covers the molecular biology of the sarcomere and the calcium-driven cross-bridge cycle.

The four Phase 2 additions complete the AQA spec coverage. Lesson 5, receptors and sensory transduction, develops the Pacinian corpuscle and rod-cell mechanisms. Lesson 6, reflex arcs and the CNS-PNS organisation, maps the anatomical pathway from receptor to effector via the spinal cord. Lesson 7, plant responses, tropisms and IAA, covers phototropism, gravitropism and the redistribution of indoleacetic acid. Lesson 8, required practical muscle fatigue and reaction times, houses RP10 with the underlying experimental design and statistical analysis.

AQA 7402 Specification Coverage

AQA Biology 7402 examines Section 3.6 across all three papers, with the heaviest weight on Paper 2 (Sections 3.5-3.8). Refer to the official AQA specification document for exact wording of every learning outcome.

Sub-topic	Spec area	Typical paper weight
Neurone structure and myelination	3.6.2.1	2-4 marks
Resting potential and action potential	3.6.2.1	6-10 marks
Synaptic transmission	3.6.2.2	5-8 marks
Sliding-filament muscle contraction	3.6.4	6-10 marks
Receptors and transduction	3.6.1.2	3-6 marks
Reflex arcs, CNS and PNS	3.6.2	3-5 marks
Plant tropisms and IAA	3.6.1.1	3-5 marks
RP10 reflex / response	practical	4-8 marks

These weights are estimates modelled on the structure of recent 7402 papers. What is reliable is that an ionic-basis question on the action potential, a synaptic-transmission long-answer item and a sliding-filament cycle diagram appear on essentially every series.

Neurone Structure and Myelination

The opening lesson on neurone structure establishes the morphological language used throughout the course. A neurone has three functional regions — the cell body (containing the nucleus and biosynthetic machinery), the dendrites (branched inputs that gather signals from upstream neurones) and the axon (the single, often elongated output that carries action potentials to downstream targets). AQA requires three neurone types: motor (cell body in the CNS, long axon to an effector), sensory (cell body in a dorsal root ganglion outside the CNS, pseudo-unipolar morphology with a single process splitting into peripheral and central branches) and relay (entirely within the CNS, short axons connecting other neurones).

Myelination — the wrapping of axons by Schwann cells (PNS) or oligodendrocytes (CNS) — creates an insulating sheath that confines voltage-gated channel activity to gaps called nodes of Ranvier. Action potentials therefore jump from node to node, a process called saltatory conduction, which is dramatically faster than continuous conduction in unmyelinated axons. A mammalian myelinated motor axon conducts at around 100 m s⁻¹; an unmyelinated invertebrate axon of comparable diameter conducts at around 1 m s⁻¹. Multiple sclerosis — a demyelinating autoimmune disease — illustrates the functional consequences of myelin loss.

A common pitfall is to describe the myelin sheath as covering the entire axon. It does not — the gaps between Schwann cells are the conduction sites without which saltatory conduction is impossible.

Resting Potential and Action Potential

The action potential and resting potential lesson develops the ionic basis of neuronal signalling. The resting potential of approximately -70 mV is maintained by the combined action of the sodium-potassium pump (which actively extrudes three Na⁺ and imports two K⁺ per ATP hydrolysed) and the selective permeability of the resting membrane to K⁺ via leak channels. The pump establishes the ionic gradients; the leak channels let K⁺ flow out down its concentration gradient until the electrical gradient (inside negative) balances the chemical gradient — the Nernst equilibrium for K⁺.

An action potential is a transient, all-or-nothing reversal of the membrane potential triggered when a depolarising stimulus reaches the threshold of approximately -55 mV. Voltage-gated sodium channels open, Na⁺ floods in, the inside becomes positive (overshoot to around +40 mV). At the peak, the sodium channels inactivate and voltage-gated potassium channels open, K⁺ floods out, the membrane repolarises and briefly hyperpolarises before the resting state is restored.

The refractory period — during which sodium channels are inactivated and cannot reopen — has two functional consequences. It enforces a maximum firing frequency (the absolute refractory period limits the upper bound) and it ensures that action potentials propagate unidirectionally (the upstream membrane cannot fire again until the downstream signal has moved on). The all-or-nothing principle means stimulus intensity is coded by frequency of action potentials, not by amplitude — a fundamental insight that strong candidates apply across all sensory questions.

A common pitfall is to describe the sodium-potassium pump as causing the action potential. The pump establishes the gradients; the action potential is driven by the voltage-gated channels.

Synaptic Transmission and Neurotransmitters

Synaptic transmission handles the chemical link between neurones. When an action potential reaches the presynaptic terminal, voltage-gated calcium channels open and Ca²⁺ flows in. The calcium triggers the fusion of synaptic vesicles with the presynaptic membrane, releasing neurotransmitter into the synaptic cleft by exocytosis.

The classic AQA neurotransmitter is acetylcholine at the cholinergic synapse. ACh diffuses across the cleft (typically 20-40 nm) and binds to nicotinic acetylcholine receptors on the postsynaptic membrane. These are ligand-gated cation channels — binding opens the channel, Na⁺ flows in, the postsynaptic membrane depolarises. If enough postsynaptic receptors fire (spatial or temporal summation), the threshold is reached and an action potential propagates down the postsynaptic neurone. Acetylcholinesterase in the cleft hydrolyses ACh into choline and acetate, terminating the signal and allowing the receptor to reset.

Excitatory synapses depolarise the postsynaptic membrane; inhibitory synapses (typically using GABA or glycine in the mammalian CNS) hyperpolarise it. The postsynaptic neurone integrates the inputs — a process called summation — and fires only when the net depolarisation reaches threshold. Spatial summation combines simultaneous inputs from multiple synapses; temporal summation combines successive inputs from a single high-frequency synapse.

Synapses also enforce one-way transmission (only the presynaptic terminal contains vesicles) and underpin plasticity — the long-lasting changes in synaptic strength that form the cellular basis of memory. A common pitfall is to describe transmission as the action potential "crossing" the synapse. It does not — the electrical signal stops at the presynaptic terminal and a chemical signal carries the information across.

Muscle Contraction: The Sliding-Filament Model

Sliding-filament muscle contraction develops the molecular biology of skeletal muscle. A muscle fibre is a multinucleate cell containing many parallel myofibrils, each made up of repeating contractile units called sarcomeres. Each sarcomere is bounded by Z-lines and contains interdigitating thin filaments (actin, with tropomyosin and troponin) and thick filaments (myosin).

The sliding-filament theory — independently proposed by Huxley and Hanson, and Huxley and Niedergerke, in 1954 — states that during contraction the thin and thick filaments slide past each other, shortening the sarcomere. The filaments themselves do not change length. The shortening is driven by the cross-bridge cycle: a myosin head binds actin, undergoes a power stroke (pulling the actin towards the centre of the sarcomere), releases on binding ATP, hydrolyses the ATP to reset, then rebinds further along the actin.

The trigger for contraction is calcium. An action potential propagates down a motor neurone, releases ACh at the neuromuscular junction, depolarises the muscle fibre membrane (sarcolemma), and propagates into the fibre via T-tubules. Depolarisation of the T-tubule triggers release of Ca²⁺ from the sarcoplasmic reticulum. Ca²⁺ binds troponin, troponin displaces tropomyosin from the myosin binding sites on actin, and cross-bridge cycling begins. When stimulation stops, Ca²⁺ is actively pumped back into the sarcoplasmic reticulum, tropomyosin re-blocks the binding sites, and contraction ends.

ATP is required at two points: to power the myosin power stroke (or strictly, to release the myosin from actin after the stroke) and to power the Ca²⁺ pump. Rigor mortis — the stiffness of skeletal muscle after death — illustrates the consequence of ATP depletion: myosin remains bound to actin in a fixed cross-bridge state until proteolysis breaks the link.

A common pitfall is to claim the actin and myosin filaments shorten during contraction. They do not — the sarcomere shortens because the filaments slide.

Receptors and Sensory Transduction

Receptors and sensory transduction develops the conversion of physical stimuli into nervous signals. AQA requires two specific receptor examples — the Pacinian corpuscle (a mechanoreceptor in the dermis that responds to pressure) and the rod and cone cells of the retina (photoreceptors).

The Pacinian corpuscle consists of a sensory neurone ending wrapped in concentric layers of connective tissue, like an onion. Pressure on the corpuscle deforms the lamellae, which stretches the neurone membrane and opens stretch-mediated sodium channels. Na⁺ flows in, the membrane depolarises — producing a generator potential — and if the generator potential exceeds threshold, an action potential propagates down the sensory neurone. The generator potential is graded with stimulus intensity; the action potentials it triggers are all-or-nothing but vary in frequency. This is the universal coding scheme: stimulus intensity → graded generator potential → action potential frequency.

Rod cells are highly sensitive monochromatic photoreceptors in the peripheral retina; cone cells are less sensitive trichromatic photoreceptors concentrated in the fovea. Multiple rods converge onto a single bipolar neurone — high sensitivity but low acuity. Each cone (in the fovea) connects to its own bipolar neurone — low sensitivity but high acuity. The photopigment in rods is rhodopsin, which bleaches on absorption of a photon and ultimately hyperpolarises the rod membrane (an unusual case where the stimulated state of the receptor is hyperpolarisation, not depolarisation).

A common pitfall is to describe rods as colour receptors or cones as low-light receptors. The reverse is true on both counts.

Reflex Arcs and the CNS-PNS Organisation

Reflex arcs and CNS-PNS organisation maps the anatomical pathway from receptor to effector. A reflex is a rapid, involuntary, stereotyped response to a stimulus, mediated by a minimal neural circuit. The classic example is the knee-jerk reflex (a stretch reflex with just two neurones — sensory and motor — and a single synapse in the spinal cord) and the withdrawal reflex to a painful stimulus (a three-neurone arc with a relay neurone in the spinal cord adding integration capacity).

The reflex arc bypasses the brain — sensory information enters the spinal cord, synapses on a motor neurone (directly or via a relay), and triggers an effector response within a few tens of milliseconds. The brain is informed in parallel by ascending pathways but is not in the causal chain of the response.

The central nervous system (CNS) comprises the brain and spinal cord — protected by the skull, vertebrae, meninges and cerebrospinal fluid. The peripheral nervous system (PNS) comprises everything else — the cranial and spinal nerves carrying sensory information to the CNS and motor commands away from it. The motor branch of the PNS divides into the somatic (voluntary, skeletal muscle) and autonomic (involuntary, smooth and cardiac muscle and glands) divisions; the autonomic divides further into the sympathetic (fight or flight, noradrenergic) and parasympathetic (rest and digest, cholinergic) branches.

A common pitfall is to claim reflexes are "always faster than voluntary responses". They generally are, but the difference is one of circuit length, not of intrinsic speed — a one-synapse reflex arc and a voluntary cortical response use the same neurones at the same conduction velocities.

Plant Responses, Tropisms and IAA

Plant responses, tropisms and IAA covers the AQA-required plant-signalling content. A tropism is a directional growth response to a directional stimulus. Phototropism is the growth response to light (shoots positively phototropic, roots negatively); gravitropism is the response to gravity (shoots negatively gravitropic, roots positively).

The classic mechanistic explanation invokes indoleacetic acid (IAA) — an auxin. In shoots, IAA is produced at the apex and transported basipetally. Under unilateral illumination, IAA accumulates on the shaded side, where it stimulates cell elongation (by loosening cell-wall components and promoting water uptake). The shaded side grows faster than the lit side, and the shoot bends towards the light. In roots, the same redistribution puts more IAA on the lower side under gravity, but in roots high IAA concentrations inhibit elongation — so the lower side grows more slowly and the root bends downward.

The Darwin (father-and-son), Boysen-Jensen, Paál and Went experiments from the late nineteenth and early twentieth centuries established the existence of a diffusible growth substance and form a recurring source of practical-application questions. AQA does not require detailed knowledge of every historical experiment, but does require students to interpret experimental designs that block, divert or replace the auxin signal.

A common pitfall is to claim IAA causes faster growth in roots as it does in shoots. The dose-response curve is opposite for the two organs.

Required Practical 10: Muscle Fatigue and Reaction Times

The RP10 lesson covers the AQA required practical on the effect of a factor on muscle fatigue or human reaction time. Typical designs include the dropped ruler reaction-time test (the height through which a ruler falls before being caught is converted to time using s = ½gt²); repeated hand-grip dynamometer fatigue tests with varying rest intervals; and the influence of distraction, caffeine or visual versus auditory stimuli on reaction time.

The lesson develops the underlying statistics — calculating means and standard deviations, identifying outliers, choosing an appropriate inferential test (Student's t-test for two-group comparisons, Mann-Whitney U for non-parametric data). It also covers ethics — informed consent, withdrawal rights, age and medical exclusions, anonymisation of data — which is a recurring source of AO3 marks on Paper 3.

Apparatus uncertainty matters. A standard 30 cm ruler reads to ±0.5 mm; a drop distance of 200 mm carries 0.25 percent uncertainty in distance and a corresponding 0.13 percent in time (because t ∝ √s). The dominant uncertainty is human reaction-time variability between trials, which is reduced by repeated measurements and averaging rather than by changing apparatus.

Cross-Topic Synoptic Links

Nervous coordination connects to energy transfers through the ATP demand of the sodium-potassium pump, the synaptic vesicle cycle and the cross-bridge cycle — neuronal and muscular tissue are among the highest-consumption tissues in the body precisely because of this constant ionic and mechanical work. It connects to homeostasis through the parallel between neural and endocrine signalling — both are coordination systems, but with very different time-courses and spatial specificities. It connects to musculoskeletal and immune systems through the detailed anatomy of skeletal muscle and the neuromuscular junction.

The membrane biology of cells — particularly the fluid mosaic model, membrane transport proteins and the principles of facilitated diffusion and active transport — is presupposed throughout this course. The protein structure of biological molecules underpins the receptor, channel and contractile-protein architecture.

Required Practical Anchors

This course owns one of the twelve AQA required practicals. RP10 (investigating the effect of a factor on human reaction time or muscle fatigue) is housed in the required practical lesson. Practical content from this course is examined on all three papers, with the heaviest weight on Paper 3.

How to Revise This Topic

The most effective revision for Section 3.6 (first half) combines mechanism-drawing with retrieval practice. Drawing the action potential graph from memory — with all ionic events labelled at every time point — is the single highest-yield retrieval task you can run for this topic, because it appears in some form on essentially every Paper 2. Drawing the cross-bridge cycle as a five-step circular diagram is the second highest-yield task; the same diagram with arrows reversed answers the rigor mortis question.

Apply spaced repetition (Ebbinghaus's forgetting curve) by revisiting the action-potential graph at expanding intervals — daily for week 1, every three days for weeks 2-3, weekly thereafter. Interleave synaptic-transmission and sliding-filament questions in the same session to force discrimination between the two ATP-dependent mechanisms. Use the exam preparation course for command-word drilling, particularly "explain" versus "describe" — this topic is dense in chains of reasoning that AQA reliably rewards when written as numbered cause-effect sentences.

Closing

Nervous coordination is the topic that separates students who can describe biology from students who can reason about it. Start with the neurone structure and action potential lessons to anchor the ionic vocabulary, then build outwards through synapses, muscle, receptors, reflexes and plant responses. Finish with the RP10 practical for the experimental-design and statistical fluency that Paper 3 reliably rewards. The full Nervous Coordination course is course 6 of 11 in the LearningBro AQA A-Level Biology learning path, and the mechanistic discipline it trains will carry through into every subsequent topic.