Nervous Coordination

This lesson is mapped to AQA 7402 Section 3.6.2 — Nervous coordination / neurones (refer to the official AQA specification document for exact wording). The nervous system is the body's high-speed signalling network: it detects changes in the internal and external environment, integrates that information in the central nervous system, and triggers rapid, coordinated responses by skeletal muscle and glands. Unlike the endocrine system — which uses bloodborne chemical messengers that act over seconds to days — the nervous system operates in milliseconds, using specialised electrically-excitable cells called neurones. A withdrawal of the hand from a hot pan is complete in around 200 ms; a sprinter's reaction to a starter's pistol takes a similar interval. None of this would be possible without the structural specialisations covered in this lesson.

The architecture of the neurone — long, thin, electrically insulated, electrochemically polarised — represents one of the most striking adaptations in animal biology. The squid giant axon work associated with Hodgkin and Huxley in the mid-twentieth century (paraphrased here as their reconstruction of the action potential from voltage-clamp ionic-current measurements) supplied the experimental basis for everything that follows in the next three lessons. Sir Charles Sherrington's framework of the nervous system as an integrative network of excitatory and inhibitory inputs — rather than a simple wire-by-wire telegraph — remains the conceptual scaffold AQA examiners expect at A* level.

Key Definition: A neurone is a specialised cell adapted for the rapid transmission of electrical impulses (action potentials) from one part of the body to another.

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The Organisation of the Nervous System

The mammalian nervous system is divided into two main parts:

1. Central Nervous System (CNS) — the brain and spinal cord. The CNS acts as the coordinator, processing sensory information and initiating motor responses. Its outer cortex contains the cell bodies of relay neurones (grey matter); its interior is dominated by myelinated axons (white matter).
2. Peripheral Nervous System (PNS) — all the nerves outside the CNS, including cranial and spinal nerves. The PNS connects receptors and effectors to the CNS via bundles of sensory and motor axons wrapped in connective tissue.

The PNS is further subdivided:

• Somatic nervous system — controls voluntary actions (skeletal muscle). One motor neurone runs from the CNS directly to the muscle.
• Autonomic nervous system — controls involuntary actions (smooth muscle, cardiac muscle, glands). Two neurones run in series, synapsing at a peripheral ganglion. This is further divided into the sympathetic (the "fight or flight" framing — paraphrasing Cannon's mid-twentieth-century synthesis) and parasympathetic ("rest and digest") divisions, which act in opposition on most visceral targets.

graph TD
    A["Nervous system"] --> B["CNS<br/>brain + spinal cord"]
    A --> C["PNS<br/>cranial + spinal nerves"]
    C --> D["Somatic<br/>voluntary skeletal muscle"]
    C --> E["Autonomic<br/>involuntary visceral"]
    E --> F["Sympathetic<br/>fight or flight"]
    E --> G["Parasympathetic<br/>rest and digest"]

    style B fill:#27ae60,color:#fff
    style C fill:#3498db,color:#fff
    style F fill:#e74c3c,color:#fff
    style G fill:#16a085,color:#fff

This top-down view is the scaffold you should sketch at the start of any nervous-system question. The somatic / autonomic and sympathetic / parasympathetic distinctions are returned to in detail in lesson 5 of this course.

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Types of Neurone

There are three main types of neurone, each with a distinct role and structure:

1. Sensory Neurones (afferent)

• Carry impulses from receptors to the CNS.
• Have a long dendron that carries the impulse from the receptor towards the cell body, and a short axon that carries the impulse from the cell body into the CNS.
• The cell body is located off to one side of the neurone (in a dorsal root ganglion adjacent to the spinal cord), not at the end. This is a defining anatomical feature in the AQA mark scheme.
• A single dorsal root ganglion contains the cell bodies of all the sensory neurones entering one segment of the spinal cord.

2. Relay Neurones (interneurones)

• Found entirely within the CNS (brain and spinal cord).
• Connect sensory neurones to motor neurones.
• Have short dendrites and short axons, forming complex interconnections — a single relay neurone may receive inputs from thousands of presynaptic cells and project to thousands more.
• Allow integration and processing of information — this is the substrate of every higher function, from reflex modulation to memory.
• Mostly unmyelinated because their axons are short and conduction speed is not the limiting factor.

3. Motor Neurones (efferent)

• Carry impulses from the CNS to effectors (muscles or glands).
• Have a long axon (up to 1 m in humans — the longest cells in the body) and many short dendrites that receive impulses from relay neurones.
• The cell body is located within the CNS (in the ventral horn of the spinal cord for spinal motor neurones).
• The axon exits the CNS via a ventral root and travels in a peripheral nerve to its target muscle fibres.

Feature	Sensory Neurone	Relay Neurone	Motor Neurone
Direction of impulse	Receptor → CNS	Within CNS	CNS → Effector
Cell body position	Off to one side (dorsal root ganglion)	Within CNS	Within CNS (ventral horn)
Axon length	Long dendron, short axon	Short axon	Long axon
Myelinated?	Usually yes	Usually no	Usually yes
Typical conduction speed	30–120 m s⁻¹ (myelinated)	Local, ~0.5 m s⁻¹	30–120 m s⁻¹ (myelinated)

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Structure of a Typical Motor Neurone

A motor neurone has the following key structures:

• Cell body (soma): Contains the nucleus, rough endoplasmic reticulum (Nissl granules for protein synthesis), Golgi apparatus, mitochondria, and other organelles. Many ribosomes are present to synthesise the membrane proteins (ion channels, pumps, receptors) and neurotransmitters required throughout the cell.
• Dendrites: Short, branched extensions of the cell body that receive impulses from other neurones via synapses. A single motor neurone may bear several thousand synaptic inputs across its dendritic tree.
• Axon: A single, long cytoplasmic extension that carries the action potential away from the cell body towards the effector. The cytoplasm within the axon is called axoplasm, and the membrane is called the axon membrane (axolemma). The axon contains microtubule and neurofilament cytoskeletal tracks along which membrane proteins and vesicles are transported from the cell body to the synaptic terminals (anterograde transport) and back (retrograde transport).
• Axon hillock: The region where the axon joins the cell body; this is the trigger zone where action potentials are initiated. It has the highest density of voltage-gated Na⁺ channels in the cell, so it has the lowest threshold for firing.
• Axon terminal (synaptic knob): The swollen end of the axon that forms a synapse with the next neurone or an effector. Contains synaptic vesicles filled with neurotransmitter and many mitochondria to provide ATP for neurotransmitter synthesis, vesicle recycling, and active calcium pumping.

graph LR
    A["Dendrites<br/>receive EPSPs / IPSPs"] --> B["Cell body<br/>nucleus, ER, ribosomes"]
    B --> C["Axon hillock<br/>trigger zone"]
    C --> D["Myelinated axon<br/>saltatory conduction"]
    D --> E["Axon terminal<br/>synaptic vesicles, Ca²⁺ channels"]
    E --> F["Neuromuscular junction<br/>skeletal muscle"]

    style C fill:#e67e22,color:#fff
    style D fill:#3498db,color:#fff
    style E fill:#27ae60,color:#fff

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The Myelin Sheath

Many neurones in the PNS are myelinated — they are surrounded by a fatty, insulating layer called the myelin sheath.

Formation

• The myelin sheath is formed by Schwann cells in the PNS (and by oligodendrocytes in the CNS).
• Each Schwann cell wraps tightly around a section of the axon multiple times, forming concentric layers of its plasma membrane.
• The cytoplasm of the Schwann cell is squeezed out, leaving behind layers of phospholipid bilayer that act as an electrical insulator (lipid is non-conducting; the bilayer also has very low permeability to ions because the high-density myelin contains few ion channels).
• Each Schwann cell covers approximately 1 mm of axon. A single PNS oligodendrocyte equivalent in the CNS may myelinate up to 50 different axon segments.

Nodes of Ranvier

• Between adjacent Schwann cells there are small gaps (approximately 2–3 µm wide) where the axon membrane is exposed. These gaps are called nodes of Ranvier.
• At the nodes, the axon membrane contains a very high density of voltage-gated sodium ion channels (approximately 1,200 per µm² of membrane, compared with around 25 per µm² in unmyelinated axon membrane).
• The myelin sheath prevents ion exchange across the membrane in the myelinated regions (internodes), so depolarisation can only occur at the nodes.

Saltatory Conduction

• In myelinated neurones, the action potential effectively jumps from one node of Ranvier to the next. This is called saltatory conduction (from the Latin saltare, meaning "to jump").
• This greatly increases the speed of impulse transmission — myelinated mammalian motor neurones can conduct impulses at up to 120 m s⁻¹, compared with only 0.5–2 m s⁻¹ in unmyelinated neurones of the same diameter.
• Saltatory conduction also reduces the metabolic cost of transmitting impulses because fewer sodium and potassium ions need to be actively pumped back across the membrane after each impulse (only the nodal region is depolarised).

Exam Tip: Be precise with terminology. The myelin sheath is formed by Schwann cells (PNS) or oligodendrocytes (CNS). The gaps between Schwann cells are nodes of Ranvier. The impulse "jumps" between nodes — this is saltatory conduction. A common A* discriminator is naming the saltatory mechanism as local current flow between adjacent nodes rather than literal physical jumping — the depolarisation at one node spreads passively through the axoplasm to the next.

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Factors Affecting the Speed of Nerve Impulse Transmission

Several factors influence how quickly an action potential travels along a neurone:

Factor	Effect on Speed	Explanation
Myelination	Increases speed	Saltatory conduction means the impulse jumps between nodes of Ranvier; axoplasmic current flows over longer distances per regeneration event
Axon diameter	Larger diameter = faster	Less resistance to ion flow in the axoplasm (analogous to a thicker wire). Squid giant axons (~1 mm diameter) achieve ~25 m s⁻¹ unmyelinated; this is why squid evolved giant axons to drive escape jets
Temperature	Higher temperature = faster (up to ~40 °C in mammals)	Increased kinetic energy of ions and faster opening kinetics of voltage-gated channels; above ~45 °C, channel proteins denature and conduction fails

The squid giant axon strategy and the vertebrate myelination strategy are alternative evolutionary solutions to the same problem: how to conduct quickly without making every axon a millimetre thick. Myelination is the more space-efficient solution and enabled the dense vertebrate CNS.

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The Reflex Arc — A Preview

A reflex arc is the simplest nerve pathway, enabling a rapid, involuntary response to a stimulus. The pathway is:

Stimulus → Receptor → Sensory neurone → Relay neurone (in CNS) → Motor neurone → Effector → Response

Features of Reflex Arcs

• Rapid — the pathway involves few synapses (each synapse adds ~0.5–1 ms of delay), so the response is fast.
• Involuntary — the response occurs without conscious thought, which is important for protective reflexes (e.g. withdrawing the hand from a hot surface).
• Stereotyped — the same stimulus reliably produces the same response.

Example: The Knee-Jerk Reflex

1. Tapping the patellar tendon stretches the quadriceps muscle.
2. Stretch receptors (muscle spindles) in the quadriceps are stimulated.
3. Impulses travel along sensory neurones to the spinal cord.
4. In the spinal cord, the sensory neurone synapses directly with a motor neurone (this is a monosynaptic reflex — no relay neurone).
5. The motor neurone stimulates the quadriceps to contract, causing the lower leg to kick forward.

Example: The Withdrawal Reflex

1. A pain receptor in the finger detects a painful stimulus (e.g. a hot object).
2. Impulses travel along a sensory neurone to the spinal cord.
3. The sensory neurone synapses with a relay neurone in the grey matter of the spinal cord.
4. The relay neurone synapses with a motor neurone.
5. The motor neurone stimulates the biceps (effector) to contract, pulling the hand away.
6. Simultaneously, the relay neurone sends ascending impulses to the brain so the person becomes aware of the stimulus.

A fuller treatment of reflex arcs — including the crossed extensor reflex that maintains balance — is covered in lesson 5 of this course.

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Clinical Relevance: Demyelination

Multiple sclerosis (MS) is an autoimmune disease in which the immune system attacks and destroys the myelin sheath surrounding neurones in the CNS. Loss of myelin disrupts saltatory conduction, slowing or blocking nerve impulse transmission. Symptoms include muscle weakness, loss of coordination, numbness, visual disturbances (optic neuritis), and cognitive fatigue. MS provides a "natural experiment" demonstrating that myelin is required for normal CNS function — an A* response can deploy this clinical detail to evaluate the importance of saltatory conduction.

Guillain-Barré syndrome is a PNS counterpart, where the immune system attacks Schwann-cell myelin, producing flaccid paralysis that ascends from the limbs. Recovery occurs over weeks as Schwann cells re-myelinate the spared axons.

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A-Level Deep Dive

Spec mapping

This content sits in AQA 7402 Section 3.6.2 — nervous coordination (refer to the official AQA specification document for exact wording). It is examined on Paper 2 (3.5–3.8 content) and synoptically on Paper 3. Always handle the nervous system at three levels: cellular (neurone structure, ion channels), circuit (reflex arc, integration at synapses), system (CNS / PNS organisation; somatic vs autonomic).

Synoptic links

This lesson connects to:

1. AQA 7402 Section 3.2.3 — Transport across membranes (course 2): Schwann-cell membrane insulation depends on phospholipid bilayer non-conductance; the voltage-gated Na⁺ channels at nodes of Ranvier are integral membrane proteins of the type covered in section 3.2.
2. AQA 7402 Section 3.6.2 — action potential, synapse, muscle (lessons 1, 2, 3 of this course): every later lesson builds directly on the structural framework established here.
3. AQA 7402 Section 3.4.2 — protein synthesis (course 4): the cell body's many ribosomes synthesise the channel proteins that populate the axon membrane; long axons require sustained anterograde transport from the soma.

Mark-scheme literacy

Neurone-structure questions split AO marks predictably:

AO	Typical share	Earned by
AO1 (knowledge)	50–60%	Naming neurone types, identifying labelled diagram structures, defining saltatory conduction
AO2 (application)	30–40%	Linking structural features (myelination, diameter, dendrite branching) to function (speed, integration)
AO3 (analysis / evaluation)	10–20%	Evaluating evolutionary trade-offs; using clinical evidence (MS, Guillain-Barré) to test the role of myelin

Reward language includes "saltatory conduction depends on the high density of voltage-gated Na⁺ channels at the node of Ranvier", "myelin reduces capacitance of the internodal membrane", "the cell body is in the dorsal root ganglion off the main path of the impulse". Mark-loss patterns include calling the myelin sheath a "fat" or "covering" without specifying lipid composition / Schwann-cell origin, conflating Schwann cells with oligodendrocytes (location matters), and treating reflex arcs as if they never involve the brain (the brain is informed in parallel even when it does not initiate the response).

Specimen question modelled on the AQA paper format

Question (6 marks): Describe how the structure of a myelinated motor neurone is adapted for the rapid transmission of nerve impulses.

Mark scheme decomposition:

Mark	AO	Awarded for
1	AO1	Long axon allows impulses to be transmitted over long distances without relay synapses
2	AO1	Myelin sheath formed by Schwann cells, acts as electrical insulator
3	AO1	Nodes of Ranvier expose the axon membrane at regular intervals
4	AO2	Saltatory conduction — action potential "jumps" between nodes
5	AO2	High density of voltage-gated Na⁺ channels at nodes / large axon diameter reduces internal resistance
6	AO3	Synthesis: cumulative effect is conduction speeds up to ~120 m s⁻¹; metabolic cost is reduced because only nodal regions repolarise

Split: AO1 = 3, AO2 = 2, AO3 = 1.

Grade C model answer (~150 words)

A motor neurone has a long axon that carries impulses from the spinal cord to the effector. The axon is surrounded by a myelin sheath made of Schwann cells which act as insulation. Between the Schwann cells are gaps called nodes of Ranvier. At the nodes, sodium ions can move across the membrane, so the action potential jumps from node to node. This is called saltatory conduction and makes the impulse faster. The axon also has a large diameter which makes it faster too because there is less resistance. Without myelin, the impulse would have to depolarise the whole length of the membrane, which would take longer. So myelinated neurones are adapted to send impulses quickly.

Examiner commentary: M1 long axon, M1 myelin / Schwann, M1 nodes of Ranvier, M1 saltatory conduction. Around 4/6. The candidate omits the mechanism of why myelin speeds conduction (it reduces depolarisation events / metabolic cost) and never reaches the AO3 synthesis. A typical mid-band response that names structures but stops short of explaining cause.

Grade A* model answer (~245 words)

The motor neurone exemplifies the principle that structure dictates conduction velocity. Its long axon — up to a metre in human lower-limb motor neurones — allows direct transmission from the ventral horn of the spinal cord to the target muscle fibre, avoiding intermediate synapses that would impose ~0.5–1 ms delays each. The axon is wrapped by concentric layers of Schwann-cell plasma membrane, the myelin sheath, which functions as a lipid-bilayer electrical insulator: ion exchange across the internodal membrane is essentially abolished, and membrane capacitance is reduced, so local currents flow further before dissipating.

At regular ~1 mm intervals, nodes of Ranvier expose the axon membrane to extracellular fluid. These nodes carry an exceptionally high density of voltage-gated Na⁺ channels (~1,200 µm⁻²), so depolarising current arriving from the previous node rapidly triggers a new action potential. The impulse therefore appears to "jump" from node to node — saltatory conduction — at up to 120 m s⁻¹ in large mammalian axons, around fifty times faster than in unmyelinated axons of comparable diameter.

A large axon diameter complements this strategy by reducing axoplasmic resistance to current flow. The cumulative effect is twofold: rapid transmission essential for reflex protection and coordinated movement, and reduced metabolic cost because only nodal regions, not the entire internode, must restore ion gradients after each impulse. The evolutionary alternative — invertebrate giant axons up to 1 mm wide — achieves comparable speed but at vastly greater anatomical cost.

Examiner commentary: Full 6/6. M1 long axon with delay analysis, M1 myelin and capacitance, M1 nodes and channel density, M1 saltatory conduction quantified, M1 axon diameter, M1 (AO3) evolutionary synthesis comparing myelination with invertebrate giant axons. The quantitative anchors (1 mm internodes, 1,200 µm⁻² channel density, 120 m s⁻¹) and the explicit "structure dictates velocity" framing are the moves that secure the top band.

Common errors and mark-loss patterns

Pedagogical observations — not fabricated statistics:

• Calling myelin "fat" without specifying it is layered Schwann-cell plasma membrane. The lipid bilayer is the relevant insulator, not generic fat.
• Confusing Schwann cells with oligodendrocytes. Schwann cells myelinate PNS axons; oligodendrocytes myelinate CNS axons. Mark schemes mark both correctly named.
• Stating that the impulse literally jumps. The action potential is regenerated at each node by ion movement; what "jumps" is the site of depolarisation, not the impulse as a discrete entity.
• Forgetting where the cell body sits in a sensory neurone. The dorsal root ganglion is off the impulse path, which is anatomically distinctive — examiners frequently test this on labelled diagrams.
• Treating the autonomic nervous system as one thing. Sympathetic and parasympathetic divisions are antagonistic on most targets — name them.

A-Level-depth misconceptions

Subtle errors that distinguish A from A*:

1. Confusing axon diameter with myelin thickness as the cause of speed gains. Both matter, but they act independently — axon diameter reduces axoplasmic resistance; myelin reduces transmembrane capacitance.
2. Treating reflex arcs as "brainless". Even the simplest reflex relays parallel information to the brain via ascending tracts — the brain merely cannot intervene quickly enough to prevent the initial response.
3. Saying "neurones transmit electrical signals" without specifying that the signal is a regenerative event at the membrane, not a passive current flowing along a wire. The action potential is rebuilt at every micrometre of unmyelinated membrane or at every node of myelinated membrane.
4. Confusing efferent and afferent. Mnemonic: Afferent = Arrives at CNS; Efferent = Exits CNS.

Going further

• Kandel, Schwartz & Jessell — Principles of Neural Science (chapters on cellular and molecular neuroscience) — the standard graduate text on neurone structure and ion channels, deployed in undergraduate medicine and neuroscience.
• Nicholls, Martin & Fuchs — From Neuron to Brain — accessible undergraduate progression from single neurones to circuits.
• Pearson / Hodder / Oxford AQA A-Level Biology — chapter on nervous coordination.
• Oxbridge-style interview prompt: "If myelin evolved independently three times in vertebrate lineages, what does that suggest about its selective value? Could you design an experimental test of the conduction-speed hypothesis?"

Extended Worked Example — 9-mark Discuss Question

Question: Compare the structure and function of sensory and motor neurones. Explain how each structure is adapted to its function in the reflex arc.

Indicative content (AO1 = 3, AO2 = 4, AO3 = 2):

Element	AO
Sensory neurone: carries impulses from receptor to CNS (afferent)	AO1
Motor neurone: carries impulses from CNS to effector (efferent)	AO1
Sensory: long dendron + short axon; cell body off to one side (dorsal root ganglion)	AO1
Motor: many short dendrites + long axon; cell body within CNS (ventral horn)	AO1
Both myelinated, both with dense voltage-gated Na⁺ channels at nodes of Ranvier	AO2
Cell body of sensory neurone is off the impulse path — does not delay conduction	AO2
Motor neurone receives convergent input on dendrites; integration at the cell body	AO2
Adaptive value: sensory carries one signal long-distance from periphery; motor integrates many CNS inputs and delivers one signal long-distance to effector	AO3
Synthesis: both architectures support fast long-range conduction (myelination) but the sensory cell-body location and the motor dendritic branching solve different problems	AO3

Grade C model answer (~210 words)

A sensory neurone carries impulses from a receptor to the CNS. It has a long dendron that goes from the receptor toward the cell body, and a short axon that goes from the cell body into the CNS. The cell body is off to one side, in the dorsal root ganglion.

A motor neurone carries impulses from the CNS to an effector like a muscle. It has many short dendrites and one long axon that goes from the CNS to the muscle. The cell body is in the CNS.

Both types of neurone have a myelin sheath made of Schwann cells, with nodes of Ranvier between the Schwann cells. The myelin sheath insulates the axon and the nodes have voltage-gated sodium channels, so the impulse can jump from node to node (saltatory conduction). This makes the impulse travel faster.

The differences in structure suit the function: the sensory neurone only needs to carry one signal from the receptor, so a simple long dendron is enough. The motor neurone receives signals from many other neurones, so it has many dendrites to collect those inputs.

Examiner commentary: M1 sensory function, M1 motor function, M1 sensory structure, M1 motor structure, M1 myelin / nodes, M1 saltatory conduction, M1 architectural difference linked to function. Around 7/9.

Grade A* model answer (~360 words)

Sensory and motor neurones share a common cellular architecture — a cell body with a nucleus and rough ER, a myelinated axon with nodes of Ranvier, and an excitable membrane bearing voltage-gated Na⁺ and K⁺ channels — but they differ in their dendritic, axonal, and cell-body geometries in ways that exactly match their roles in the reflex arc.

Sensory (afferent) neurones carry impulses unidirectionally from a peripheral receptor to the dorsal horn of the spinal cord. The structural signature is a long dendron running from the receptor toward the cell body and a short axon running from the cell body into the CNS. The cell body sits in the dorsal root ganglion, off the main impulse path — the impulse propagates through a T-junction at the cell body. This unusual geometry minimises conduction delay (the impulse does not need to pass through the cell body) and explains why sensory neurones have one of the fastest conduction velocities in the body (Ia afferents reach ~80 m s⁻¹).

Motor (efferent) neurones carry impulses from the ventral horn of the spinal cord to a peripheral effector (skeletal muscle for somatic motor neurones). The structural signature is many short branching dendrites receiving convergent synaptic inputs from thousands of presynaptic cells, and a long axon terminating at the neuromuscular junction. The dendritic tree is the receiver; the cell body integrates inputs at the axon hillock (the trigger zone); the axon delivers the integrated decision to the effector. Up to a metre of myelinated axon in the lower limb is required to reach the target muscle fibre.

Both architectures share myelination and saltatory conduction because both must conduct rapidly over long distances, but the receiver end differs profoundly: sensory neurones have one peripheral terminal (one receptor); motor neurones have an enormous receptive surface (thousands of synapses on the dendritic tree). The architectural differences reflect a single design principle — structure matches function — and the reflex arc threads them together: one fast input line, one fast output line, joined by an integrator (the relay neurone or direct synapse) in the CNS.

Examiner commentary: Full 9/9. The candidate names the dorsal root ganglion T-junction, partitions the dendritic vs axonal asymmetry, quantifies conduction velocity, and synthesises the "structure matches function" principle.

Required practical reference

This lesson underpins Required Practical 10 (factor affecting reflex / response — covered in detail in lesson 7 of this course). Reaction-time measurements rest on the conduction delays introduced by every neurone, synapse, and effector in the reflex chain. Quantitatively, a 200 ms reaction time partitions into ~30 ms sensory-receptor delay, ~50 ms CNS processing, ~50 ms motor conduction, and ~70 ms muscle latency — a useful sanity check for student data.

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Summary

• The nervous system consists of the CNS (brain + spinal cord) and PNS (somatic + autonomic), using three types of neurone: sensory (afferent), relay (interneurone), and motor (efferent).
• Motor neurones have a cell body with dendrites, a long myelinated axon, and synaptic knobs containing many mitochondria and synaptic vesicles.
• The myelin sheath, formed by Schwann cells in the PNS and oligodendrocytes in the CNS, insulates the axon and enables saltatory conduction at nodes of Ranvier.
• Saltatory conduction dramatically increases the speed of impulse transmission (up to ~120 m s⁻¹) and reduces metabolic cost.
• Reflex arcs provide rapid, involuntary responses to stimuli; they are revisited in detail in lesson 5.
• Speed of transmission depends on myelination, axon diameter, and temperature. Demyelinating diseases (MS, Guillain-Barré) demonstrate the functional importance of myelin.

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AQA alignment: This lesson is aligned with the AQA GCE A-Level Biology (7402) specification, Section 3.6.2 — Nervous coordination / neurone structure. For the most accurate and up-to-date information, refer to the official AQA specification document.