Synaptic Transmission — AQA A-level Biology

SPEC 3.6.2.1 - NERVE IMPULSES

Nerve Impulses

Resting potential, action potential, all-or-nothing principle, refractory period & myelination

Structure of a Myelinated Motor Neurone

The AQA spec requires you to know the structure of a myelinated motor neurone. Each labelled component has a specific function.

Key Components

Dendrites — receive signals from other neurones; short, branched extensions of the cell body. Cell body (soma) — contains the nucleus; metabolic centre. Axon — carries impulse away from cell body; may be very long (up to 1 m). Myelin sheath — formed by Schwann cells wrapped around the axon; acts as electrical insulator. Nodes of Ranvier — gaps between Schwann cells where the axon is exposed. Synaptic knob — terminal bulb containing synaptic vesicles.

The Resting Potential

Resting Potential

The electrical potential difference across the axon membrane when it is not transmitting an impulse. The inside of the axon is approximately −70 mV relative to the outside. The membrane is said to be polarised.

The resting potential is maintained by two mechanisms acting together:

Na⁺/K⁺ ATPase Pump

Continuously pumps 3 Na⁺ ions out and 2 K⁺ ions in for every cycle, using ATP. This creates:

High [Na⁺] outside the axon

High [K⁺] inside the axon

Net positive charge outside (more positive ions pumped out than in)

Differential Membrane Permeability

At rest, the membrane is more permeable to K⁺ than to Na⁺. K⁺ leaks out through open potassium channels down its concentration gradient. Because K⁺ carries positive charge out, the inside becomes more negative.

The electrochemical gradient and concentration gradient combine to maintain the −70 mV resting potential.

Key Phrase

The spec requires: "differential membrane permeability, electrochemical gradients, and the movement of sodium ions and potassium ions." Use all three phrases when explaining the resting potential.

The Action Potential

Action Potential

A rapid, temporary reversal of the membrane potential caused by a stimulus that exceeds threshold. The inside of the axon briefly becomes positive (approximately +40 mV) before returning to −70 mV. Action potentials obey the all-or-nothing principle.

1

Stimulus reaches threshold — a sufficient stimulus depolarises the membrane to the threshold level. Voltage-gated Na⁺ channels open.

2

Depolarisation — Na⁺ rushes into the axon down its electrochemical gradient. The inside becomes positive (~+40 mV).

3

Repolarisation — Na⁺ channels inactivate. Voltage-gated K⁺ channels open. K⁺ rushes out of the axon, restoring negative charge inside.

4

Hyperpolarisation — K⁺ channels are slow to close; the membrane briefly overshoots to more negative than −70 mV.

5

Resting potential restored — the Na⁺/K⁺ pump restores original ion distribution; the resting potential of −70 mV is re-established.

Action Potential — Voltage Trace

All-or-Nothing Principle

If a stimulus does not reach threshold — no action potential fires. Once threshold is reached, the action potential always fires with the same amplitude. A larger stimulus does NOT produce a larger action potential — only a higher frequency of action potentials.

Propagation Along an Axon

An action potential in one region of the axon membrane triggers an action potential in the next region — this is how the impulse travels along the axon.

1

An action potential fires at one point — the inside of that region becomes positive (+40 mV).

2

Local currents flow from the depolarised region to the adjacent resting region ahead of the action potential, depolarising it to threshold.

3

The adjacent region fires its own action potential — the impulse moves forward.

4

The region behind the action potential is in the refractory period — Na⁺ channels are inactivated and cannot be re-opened. The action potential cannot travel backwards.

Why Impulses Are Unidirectional

The refractory period prevents the impulse from travelling backwards. The region just behind the action potential cannot be re-depolarised because its voltage-gated Na⁺ channels are still inactivated. The only direction the impulse can travel is forward — toward unexcited membrane.

The Refractory Period

Refractory Period

The period following an action potential during which the axon membrane cannot be stimulated to generate another action potential. It occurs because the voltage-gated Na⁺ channels are inactivated and cannot immediately re-open.

The refractory period has three functions that AQA mark schemes require:

Function 1 — Discrete Impulses

The refractory period ensures that each action potential is a separate, discrete event. Without it, a single stimulus could cause continuous depolarisation rather than individual impulses. Each action potential must be followed by full recovery before the next can fire.

Function 2 — Limiting Frequency

The refractory period sets a maximum frequency of action potentials. A neurone cannot fire more frequently than its refractory period allows. Maximum frequency = 1 ÷ refractory period (in seconds) Example: Refractory period = 2 ms = 0.002 s → Max frequency = 1 ÷ 0.002 = 500 Hz

Third Function — Unidirectional Propagation

The refractory period also ensures impulses travel in one direction only. The membrane behind the action potential cannot be re-excited — so the impulse can only propagate forward. Many students miss this third function.

Saltatory Conduction in Myelinated Axons

Saltatory Conduction

The jumping of an action potential from one node of Ranvier to the next along a myelinated axon. "Saltatory" comes from the Latin saltare — to jump. Much faster than continuous conduction along an unmyelinated axon.

Why Saltatory Is Faster

In an unmyelinated axon, ion exchange must occur at every point along the membrane — slow and continuous. In a myelinated axon, the myelin insulates the axon between nodes, so depolarisation only occurs at nodes of Ranvier. The impulse effectively jumps from node to node, dramatically increasing speed. Saltatory conduction also requires less ATP because Na⁺/K⁺ pumps only need to work at nodes.

Factors Affecting Speed of Conduction

FactorEffect on SpeedReason

Myelination↑↑ Greatly increases speedSaltatory conduction — AP jumps from node to node; no ion exchange between nodes; also uses less ATP

Axon diameter (↑)↑ Increases speedWider axon has less internal electrical resistance; the local current spreads further and faster to depolarise the next region

Temperature (↑)↑ Increases speed (up to optimum)Higher temperature increases the rate of ion diffusion across the membrane and increases the activity of ion channels; ions move faster, so depolarisation propagates more rapidly

Temperature — Common Error

Temperature increases conduction speed up to the optimum temperature for the ion channel proteins. Beyond the optimum, proteins denature and speed falls sharply. Never state that higher temperature always increases conduction speed without this caveat.

Calculation — MS 0.2

The spec (MS 0.2) requires calculation of maximum frequency of impulse conduction from the refractory period. If refractory period = 2 ms: Maximum frequency = 1 ÷ 0.002 s = 500 impulses per second (Hz).

Myelinated vs Unmyelinated Axons

FeatureMyelinated AxonUnmyelinated Axon

Myelin sheathPresent (Schwann cells)Absent

Nodes of RanvierPresent (gaps between Schwann cells)Absent

Where AP occursOnly at nodes of RanvierAlong entire membrane

Type of conductionSaltatory (node-to-node jumping)Continuous wave along membrane

SpeedFast (~70–120 m/s for large myelinated)Slow (~0.5–2 m/s)

ATP usageLess (Na⁺/K⁺ pump only at nodes)More (pump along whole axon)

ExamplesMotor neurones, sensory neurones carrying touch and proprioceptionSome autonomic neurones, slow pain fibres

Key Principle

Myelin increases speed AND reduces energy cost. Both are important physiologically — fast conduction for rapid responses, low ATP use for sustained function.

Common Exam Errors

Error 1 — "The impulse IS the movement of ions"

The impulse is an electrochemical wave — a change in membrane potential that propagates along the axon. It is NOT the physical movement of Na⁺ or K⁺ ions along the axon. Ions move across the membrane (in/out), not along it. The membrane potential change travels along the axon.

Error 2 — Confusing depolarisation and repolarisation ions

Depolarisation: Na⁺ moves into the axon (voltage-gated Na⁺ channels open). Repolarisation: K⁺ moves out of the axon (voltage-gated K⁺ channels open). Many students reverse these or say "K⁺ moves in during depolarisation."

Error 3 — Missing refractory period functions

The refractory period has three functions: (1) produces discrete impulses, (2) limits maximum frequency, (3) ensures unidirectional propagation. Mentioning only one or two loses marks on "state two functions" questions. Know all three.

Error 4 — "Myelin speeds up conduction by reducing resistance"

While true, this alone is insufficient. Full credit requires: myelin insulates → depolarisation only occurs at nodes → saltatory conduction → AP jumps node to node → faster. The mechanism word "saltatory" is expected by AQA mark schemes.

DEEP Framework — Extended Answer

Model answer for: "Explain how an action potential is generated and propagated along a myelinated axon." [6 marks]

DEEP Chain

D

Define: An action potential is a rapid, temporary reversal of membrane potential. At rest, the inside of the axon is negative (approximately −70 mV) due to differential membrane permeability and the Na⁺/K⁺ ATPase pump.

E

Explain generation: When a stimulus depolarises the membrane to threshold, voltage-gated Na⁺ channels open. Na⁺ rushes into the axon down its electrochemical gradient — the inside becomes positive (+40 mV). Na⁺ channels then inactivate; K⁺ channels open and K⁺ flows out, repolarising the membrane. K⁺ channels are slow to close, causing a brief hyperpolarisation; they then close and the resting potential of −70 mV is restored.

E

Explain propagation (myelinated): In a myelinated axon, the myelin sheath (formed by Schwann cells) insulates the axon between nodes of Ranvier. Depolarisation can only occur at nodes. The local current from a depolarised node spreads to the next node, depolarising it to threshold and triggering an AP there. This is saltatory conduction — the AP jumps node to node. The region behind each node is in the refractory period and cannot be re-excited, ensuring unidirectional propagation.

P

Practical significance: Saltatory conduction in myelinated axons is dramatically faster than continuous conduction in unmyelinated axons (up to 120 m/s vs ~1 m/s) and uses less ATP (Na⁺/K⁺ pump only works at nodes). This explains why disorders affecting myelin (e.g. multiple sclerosis) slow nerve conduction and impair motor and sensory function.

Practice Question

A neurophysiologist records action potentials from two axons: Axon A is myelinated with a diameter of 12 μm; Axon B is unmyelinated with a diameter of 1 μm. The refractory period of Axon A is 1 ms.

(a) Calculate the maximum frequency of action potentials that Axon A can transmit. Show your working and state the unit. [2 marks]

(b) Explain why Axon A conducts impulses faster than Axon B. You must refer to the myelin sheath, nodes of Ranvier, and saltatory conduction. [4 marks]

(c) State two functions of the refractory period. [2 marks]

a:

Maximum frequency = 1 ÷ refractory period = 1 ÷ 0.001 s = 1000 Hz (1000 impulses per second).

b:

Axon A has a myelin sheath formed by Schwann cells, which acts as an electrical insulator. Depolarisation (and therefore action potentials) can only occur at the nodes of Ranvier — gaps between Schwann cells where the axon membrane is exposed. The local current from one depolarised node spreads directly to the next node, depolarising it to threshold. This is saltatory conduction — the action potential jumps from node to node. Because ion exchange does not occur along the myelinated internodal segments, conduction is much faster than in Axon B where depolarisation must occur continuously along the entire membrane.

c:

(1) The refractory period ensures that action potentials are discrete, separate impulses — preventing continuous depolarisation. (2) It limits the maximum frequency of action potentials that can be transmitted. [Also acceptable: ensures unidirectional propagation of the impulse, because the membrane behind the action potential cannot be re-excited.]

Synoptic Links

Synoptic Link — Paper 3

Na⁺/K⁺ ATPase pump → Spec 3.2 (Cell membranes & transport): The Na⁺/K⁺ pump is an active transport mechanism using ATP hydrolysis. A synoptic question could ask you to explain the resting potential in terms of both active transport (the pump) and facilitated diffusion (K⁺ leakage through open channels).

Synoptic Link — Paper 3

Generator potential → this lesson: In Lesson 6.02 you learned that the generator potential in a Pacinian corpuscle, if it reaches threshold, triggers an action potential in the sensory neurone. That action potential is exactly the mechanism described in this lesson. The generator potential is the graded precursor; the action potential is the all-or-nothing propagated signal.

Synoptic Link — Paper 3

Myelination and multiple sclerosis (MS): MS is an autoimmune disease in which the myelin sheath is progressively destroyed. This disrupts saltatory conduction — action potentials can no longer jump between nodes. Signal transmission becomes slow or fails entirely. A comprehension or extended writing question could ask you to explain the symptoms of MS in terms of the nerve impulse mechanism.

Summary

Key Concepts — Lesson 6.04

ConceptKey Point

Resting potential~−70 mV inside; maintained by Na⁺/K⁺ pump (3 Na⁺ out, 2 K⁺ in) and differential permeability (K⁺ leaks out)

Action potentialStimulus → threshold → voltage-gated Na⁺ channels open → Na⁺ in → depolarisation (+40 mV) → Na⁺ channels close; K⁺ channels open → K⁺ out → repolarisation

All-or-nothingAP either fires at full amplitude (if threshold reached) or not at all; intensity coded by frequency

Refractory periodNa⁺ channels inactivated: (1) produces discrete impulses, (2) limits max frequency, (3) ensures unidirectional propagation

Saltatory conductionMyelinated axon: AP jumps node to node; faster; uses less ATP than continuous conduction

Conduction speed factors↑ myelination, ↑ axon diameter, ↑ temperature (to optimum) all increase speed

Max frequency calculationMax frequency (Hz) = 1 ÷ refractory period (s). e.g. 2 ms → 1 ÷ 0.002 = 500 Hz

Top Exam Tip

For any action potential question, state the ion, the direction of movement (in or out), the type of channel (voltage-gated), and the consequence for membrane potential. The examiner wants a mechanistic chain: channel type → ion movement direction → membrane potential change. For saltatory conduction, always use the words "saltatory" and "node of Ranvier" — they are mark-scheme keywords.