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This lesson covers the structure and function of neurones and the mechanism of nerve impulse transmission as required by the Edexcel A-Level Biology specification (9BI0), Topic 9 -- Control Systems. You need to understand the types of neurones, the resting potential, and the action potential in detail.
The nervous system is divided into two main parts:
| Division | Components | Function |
|---|---|---|
| Central Nervous System (CNS) | Brain and spinal cord | Processing and integration of information |
| Peripheral Nervous System (PNS) | Sensory and motor neurones | Carries impulses to and from the CNS |
The PNS is further divided into:
The autonomic nervous system has two branches:
There are three main types of neurone:
| Type | Direction of Impulse | Structure | Function |
|---|---|---|---|
| Sensory neurone | Receptor → CNS | Long dendron, cell body in the middle, short axon | Carries impulses from receptors to the CNS |
| Relay neurone (interneurone) | Within the CNS | Short dendrites, short axon | Connects sensory and motor neurones |
| Motor neurone | CNS → Effector | Short dendrites, cell body at one end, long axon | Carries impulses from the CNS to effectors (muscles/glands) |
A typical motor neurone has:
Exam Tip: In diagrams, make sure you can identify all the labelled parts of a motor neurone. The myelin sheath is formed by Schwann cells -- do not describe it simply as a 'covering'. State that Schwann cells wrap around the axon multiple times to form layers of membrane.
When a neurone is not transmitting an impulse, it has a resting potential of approximately -70 mV. The inside of the axon is negatively charged relative to the outside. The membrane is said to be polarised.
This resting potential is maintained by:
The sodium-potassium pump (Na+/K+-ATPase) -- an active transport protein that pumps 3 Na+ ions out of the axon and 2 K+ ions in for every ATP molecule hydrolysed. This creates a net movement of positive charge out of the axon.
Potassium leak channels -- the axon membrane is more permeable to K+ than Na+ at rest. K+ ions diffuse out of the axon down their concentration gradient through these channels, making the inside more negative.
The presence of large organic anions (e.g. proteins) inside the axon, which cannot diffuse out and contribute to the negative charge inside.
| Factor | Effect |
|---|---|
| Na+/K+-ATPase | Pumps 3 Na+ out, 2 K+ in (net loss of positive charge) |
| K+ leak channels | K+ diffuses out (makes inside more negative) |
| Organic anions | Trapped inside; add to negative interior charge |
| Na+ channels | Mostly closed at rest |
Exam Tip: When explaining the resting potential, always state that the Na+/K+ pump is an example of active transport (requires ATP). The pump moves ions against their concentration gradient. K+ leak channels allow passive movement down the concentration gradient.
An action potential is a rapid reversal of the resting potential that travels along the axon as a nerve impulse.
| Stage | Membrane Potential | Events |
|---|---|---|
| Resting state | -70 mV | Na+ channels closed; K+ channels closed; Na+/K+ pump active |
| Depolarisation | -70 mV → -55 mV (threshold) | A stimulus causes some Na+ channels to open; Na+ enters the axon |
| Rapid depolarisation | -55 mV → +40 mV | Voltage-gated Na+ channels open; Na+ floods in; inside becomes positive |
| Repolarisation | +40 mV → -70 mV | Na+ channels close (inactivated); voltage-gated K+ channels open; K+ flows out |
| Hyperpolarisation | Below -70 mV (overshoot) | K+ channels slow to close; slight excess of K+ leaves the axon |
| Restoration | Back to -70 mV | Na+/K+ pump restores the original ion distribution |
The threshold potential is approximately -55 mV. If a stimulus is strong enough to depolarise the membrane to this level, an action potential is triggered. Below this threshold, no action potential occurs.
The all-or-nothing principle states that a neurone either fires a complete action potential or does not fire at all. There is no 'partial' action potential.
How does the body distinguish different stimulus intensities?
Exam Tip: The all-or-nothing principle is a very common exam topic. Remember that the amplitude of each action potential is always the same (+40 mV). The intensity of a stimulus is encoded by frequency of action potentials, not their size.
After an action potential, there is a brief period during which the neurone cannot fire again:
| Period | Duration | Explanation |
|---|---|---|
| Absolute refractory period | ~1 ms | Na+ channels are inactivated and cannot reopen; no action potential is possible regardless of stimulus strength |
| Relative refractory period | ~2-3 ms | Some Na+ channels have reset; a stronger-than-normal stimulus can trigger an action potential |
Importance of the refractory period:
In myelinated neurones, the myelin sheath acts as an electrical insulator. Ion exchange (depolarisation and repolarisation) can only occur at the nodes of Ranvier where the axon membrane is exposed.
The action potential therefore 'jumps' from node to node -- this is called saltatory conduction.
| Feature | Myelinated Neurone | Non-myelinated Neurone |
|---|---|---|
| Speed | Very fast (up to 120 m/s) | Slower (0.5-2 m/s) |
| Mechanism | Saltatory conduction (jumps between nodes) | Continuous conduction along the whole axon |
| Energy use | Less ATP required (fewer ion exchanges) | More ATP required |
| Factor | Effect |
|---|---|
| Myelination | Myelinated axons conduct much faster (saltatory conduction) |
| Axon diameter | Wider axons conduct faster (less resistance to ion flow) |
| Temperature | Higher temperature increases speed (up to a point) -- enzymes and membrane proteins work faster |
Exam Tip: When explaining saltatory conduction, state that the action potential jumps from node to node (not 'gap to gap'). The myelin sheath prevents ion exchange between nodes, so local circuits depolarise the next node instead.
The mechanism by which depolarisation spreads along an axon involves local circuits:
In myelinated neurones, the local circuits extend from one node of Ranvier to the next, greatly increasing the speed of transmission.
This lesson sits in Edexcel 9BI0 Topic 8 — Grey Matter (Coordination, Response and Gene Technology), specifically the sub-strand on the cellular basis of nervous communication. The relevant content statements paraphrase to: describe the structure and function of sensory, relay and motor neurones; explain the resting potential in terms of differential membrane permeability and the Na+/K+ pump; explain the action potential as a sequence of voltage-gated ion-channel events (Na+ influx → K+ efflux); and explain saltatory conduction in myelinated axons (refer to the official Pearson Edexcel 9BI0 specification document for exact wording). The material is examined on Paper 2 — Energy, Exercise and Coordination and reactivates synoptically through Topic 2 (membrane channels), Topic 1 (voltage-gated channels as integral membrane proteins with a selectivity filter and S4 voltage sensor), Topic 5 (the Na+/K+ pump is fuelled by mitochondrial ATP) and Topic 7 (ion-gradient and Nernst-style reasoning).
Question (8 marks): A motor neurone has a resting potential of approximately -70 mV. When stimulated, an action potential is generated.
(a) Explain how the resting potential of -70 mV is established and maintained across the axon membrane. (4)
(b) Describe the sequence of ion-channel events that produces a single action potential, from threshold to the restoration of the resting potential. (4)
Solution with mark scheme:
(a) Step 1 — Na+/K+ pump. The Na+/K+-ATPase pumps 3 Na+ out for every 2 K+ in, hydrolysing one ATP per cycle. The 3:2 stoichiometry produces a net outward flow of positive charge and a steep ionic asymmetry: high [Na+] outside, high [K+] inside.
M1 (AO1) — naming the Na+/K+ pump and its 3:2 stoichiometry. "The pump moves ions" without direction or stoichiometry loses M1.
Step 2 — K+ leak channels. At rest, the membrane is ~25× more permeable to K+ than Na+ because K+ leak channels are open while voltage-gated Na+ channels are closed. K+ therefore diffuses out down its concentration gradient, making the inside more negative.
A1 (AO2) — linking differential permeability (K+ » Na+) to the negative interior.
Step 3 — organic anions. Large impermeant organic anions (proteins, phosphate groups) inside the axon contribute to the negative interior charge.
M1 (AO1) — naming impermeant anions.
Step 4 — equilibrium. -70 mV is reached when the electrical pull of the negative interior on K+ balances K+'s outward concentration gradient (Nernst-style equilibrium).
A1 (AO2) — linking -70 mV to a dynamic balance rather than a fixed property.
(b) Step 1 — depolarisation. At threshold (~-55 mV), voltage-gated Na+ channels open; Na+ floods in down both gradients, depolarising the membrane to +30 to +40 mV.
M1 (AO1) — voltage-gated Na+ opening and Na+ influx.
Step 2 — Na+ inactivation and K+ opening. Voltage-gated Na+ channels inactivate (a separate inactivation gate); slightly later, voltage-gated K+ channels open. K+ leaves, repolarising towards -70 mV.
A1 (AO2) — explicit Na+ inactivation and delayed K+ opening as distinct steps.
Step 3 — hyperpolarisation. K+ channels close slowly, so K+ efflux briefly overshoots to ~-80 mV.
M1 (AO2) — naming hyperpolarisation and explaining it via slow K+ channel closure.
Step 4 — restoration. The Na+/K+ pump and K+ leak channels return the membrane to -70 mV; absolute refractory period ~1 ms.
A1 (AO3) — explicit pump + refractory link rather than "everything goes back to normal".
Total: 8 marks (M2 A2 in (a), M2 A2 in (b)).
Question (6 marks): Explain why an action potential travels along a myelinated mammalian motor neurone faster than along a non-myelinated axon of the same diameter.
Mark scheme decomposition by AO:
| Mark | AO | Awarded for |
|---|---|---|
| 1 | AO1 | Identifying the myelin sheath as an electrical insulator formed by Schwann cells wrapping the axon. |
| 2 | AO1 | Identifying the nodes of Ranvier as gaps where the axon membrane is exposed and ion exchange can occur. |
| 3 | AO2 | Stating that voltage-gated Na+ and K+ channels are concentrated at the nodes, so action potentials only regenerate at nodes (not along internodes). |
| 4 | AO2 | Linking local circuits / passive current spread between nodes to saltatory conduction ("the action potential jumps from node to node"). |
| 5 | AO2 | Contrasting with non-myelinated axons, in which the action potential must regenerate continuously at every point along the membrane (slower because each new patch must be fully depolarised). |
| 6 | AO3 | Synthesis / evaluation — linking the speed advantage to biological function, e.g. "saltatory conduction allows reflex withdrawal in ~50 ms over a metre, which would be too slow without myelination." Equivalent: linking faster speed to lower ATP cost (fewer ions exchanged per AP). |
Total: 6 marks split AO1 = 2, AO2 = 3, AO3 = 1. This is a typical Edexcel "explain why" extended response — the AO3 mark is reserved for the candidate who links mechanism to functional consequence rather than restating saltation.
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