Key Idea: Neural signalling is how the nervous system carries information fast. The one idea the whole topic hangs on is that a message travels two ways: as an electrical signal inside a neuron, and as a chemical signal between neurons. From that single idea everything else follows — how a neuron is built, the resting potential it is held at, the action potential that fires along it, what makes that impulse fast, how it crosses the gap at a synapse, and how all of this builds a fast reflex. Topic 3.5 (C2.2) is a Paper 1 data favourite (read an action-potential trace or compare axon cross-sections) and a regular Paper 2 / Paper 3 topic (describe synaptic release, or outline a reflex arc).
🧠 Neurons & the nervous system
A neuron (nerve cell) carries a one-way nerve impulse: in through the dendrites, into the cell body (which holds the nucleus), along the axon, and out through the axon terminals. A fatty myelin sheath insulates the axon and speeds the impulse, with nodes of Ranvier in the gaps.
A motor neuron: the signal is received by the dendrites, passes through the cell body (with its nucleus), runs along the myelinated axon — insulated by the myelin sheath and broken by the nodes of Ranvier — and leaves at the axon terminals.
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Key Idea: Sensory neurons carry impulses TO the CNS (from receptors). Motor neurons carry impulses FROM the CNS (to effectors — muscles and glands). The central nervous system (CNS) is the brain and spinal cord; the peripheral nervous system (PNS) is all the other nerves.
| Feature | Sensory neuron | Motor neuron |
|---|---|---|
| Carries impulses | to the CNS | from the CNS |
| Starts from | receptors (the senses) | the CNS (brain/spinal cord) |
| Ends at | the CNS | effectors (muscles, glands) |
| Job | detect a stimulus | produce the response |
Sensory → Senses send signals in. Motor → Moves the body by sending signals out to the muscles. The CNS is the Core: brain + spinal cord.
🔋 The resting potential
A neuron that is not firing is held ready at its resting potential: the inside is negative (about −70 mV) compared with the outside. This is set up by the sodium-potassium pump, which actively transports 3 Na⁺ out and 2 K⁺ in per cycle, against their gradients, using ATP from respiration. More positive ions leave than enter, so the inside loses charge — then K⁺ leaks back out, making it more negative still.
| Feature | Sodium-potassium pump | Potassium leak |
|---|---|---|
| Ion movement | Na⁺ out, K⁺ in (3:2) | K⁺ out |
| Gradient | against (uphill) | down (downhill) |
| Energy | uses ATP (active transport) | none (passive) |
| Effect | sets up the gradient | inside more negative |
The pump runs on ATP made by respiration. A neuron starved of oxygen or glucose loses its resting potential: no respiration → no ATP → the pump stops → the ion gradients run down.
⚡ Action potentials
When a stimulus reaches threshold, the membrane depolarises — Na⁺ rushes IN and the inside rises to about +40 mV. Then it repolarises — K⁺ moves OUT and the inside returns to −70 mV. That up-and-down spike is one action potential. Every action potential is the same size (all-or-none); a stronger stimulus fires them more often, not bigger. The impulse regenerates itself region by region and travels one way along the axon.
One action potential: the membrane rests at about −70 mV, depolarises to about +40 mV as Na⁺ rushes IN, then repolarises back down as K⁺ moves OUT — past a dashed threshold line.
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Depolarisation (rising): **Na⁺ ions rush IN**. Voltage-gated **Na⁺ channels open**. Inside becomes **positive** (up to about +40 mV). The **upstroke** of the trace.
Repolarisation (falling): **K⁺ ions move OUT**. Voltage-gated **K⁺ channels open**. Inside becomes **negative** again (back to −70 mV). The **downstroke** of the trace.
Rising line = depolarisation = Na⁺ IN. Falling line = repolarisation = K⁺ OUT. Always link the ion to the voltage change — never just say 'the line goes up'.
🏃 Conduction speed & myelination
An impulse travels faster when the axon is myelinated, uses saltatory conduction, and is wide. The myelin sheath insulates the axon, so depolarisation happens only at the nodes of Ranvier — the impulse jumps from node to node instead of creeping along continuously. A wider axon also conducts faster because it has less internal resistance.
| Axon type | Myelin | Conduction | Speed |
|---|---|---|---|
| Myelinated, wide | present | saltatory (jumps node to node) | fastest |
| Myelinated, thin | present | saltatory | fast |
| Unmyelinated, wide | absent | continuous | slow |
| Unmyelinated, thin | absent | continuous | slowest |
'Saltatory' comes from the Latin for 'to jump' — the impulse leaps from gap to gap. And for diameter, a wide motorway moves traffic faster than a narrow lane: a wide axon moves the impulse faster than a narrow one.
🔗 Synaptic transmission
Neurons do not touch — they meet at a synapse with a tiny gap (the synaptic cleft). The signal crosses as a chemical (a neurotransmitter), not electrically. At the presynaptic membrane an impulse lets Ca²⁺ in, vesicles fuse, and neurotransmitter is released by exocytosis. It diffuses across and binds receptors on the postsynaptic membrane; Na⁺ enters, the membrane depolarises (an EPSP), and a new impulse fires.
At a synapse the impulse reaches the presynaptic neuron; vesicles release neurotransmitter (amber dots) into the cleft; it diffuses across, binds receptors on the postsynaptic neuron and triggers a new impulse.
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| Feature | Presynaptic membrane | Postsynaptic membrane |
|---|---|---|
| Holds | vesicles of neurotransmitter | receptor proteins |
| Does what | releases neurotransmitter (exocytosis) | receives it; binding opens channels |
| Key ion | Ca²⁺ enters → vesicles fuse | Na⁺ enters → depolarisation (EPSP) |
| Role | sender (signal leaves) | receiver (new impulse starts) |
Key Idea: The neurotransmitter crosses by diffusion, which is fast only over a short distance. A narrow cleft therefore gives a short diffusion distance, so transmission is fast — the standard 'suggest an advantage' answer.
🦵 Reflexes & receptors
A reflex is a fast, automatic response that travels a fixed reflex arc: stimulus → receptor → sensory neuron → relay neuron (CNS) → motor neuron → effector → response. It is quick because the signal short-cuts through the spinal cord instead of the brain. The synapses between the neurons sit inside the CNS.
The reflex arc: stimulus → receptor → sensory neuron → relay neuron (CNS) → motor neuron → effector → response, all without waiting for the brain.
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| Receptor type | Stimulus it detects | Everyday example |
|---|---|---|
| Mechanoreceptor | Touch, pressure, texture | Feeling whether food is hard or soft |
| Thermoreceptor | Temperature (hot/cold) | Sensing that a drink is hot |
| Chemoreceptor | Chemicals (taste/smell) | Tasting that food is sweet or salty |
| Photoreceptor | Light | Detecting bright light in the eye |
In a pain reflex, the receptor is a sensory nerve ending, the effector is a muscle, and the synapses sit in the CNS (spinal cord) — never in the skin or the muscle.
✍️ Worked examples
IB-style question — distinguish sensory and motor neurons
Distinguish between the functions of a sensory neuron and a motor neuron, and name the two components of the central nervous system. [3]
Model answer:
A sensory neuron carries impulses from receptors towards the CNS (it brings information in from the senses).
A motor neuron carries impulses from the CNS to effectors — the muscles and glands that produce a response.
The central nervous system is made of the brain and the spinal cord. (Mark 1: sensory → towards CNS. Mark 2: motor → away from CNS. Mark 3: brain AND spinal cord.)
Sensory neurons carry impulses from receptors to the CNS; motor neurons carry impulses from the CNS to effectors. The CNS is the brain and the spinal cord.
IB-style question — outline how ATP establishes the resting potential
Outline how ATP produced in respiration is used to establish the resting potential of a neuron. [2]
How to score both marks:
ATP powers the sodium-potassium pump, which actively transports 3 Na⁺ out and 2 K⁺ in per cycle.
Because the ions are pumped against their concentration gradients (which needs the ATP), more positive ions leave than enter, so the inside becomes negative (≈ −70 mV). (Mark 1: ATP runs the pump / active transport. Mark 2: ions moved against the gradient, leaving the inside negative.)
ATP from respiration powers the sodium-potassium pump, which actively transports Na⁺ out and K⁺ in against their gradients; more positive ions leave than enter, so the inside becomes negative — the resting potential.
IB-style question — explain a point on the trace
On an oscilloscope trace, at point Y the membrane potential is rising steeply from −70 mV towards +40 mV. Explain what is happening to the axon membrane at point Y. [3]
How to score all three marks:
Point Y is on the rising part of the trace, so the membrane is undergoing depolarisation.
Voltage-gated sodium channels open, so Na⁺ ions move INTO the axon.
Because Na⁺ is positive, the inside becomes less negative and then positive, so the membrane potential rises from −70 mV towards +40 mV. (Mark 1: depolarisation. Mark 2: Na⁺ channels open / Na⁺ enters. Mark 3: inside becomes positive, so potential rises.)
Point Y is depolarisation: voltage-gated Na⁺ channels open and Na⁺ moves into the axon, making the inside positive, so the membrane potential rises from −70 mV towards +40 mV.
IB-style question — compare two axon cross-sections
Axon P is narrow with no myelin sheath. Axon Q is wide and myelinated. Deduce which axon conducts a nerve impulse more slowly, and explain your reasoning. [3]
Model answer:
Axon P conducts the impulse more slowly.
Axon P has no myelin, so its impulse travels continuously along the whole membrane instead of jumping between nodes — slower than the saltatory conduction in Q.
Axon P is also narrow, giving it more internal resistance to the flow of charge, which slows the impulse further. (Mark 1: P. Mark 2: P unmyelinated → continuous. Mark 3: P narrow → more resistance.)
Axon P — it is unmyelinated, so it conducts continuously rather than by faster saltatory conduction, and it is narrow, so it has more internal resistance; both make it the slower of the two.
IB-style question — describe neurotransmitter release
Describe how a neurotransmitter is released at the presynaptic membrane when a nerve impulse arrives. [3]
How to score all three marks:
The arriving impulse opens channels, so calcium ions (Ca²⁺) enter the presynaptic neuron.
The calcium causes synaptic vesicles (which hold the neurotransmitter) to fuse with the presynaptic membrane.
The vesicles release the neurotransmitter into the synaptic cleft by exocytosis. (Mark 1: Ca²⁺ enters. Mark 2: vesicles fuse with the membrane. Mark 3: neurotransmitter released by exocytosis into the cleft.)
The impulse causes Ca²⁺ to enter the presynaptic neuron; synaptic vesicles fuse with the presynaptic membrane; they release neurotransmitter into the synaptic cleft by exocytosis.
✅ Quick self-check
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What path does an impulse take through a neuron, and how do sensory and motor neurons differ? Dendrites → cell body → axon → axon terminals (one direction). Sensory neurons carry impulses TO the CNS from receptors; motor neurons carry impulses FROM the CNS to effectors (muscles and glands).
What is the resting potential, and how is it set up? The inside of a resting neuron is about −70 mV (negative). The sodium-potassium pump moves 3 Na⁺ out and 2 K⁺ in against their gradients using ATP, so more positive ions leave than enter; K⁺ leaking back out makes it more negative still.
What happens during depolarisation and repolarisation? Depolarisation: Na⁺ channels open and Na⁺ rushes IN, so the inside rises to about +40 mV. Repolarisation: K⁺ channels open and K⁺ moves OUT, so the inside falls back to about −70 mV.
What does the all-or-none principle mean? An action potential fires fully or not at all. Below threshold nothing fires; at or above it, a fixed-size action potential fires. A stronger stimulus raises the frequency of impulses, not their size.
Why is a myelinated, wide axon fast? Myelin insulates the axon, so depolarisation only happens at the nodes of Ranvier and the impulse jumps node to node (saltatory conduction). A wider axon has less internal resistance, so the impulse moves faster still.
How does a signal cross a synapse? Ca²⁺ enters the presynaptic neuron → vesicles fuse → neurotransmitter is released by exocytosis → it diffuses across the narrow cleft → binds receptors on the postsynaptic membrane → Na⁺ enters → the membrane depolarises (EPSP) → a new impulse may fire.
What is the reflex arc, and where are the synapses? Stimulus → receptor → sensory neuron → relay neuron (in the CNS) → motor neuron → effector → response. The synapses between the neurons sit inside the CNS (grey matter of the spinal cord), which is why a reflex is fast — it bypasses the brain.
The whole spike in one picture: resting (−70 mV) → threshold → depolarisation (Na⁺ in) → peak (+40 mV) → repolarisation (K⁺ out) → resting again. Every action potential is the same size — all-or-none.
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Exam Tips
- Label confusion costs marks: dendrites are the branched RECEIVING end; the axon is the single long fibre that carries the impulse away.
- Sensory = TO the CNS, motor = FROM the CNS — state the direction explicitly to score the 'distinguish' mark. The CNS is the brain AND the spinal cord (both needed).
- For 'how ATP establishes the resting potential', name the PUMP (active transport of Na⁺ out / K⁺ in) AND the result (inside becomes negative because ions move against the gradient).
- On a trace: rising line = depolarisation (Na⁺ in); falling line = repolarisation (K⁺ out). Always tie the ion to the voltage change.
- All-or-none means a stronger stimulus fires MORE action potentials, not BIGGER ones — the spike height never changes.
- Asked where depolarisation occurs on a myelinated neuron? At the nodes of Ranvier — never under the myelin. The slowest axon is the thin, unmyelinated one.
- For 'describe neurotransmitter release [3]', give the ordered chain: Ca²⁺ enters → vesicles fuse → neurotransmitter released by exocytosis into the cleft.
- Presynaptic = vesicles + releases; postsynaptic = receptors + receives. A narrow cleft is an advantage because it gives a SHORT diffusion distance, so transmission is fast.
- In a pain reflex: receptor = sensory nerve ending, effector = a muscle, and the synapses are in the CNS (spinal cord), NOT the skin or muscle.
The whole signalling story end to end: a stimulus is detected by a receptor and routed — neuron → synapse → neuron — through the CNS to an effector that produces the response.
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Key Idea: A neuron carries a one-way impulse (dendrites → cell body → axon → axon terminals), with sensory neurons feeding the CNS and motor neurons driving the effectors. At rest the inside is −70 mV, held there by the Na⁺/K⁺ pump (active transport, ATP from respiration). A stimulus at threshold fires an action potential — Na⁺ in (depolarisation, up to +40 mV) then K⁺ out (repolarisation) — that is all-or-none and regenerates along the axon, fastest where the axon is myelinated and wide (saltatory conduction at the nodes of Ranvier). Between neurons the signal crosses a synapse as a neurotransmitter (Ca²⁺ in → vesicles fuse → exocytosis → bind receptors → Na⁺ in → EPSP). Put together, these pieces build a reflex arc — stimulus → receptor → sensory → relay (CNS) → motor → effector → response — that detects a stimulus and responds fast and automatically.