BiologyISC Class 11 · Neural Control and Coordination

The Neuron: Resting Potential, Action Potential, Conduction and Synaptic Transmission

Updated 17 October 2026 · Fact-checked

A neuron has a cell body, dendrites and an axon. At rest the membrane is polarised at about −70 mV with Na⁺ outside and K⁺ inside, maintained by the Na⁺–K⁺ pump. A stimulus above threshold opens Na⁺ channels, causing depolarisation (action potential) that travels along the axon, in myelinated fibres by saltatory conduction, and is passed across a synapse by a neurotransmitter.

Graph of membrane voltage against time showing resting potential, threshold, rising phase, peak, falling phase and undershoot, plus a real r — Follow the curve from resting potential (−70 mV) through threshold, the rising phase to the peak, and the falling phase back to rest.
Picture: Artwork by Synaptidude at en.wikipedia · CC BY-SA 3.0 · Wikimedia Commons

Notice the impulse runs from dendrites to axon terminals, jumping across the gaps (nodes of Ranvier) in the myelin, then crosses the synapse to the next neuron.

Illustration drawn for QuestionBuddy.

Understand Neuron, Nerve Impulse and Synaptic Transmission

Structure. A neuron has a cell body (cyton) with Nissl's granules, short branched dendrites that receive impulses, and an axon (long, ending in axon terminals with synaptic vesicles). Types by the number of processes: unipolar (one axon; embryonic), bipolar (one axon and one dendrite; retina), multipolar (one axon and two or more dendrites; cerebral cortex), pseudo-unipolar (dorsal root ganglia). Myelinated nerve fibres are enveloped by Schwann cells forming a myelin sheath with gaps called nodes of Ranvier; non-myelinated fibres lack the sheath.

Resting potential. At rest the axonal membrane is more permeable to K⁺ than to Na⁺ and almost impermeable to negatively charged proteins. The Na⁺–K⁺ pump uses ATP to move 3 Na⁺ out and 2 K⁺ in, so the outside is positive and the inside negative: a resting potential of about −70 mV (the membrane is polarised).

Action potential. A stimulus makes the membrane permeable to Na⁺: rapid Na⁺ influx reverses the polarity (depolarisation, up to about +30 mV, interior positive). Then voltage-gated K⁺ channels open, K⁺ moves out and the resting potential is restored (repolarisation). It follows an all-or-none law, and there is a short refractory period.

Conduction. In a non-myelinated fibre the impulse travels continuously from point to point. In a myelinated fibre the impulse jumps from one node of Ranvier to the next (saltatory conduction), which is much faster and uses less energy.

Synapse. The junction between a presynaptic neuron and a postsynaptic neuron (or effector). *Electrical synapses:* gap junctions, very fast but rare in humans. *Chemical synapses:* the impulse causes synaptic vesicles to fuse with the presynaptic membrane and release a neurotransmitter (acetylcholine, dopamine, GABA) into the synaptic cleft (about 20 nm); it binds receptors on the postsynaptic membrane, opening ion channels and generating a new potential (excitatory or inhibitory). Acetylcholinesterase destroys acetylcholine afterwards. Chemical synapses allow only one-way transmission.

Neuron, Nerve Impulse and Synaptic Transmission: key facts

Resting potential: $ab o u t - 70 mV ∣ N a^{+} o u t, K^{+} in$; The Na⁺–K⁺ pump moves 3 Na⁺ out and 2 K⁺ in.
Action potential: $D e p o l a r i s a t i o n : N a^{+} in ∣ R e p o l a r i s a t i o n : K^{+} o u t$; All-or-none; peak about +30 mV.
Conduction: Non-myelinated: continuous | Myelinated: saltatory (node to node); Faster and economical when myelinated.
Chemical synapse: 1.Impulse
2.Ca²⁺
3.vesicle fusion
4.neurotransmitter
5.receptor
6.new potential; One-way transmission.

Neuron, Nerve Impulse and Synaptic Transmission in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Why a pinprick is felt quickly

Suppose a pin pricks the big toe of a person 1.8 m tall and the impulse travels to the brain through a myelinated fibre at 60 m s⁻¹.

Find the time taken and the time if the fibre were non-myelinated at 1 m s⁻¹ (assume the path length is 1.8 m).

Try it first, then show the solutionSolution

Time (myelinated) = 1.8/60 = 0.03 s.
Time (non-myelinated) = 1.8/1 = 1.8 s.
The myelinated fibre is 60 times faster because of saltatory conduction.

Answer: 0.03 s versus 1.8 s.

Memory hook: Myelin makes the impulse jump from node to node.

Tricks to remember Neuron, Nerve Impulse and Synaptic Transmission

Short memory aids. Say each one aloud once, then use it on a question the same day.

Na in, K out
Sodium rushes in during depolarisation; potassium leaves during repolarisation.
Use it when: Action potential questions.
Chemical synapse is one-way
Neurotransmitter is released only by the presynaptic side.
Use it when: Properties of synapses.

Neuron, Nerve Impulse and Synaptic Transmission: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Resting potential · 1 mark · MCQBoard level

The resting potential of a neuron is approximately:
(a) +30 mV
(b) 0 mV
(c) −70 mV
(d) −100 mV

Try it first, then show the solutionSolution

The inside is about 70 mV more negative than the outside.

Answer: (c) −70 mV

Example 2: Neuron structure · 3 marks · Short answerBoard level

Draw (describe) a typical neuron and state the function of dendrites, axon and the myelin sheath.

Try it first, then show the solutionSolution

A neuron has a cell body with Nissl's granules, dendrites and an axon with axon terminals. (1 mark)
Dendrites receive impulses; the axon carries impulses away. (1 mark)
The myelin sheath (formed by Schwann cells, with nodes of Ranvier) insulates the axon and speeds conduction. (1 mark)

Answer: Cell body, dendrites, axon; myelin speeds up conduction.

Example 3: Action potential · 4 marks · Short answerHarder board level

Explain how an action potential is generated and how the resting potential is restored. By how many millivolts does the potential change from rest to the peak?

Try it first, then show the solutionSolution

A stimulus opens voltage-gated Na⁺ channels; Na⁺ enters and depolarises the membrane. (1 mark)
The potential reaches about +30 mV; this is the action potential (a swing of 100 mV from −70 mV). (1 mark)
The Na⁺ channels close and K⁺ channels open: K⁺ leaves and repolarises the membrane. (1 mark)
The Na⁺–K⁺ pump restores the original ion distribution. (1 mark)

Answer: Na⁺ influx then K⁺ efflux; a swing of 100 mV from −70 mV to +30 mV.

Example 4: Synaptic transmission · 4 marks · Short answerHarder board level

Describe the sequence of events at a chemical synapse. Why is transmission one-way, and what is the role of acetylcholinesterase?

Try it first, then show the solutionSolution

The arrival of the impulse causes Ca²⁺ entry and synaptic vesicles fuse with the presynaptic membrane, releasing neurotransmitter into the cleft. (1.5 marks)
The neurotransmitter binds receptors on the postsynaptic membrane, opening ion channels and producing a new potential. (1 mark)
Only the presynaptic terminal has vesicles, and only the postsynaptic membrane has receptors, so transmission is one-way. (0.75 mark)
Acetylcholinesterase breaks down acetylcholine and so ends the signal. (0.75 mark)

Answer: Vesicle fusion, binding, new potential; one-way; enzyme terminates the signal.

Example 5: Conduction speed · 5 marks · Long answerHOTS / competency

Compare the conduction in myelinated and non-myelinated fibres. A reflex requires an impulse to travel 0.9 m along a sensory fibre at 60 m s⁻¹ and 0.9 m along a motor fibre at 60 m s⁻¹, with a synaptic delay of 1 ms at each of two synapses. Calculate the total reaction time.

Try it first, then show the solutionSolution

Myelinated: impulse jumps from node to node (saltatory), faster and energy-saving; non-myelinated: continuous, slower. (2 marks)
Conduction time = (0.9 + 0.9)/60 = 0.03 s = 30 ms. (1.5 marks)
Synaptic delay = 2 × 1 ms = 2 ms. (1 mark)
Total = 30 + 2 = 32 ms. (0.5 mark)

Answer: Saltatory conduction is faster; total time 32 ms.

Where students lose marks in Neuron, Nerve Impulse and Synaptic Transmission

The slips examiners see most often, and what to write instead.

Mistake: Swapping the ions in depolarisation
Saying K⁺ enters.
Do this: Na⁺ enters during depolarisation; K⁺ leaves during repolarisation.
Usually costs: 2 marks.
Mistake: Saying the synapse is two-way
Wrong direction.
Do this: Chemical synapses transmit in one direction.
Usually costs: 1 mark.
Mistake: Forgetting nodes of Ranvier
Giving the sheath without gaps.
Do this: Mention nodes of Ranvier in saltatory conduction.
Usually costs: 1 mark.

Exam tips

Draw a neuron and a synapse with labels.
Draw the action potential graph with phases.
State the ions and the channels at each stage.
Compare myelinated and non-myelinated fibres in a table.

Practice questions on Neuron, Nerve Impulse and Synaptic Transmission

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
The gap between two neurons is called the:
(a) node of Ranvier
(b) synaptic cleft
(c) axon hillock
(d) dendrite
Hint
About 20 nm wide.
Answer
(b) synaptic cleft
Q2 · 2 marks · Short answerBoard level
What is the Na⁺–K⁺ pump and what is its role in a neuron?
Hint
It moves ions against gradients.
Answer
An active transport system using ATP that moves 3 Na⁺ out and 2 K⁺ in; it maintains the resting potential and the ionic gradients.
Q3 · 3 marks · Short answerHarder board level
Name two neurotransmitters and say where acetylcholine acts.
Hint
Recall.
Answer
Acetylcholine, dopamine, GABA (any two). Acetylcholine acts at the neuromuscular junction and in many synapses of the nervous system.
Q4 · 3 marks · Short answerHarder board level
Name the three parts of a chemical synapse and state one feature of each.
Hint
Pre, cleft and post.
Answer
Presynaptic membrane with vesicles holding the neurotransmitter; synaptic cleft (about 20 nm) across which the neurotransmitter diffuses; postsynaptic membrane with receptors that open ion channels when the neurotransmitter binds.
Q5 · 3 marks · Assertion-reasonHarder board level
Assertion (A): Myelinated nerve fibres conduct impulses faster than non-myelinated fibres. Reason (R): The impulse jumps from one node of Ranvier to the next. Choose: (a) both true and R explains A (b) both true, R does not explain A (c) A true, R false (d) A false, R true.
Hint
Saltatory conduction.
Answer
(a) Both true and R explains A.
Q6 · 4 marks · Case-basedHOTS / competency
Case study: Suppose a toxin blocks the enzyme acetylcholinesterase.
(i) State what accumulates in the synaptic cleft. (1 mark)
(ii) State the effect on the postsynaptic cell. (1 mark)
(iii) Name the type of synapse. (1 mark)
(iv) State the normal role of the enzyme. (1 mark)
Hint
Think about the termination of the signal.
Answer
(i) Acetylcholine. (ii) Continuous stimulation (the muscle stays contracted). (iii) A chemical synapse. (iv) It breaks down acetylcholine to end the signal.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Resting potential?Show answer
About −70 mV.
2.Insulating sheath?Show answer
Myelin.
3.Gaps in the sheath?Show answer
Nodes of Ranvier.
4.Enzyme breaking acetylcholine?Show answer
Acetylcholinesterase.

Neuron, Nerve Impulse and Synaptic Transmission: frequently asked questions

What is the all-or-none law?

Once the threshold stimulus is reached, a full action potential occurs; a weaker stimulus produces none.

What is the refractory period?

The short time after an action potential during which the neuron cannot respond to another stimulus.

Why are chemical synapses useful?

They allow integration, one-way flow and modulation of signals.