BiologyCBSE Class 11 · Neural Control and Coordination

Neuron Structure and the Generation and Conduction of a Nerve Impulse

Updated 17 October 2026 · Fact-checked

A neuron has dendrites, a cell body (cyton) and an axon. At rest the membrane is polarised at about -70 mV because of the Na+/K+ pump (3 Na+ out, 2 K+ in) and selective permeability. A stimulus makes the membrane freely permeable to Na+, causing depolarisation (action potential), followed by repolarisation, so the impulse travels along the axon.

Notice the impulse enters at the dendrites, passes the cell body, and travels along the axon, jumping across nodes of Ranvier to the synaptic knobs.

Illustration drawn for QuestionBuddy.

Understand Neuron and Nerve Impulse Conduction

Neuron. The structural and functional unit of the neural system. Three parts: cell body (cyton/soma) with cytoplasm, nucleus and Nissl's granules (RNA-rich, for protein synthesis); dendrites (short fibres branching from the cell body that carry impulses towards the cell body); axon (a long fibre ending in axon terminals with synaptic knobs containing vesicles of neurotransmitter, carrying impulses away from the cell body).

Types by number of processes. Multipolar (one axon, two or more dendrites; cerebral cortex), bipolar (one axon, one dendrite; retina), unipolar (cell body with one axon; embryonic stage). Myelinated axons are enveloped with Schwann cells forming a myelin sheath around the axon; gaps between myelin sheaths are nodes of Ranvier. Non-myelinated fibres are enclosed by a Schwann cell but do not form a myelin sheath (autonomic and somatic neural systems). Myelinated fibres carry impulses faster (by jumping from node to node).

Resting potential. When a neuron is not conducting, the axonal membrane is polarised: it is comparatively more permeable to K⁺ and nearly impermeable to Na⁺ and negatively charged proteins in the axoplasm. The axoplasm has high K⁺ and low Na⁺; the fluid outside has low K⁺ and high Na⁺. The sodium-potassium pump actively transports 3 Na⁺ outwards for 2 K⁺ inwards. Result: outer surface positively charged, inner surface negatively charged; the potential difference is the resting potential, about −70 mV (inside relative to outside).

Action potential (nerve impulse). At a site (A) a stimulus makes the membrane freely permeable to Na⁺; Na⁺ rushes in; the polarity reverses: outer negative, inner positive. This is depolarisation; the electrical potential difference across the membrane is the action potential. At the site, the membrane becomes impermeable to Na⁺ again and permeable to K⁺: K⁺ diffuses out, restoring the resting potential, repolarisation. The next site (B) is then depolarised by current flow from A; this repeats down the axon, so the impulse travels along it. The refractory period follows; the Na⁺/K⁺ pump restores the original ion distribution. The impulse is all-or-none.

Direction. Impulses travel in one direction at synapses; along an axon the wave spreads in both directions from the stimulus site, but only forward is possible because the previous region is in a refractory state.

Graph: membrane potential rising from −70 mV, rapid rise to positive, return to −70 mV.

Draw and label: a neuron with dendrites, cell body, nucleus, axon, myelin sheath, node of Ranvier and axon terminals.

Neuron and Nerve Impulse Conduction: key facts

Resting potential: About -70 mV
inside negative
polarised; K+ leak, Na+ barrier
Pump: $3 N a + o u t : 2 K + in$; Active transport
Action potential: $N a + in (d e p o l a r i s a t i o n) ∣ K + o u t (r e p o l a r i s a t i o n)$; Reverse polarity then recovery
Neuron types: $M u l t i p o l a r ∣ bi p o l a r ∣ u ni p o l a r$; By processes

How to answer Neuron and Nerve Impulse Conduction questions

Questions give a membrane state and ask for ion movement, or ask to compare neurons.

1Name the state: resting, depolarised or repolarised.
2State the ion that moves and its direction.
3Give the polarity of inside and outside.
4Mention the pump to restore distribution.
5For types, count dendrites and axons.

Neuron and Nerve Impulse Conduction 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.

Touching a hot cup

Suppose a person touches a hot cup and an impulse races up the arm.

Explain how the impulse travels along a neuron.

Try it first, then show the solutionSolution

The stimulus makes the membrane at that point permeable to Na+; Na+ enters, reversing polarity.
K+ then leaves, restoring the potential, while the next patch depolarises.
The wave of depolarisation repeats along the axon and faster in myelinated fibres.

Answer: Wave of depolarisation and repolarisation.

Memory hook: Na+ in, K+ out, move on.

Tricks to remember Neuron and Nerve Impulse Conduction

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

3 out, 2 in
Pump: 3 Na+ out, 2 K+ in.
Use it when: Pump
Na in, K out
Depolarise with Na in; repolarise with K out.
Use it when: Action potential
Dendrite = Door in
Dendrites receive impulses.
Use it when: Neuron

Neuron and Nerve Impulse Conduction: solved examples

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

Example 1: Resting potential · 3 marks · Short answerBoard level

Explain how the resting potential of a neuron is maintained.

Try it first, then show the solutionSolution

Membrane is more permeable to K+ and almost impermeable to Na+ and proteins (1 mark).
Na+/K+ pump moves 3 Na+ out and 2 K+ in (1 mark).
Outside positive, inside negative: about -70 mV (1 mark).

Answer: Selective permeability plus pump gives -70 mV.

Example 2: Depolarisation · 3 marks · Short answerBoard level

What happens to the membrane during depolarisation and repolarisation?

Try it first, then show the solutionSolution

Depolarisation: membrane becomes freely permeable to Na+, Na+ enters, polarity reverses (1 mark).
Repolarisation: membrane becomes permeable to K+, K+ leaves, resting potential returns (1 mark).
The pump restores ion distribution (1 mark).

Answer: Na+ in, K+ out, pump restores.

Example 3: Myelination · 3 marks · Short answerHarder board level

Why does an impulse travel faster in a myelinated axon? Name the gaps.

Try it first, then show the solutionSolution

Myelin sheath formed by Schwann cells insulates the axon (1 mark).
Impulse jumps between nodes of Ranvier (1 mark).
Fewer sites need depolarising, so conduction is faster (1 mark).

Answer: Node to node jumping.

Example 4: Neuron types · 3 marks · Short answerHarder board level

Classify the neurons: (a) in the retina (b) in the cerebral cortex (c) in the embryo. Give the number of axons and dendrites for each.

Try it first, then show the solutionSolution

Retina: bipolar, one axon and one dendrite (1 mark).
Cortex: multipolar, one axon and two or more dendrites (1 mark).
Embryo: unipolar, one axon only (1 mark).

Answer: Bipolar, multipolar, unipolar.

Example 5: Pump failure · 5 marks · Long answerHOTS / competency

Suppose ouabain poisons the Na+/K+ pump. Predict what happens over time to the resting potential and to the ability to fire impulses, and why a single impulse still works at first.

Try it first, then show the solutionSolution

Pump stops, so ion gradients run down slowly (1 mark).
Resting potential decreases towards zero (1 mark).
Neuron cannot restore Na+ and K+ gradients after impulses, so firing ceases (1 mark).
At first the existing gradients are enough for a few impulses (1 mark).
Each impulse moves few ions, so the effect is gradual (1 mark).

Answer: Gradients decay gradually; firing fails.

Where students lose marks in Neuron and Nerve Impulse Conduction

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

Mistake: Inside positive at rest
Polarity reversed.
Wrong working
Inside is positive at rest.
Right working
Inside is negative at rest.
Do this: -70 mV.
Usually costs: 1 mark.
Mistake: K+ enters in depolarisation
Ion mixed.
Wrong working
K+ enters during depolarisation.
Right working
Na+ enters; K+ leaves in repolarisation.
Do this: Na in.
Usually costs: 1 mark.
Mistake: Pump moves equal ions
Ratio ignored.
Wrong working
Pump moves 3 K+ and 2 Na+.
Right working
3 Na+ out and 2 K+ in.
Do this: 3 Na out.
Usually costs: 1 mark.

Exam tips

Always give polarity and ion movement.
Quote -70 mV.
Draw the action potential curve.
Name nodes of Ranvier for speed.

Practice questions on Neuron and Nerve Impulse Conduction

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 resting potential of a neuron is about
(a) +70 mV (b) -70 mV (c) 0 mV (d) -170 mV
Hint
Inside negative.
Answer
(b).
Q2 · 1 mark · Assertion-reasonHarder board level
Assertion (A): The Na+/K+ pump contributes to resting potential.
Reason (R): It moves 3 Na+ out for every 2 K+ in.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Net charge.
Answer
(a).
Q3 · 2 marks · Very short answerBoard level
What are Nissl's granules and where are they found?
Hint
Cell body.
Answer
RNA-containing granules in the cell body that help in protein synthesis.
Q4 · 4 marks · Case-basedHarder board level
Read and answer.
Suppose a recording shows the membrane potential rising from -70 mV to +30 mV and returning.
(a) Name the event. (1)
(b) Name the ion for the rise. (1)
(c) Name the ion for the fall. (2)
Hint
Action potential.
Answer
(a) Action potential (depolarisation then repolarisation). (b) Na+ enters. (c) K+ leaves.
Q5 · 3 marks · Short answerHOTS / competency
Suppose a drug blocks voltage gated Na+ channels in a nerve. Predict the effect on impulse generation and explain why it can be used as a local anaesthetic.
Hint
No depolarisation.
Answer
Without Na+ entry depolarisation cannot occur, so no action potential forms and pain signals are not conducted; hence use as a local anaesthetic.

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.What is a dendrite?Show answer
Fibre carrying impulses to the cell body.
2.Resting potential?Show answer
About -70 mV.
3.What forms myelin in PNS?Show answer
Schwann cells.
4.Nissl granules?Show answer
RNA-rich bodies.
5.Pump ratio?Show answer
3 Na+ out, 2 K+ in.
6.Gap in myelin?Show answer
Node of Ranvier.

Neuron and Nerve Impulse Conduction: frequently asked questions

What is a nerve impulse?

A wave of depolarisation along a neuron.

What is polarisation?

Charge difference across the membrane.

What is a bipolar neuron?

One axon and one dendrite.

What does myelin do?

Speeds conduction.

Why one direction?

Refractory region behind prevents backward spread.