Neural Neuron Impulse Synapse

8 MCQs9-step worked example
Source: NCERT Human PhysiologyPYQ coverage: NEET 2022Official key: NTA-verifiedLast updated: 25 Sep 2026

Neural Neuron Impulse Synapse, explained for NEET

Ask which ion the resting axonal membrane favours and most students say Na+, because Na+ is the ion that ends up rushing in when the impulse fires. Backwards: at rest the membrane is comparatively more permeable to K+ and nearly impermeable to Na+, so the axoplasm sits K+-rich, Na+-poor, and the sodium-potassium pump (3 Na+ out for every 2 K+ in) keeps that gradient standing — outer surface positive, inner surface negative: the resting potential (NCERT Class 11 Biology, Chapter 18, page 232-233).

A stimulus at a site makes that patch of membrane freely permeable to Na+. Na+ floods in, the polarity there flips — outer surface goes negative, inner surface goes positive — and that reversal, the action potential, is the nerve impulse. It doesn't teleport to the next patch; current flows from the depolarised site to the next resting site along the inner surface (and the reverse way along the outer surface), which is what depolarises that next site in turn. Get the direction backwards and you'll describe the wrong surface as going positive (page 233).

At the axon's end, the impulse has to cross to the next neuron. Two junction types exist. Electrical synapses let current pass directly between the two membranes — and, despite being the rare type in our system, they are always faster than chemical synapses, not slower, however much more familiar chemical transmission feels (page 234). At a chemical synapse the arriving impulse pushes synaptic vesicles to fuse with the pre-synaptic membrane, releasing neurotransmitter into the synaptic cleft; the neurotransmitter binds post-synaptic receptors, opening ion channels that generate a fresh potential — excitatory or inhibitory — in the next neuron (page 234).

Watch out: a neuron's own wiring (multipolar — one axon, two-plus dendrites, cerebral cortex; bipolar — one axon, one dendrite, retina; unipolar — one axon, usually embryonic) is a separate fact from how it fires, and NEET tests both without warning which one it wants.

Can you answer these Neural Neuron Impulse Synapse MCQs?

Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.

MCQ 1Easy RecallPractice

Which structure of a neuron transmits impulses towards the cell body?

Show answer and why every option is right or wrong

Answer: B. Dendrites are the short, repeatedly-branching fibres that carry impulses towards the cell body (NCERT Class 11 Biology, Chapter 18, page 231).

Why A is wrong: A is wrong because the axon carries impulses away from the cell body, toward a synapse or neuro-muscular junction — the opposite direction.

Why C is wrong: C is wrong because the synaptic knob is the bulb-like terminal of the axon that stores neurotransmitter vesicles; it doesn't conduct impulses toward the cell body.

Why D is wrong: D is wrong because Nissl's granules are granular bodies found in the cell body and dendrites, not a conducting structure.

Report an error · Every fix is public: corrections log

MCQ 2Easy RecallPractice

At rest, the axonal membrane is comparatively more permeable to which ion?

Show answer and why every option is right or wrong

Answer: A. The resting axonal membrane is comparatively more permeable to K+ and nearly impermeable to Na+, keeping the axoplasm K+-rich (NCERT Class 11 Biology, Chapter 18, page 232).

Why B is wrong: B is wrong because Na+ permeability is LOW at rest — it only rises sharply once a stimulus arrives, which is the trap: students associate Na+ with the neuron's electrical behaviour and assume it dominates throughout, including at rest.

Why C is wrong: C is wrong because Ca2+ permeability isn't the resting-state property NCERT describes here — the chapter contrasts K+ and Na+ specifically.

Why D is wrong: D is wrong because Cl- isn't the ion NCERT names for the resting permeability difference; the chapter's contrast is K+ versus Na+.

Report an error · Every fix is public: corrections log

MCQ 3Easy RecallPractice

A neuron has one axon and one dendrite and is found in the retina of the eye. Which type is it?

Show answer and why every option is right or wrong

Answer: D. Bipolar neurons have one axon and one dendrite and are found in the retina (NCERT Class 11 Biology, Chapter 18, page 232).

Why A is wrong: A is wrong because multipolar neurons have one axon and two OR MORE dendrites, and are found in the cerebral cortex, not the retina — swapping this count/location pair is the exact confusion the chapter's neuron-type distinction guards against.

Why B is wrong: B is wrong because unipolar neurons have a cell body with one axon only (no named dendrite) and occur usually in the embryonic stage, not the retina.

Why C is wrong: C is wrong because NCERT names only three types here — multipolar, bipolar, unipolar — and does not use this term for any of them.

Report an error · Every fix is public: corrections log

MCQ 4Direct ApplicationPractice

A stimulus is applied at site A on a resting, polarised axon. Immediately after the stimulus, what happens to the outer and inner surfaces of the membrane at site A?

Show answer and why every option is right or wrong

Answer: C. The stimulus makes site A freely permeable to Na+; the resulting Na+ influx reverses the polarity so the outer surface becomes negative and the inner surface becomes positive — this reversal is the action potential (NCERT Class 11 Biology, Chapter 18, page 233).

Why A is wrong: A is wrong because it describes the UNSTIMULATED resting state, not the site after the stimulus — polarity reverses there, it doesn't hold steady.

Why B is wrong: B is wrong because it exaggerates the resting pattern in the wrong direction instead of reversing it.

Why D is wrong: D is wrong because the membrane doesn't go neutral — it flips to the opposite polarity (negative outside, positive inside), not to zero charge.

Report an error · Every fix is public: corrections log

MCQ 5Direct ApplicationPractice

Comparing the two synapse types for how fast an impulse crosses them, which is true?

Show answer and why every option is right or wrong

Answer: B. Impulse transmission across an electrical synapse is always faster than across a chemical synapse, even though electrical synapses are the rare type in our system (NCERT Class 11 Biology, Chapter 18, page 234).

Why A is wrong: A is wrong because rarity/commonness doesn't determine speed here, and it also gets the fact backwards — electrical synapses are the rare ones, not chemical.

Why C is wrong: C is wrong because chemical synapses use vesicles and electrical synapses do not (current flows directly between membranes); the two mechanisms are not identical, and are not equally fast.

Why D is wrong: D is wrong because direct current flow between membranes describes the ELECTRICAL synapse, not the chemical one — the option swaps which synapse type does the direct-flow mechanism.

Report an error · Every fix is public: corrections log

MCQ 6Direct ApplicationPractice

The sodium-potassium pump transports 3 Na+ out of the cell for every 2 K+ it moves in. What effect does this have on the resting axonal membrane?

Show answer and why every option is right or wrong

Answer: D. Because more positive charge (3 Na+) leaves than enters (2 K+), the outer surface ends up positively charged and the inner surface negatively charged — the polarised resting state (NCERT Class 11 Biology, Chapter 18, page 232-233).

Why A is wrong: A is wrong because the unequal 3-out/2-in exchange creates a net charge difference across the membrane, not equal charge on both sides.

Why B is wrong: B is wrong because it reverses the actual polarity — the pump's net outward movement of positive charge leaves the OUTER surface positive, not the inner surface.

Why C is wrong: C is wrong because the pump changes both ion concentration AND, as a direct consequence of the 3:2 imbalance, the membrane's charge distribution — the two aren't separable here.

Report an error · Every fix is public: corrections log

MCQ 7CalculationPractice

A stimulus depolarises site A on an axon. At the neighbouring resting site B, the outer surface is still positively charged and the inner surface negatively charged. Which sequence of events correctly explains how the impulse reaches site B?

Show answer and why every option is right or wrong

Answer: A. With A depolarised and B still resting, current flows on the inner surface from A to B (and on the outer surface from B to A) to complete the circuit; this reverses the polarity at B, generating a fresh action potential there, and the sequence repeats along the axon (NCERT Class 11 Biology, Chapter 18, page 233).

Why B is wrong: B is wrong because no particle travels down the axon — it's a local current-driven polarity reversal repeated site by site, not a moving object.

Why C is wrong: C is wrong because B's depolarisation is NOT independent or simultaneous — it happens because current flows from the already-depolarised A, and it happens fractionally after A, not at the same instant.

Why D is wrong: D is wrong because neurotransmitter-vesicle release is the mechanism at a chemical SYNAPSE between two neurons, not the mechanism of conduction along a single axon between two sites on the same membrane.

Report an error · Every fix is public: corrections log

MCQ 8Concept TrapPractice

At a chemical synapse, put these events in the order they actually occur once an impulse arrives at the axon terminal.
(1) Neurotransmitter binds post-synaptic receptors
(2) Synaptic vesicles fuse with the pre-synaptic membrane
(3) Ion channels open on the post-synaptic membrane, generating a new potential
(4) Vesicles move toward the pre-synaptic membrane

Show answer and why every option is right or wrong

Answer: C. The arriving impulse moves vesicles toward the membrane (4), they fuse and release neurotransmitter (2), the neurotransmitter binds post-synaptic receptors (1), and that binding opens ion channels, generating the new potential (3) (NCERT Class 11 Biology, Chapter 18, page 234).

Why A is wrong: A is wrong because it has receptor binding happening before the vesicles even fuse and release neurotransmitter — nothing is available to bind yet.

Why B is wrong: B is wrong because it has fusion (releasing neurotransmitter) happening before the vesicles move to the membrane, which reverses the first two steps.

Why D is wrong: D is wrong because it has receptor binding (1) occurring before vesicle fusion and release (2) — there is no neurotransmitter in the cleft to bind until after fusion.

Report an error · Every fix is public: corrections log

Free NEET study resources

Get a structured 30-day study plan and a complete formula booklet — delivered to your inbox instantly.

How do you solve a Neural Neuron Impulse Synapse question? A worked example

  1. 1

    Given

    A resting, polarised axon receives a stimulus at site A. The impulse must propagate to a neighbouring site B and then cross a chemical synapse to a second neuron.

  2. 2

    Required

    Trace, in order, what happens to the membrane at site A, then at site B, and then at the chemical synapse.

  3. 3

    Concept

    A resting axon is polarised because the membrane is comparatively more permeable to K+ and nearly impermeable to Na+, kept that way by the sodium-potassium pump (3 Na+ out, 2 K+ in), giving a positive outer surface and negative inner surface — the resting potential. A stimulus makes the membrane at that site freely permeable to Na+, and impulses cross to another neuron at a synapse, either electrical (direct current flow, always faster) or chemical (vesicle-mediated).

  4. 4

    Formula

    Not applicable — this is a qualitative ion-permeability and polarity-reversal mechanism, not a computed quantity.

  5. 5

    Substitution

    Not applicable (qualitative mechanism).

  6. 6

    Calculation

    At site A: the stimulus opens Na+ permeability, Na+ floods in, and the polarity there reverses — outer surface negative, inner surface positive. This reversal is the action potential (the nerve impulse) at A.
    At site B (still resting, outer positive/inner negative): current flows on the inner surface from A to B, and on the outer surface from B to A, completing the circuit. This reverses B's polarity too, generating a fresh action potential at B — the impulse has been conducted from A to B.
    At the synapse: the impulse arriving at the axon terminal moves synaptic vesicles to the pre-synaptic membrane; they fuse and release neurotransmitter into the synaptic cleft; the neurotransmitter binds post-synaptic receptors, opening ion channels and generating a new (excitatory or inhibitory) potential in the second neuron.

  7. 7

    Final answer

    Site A depolarises first (Na+ influx reverses its polarity) → that reversal drives current flow into site B, depolarising B in turn and conducting the impulse along the axon → at the axon terminal the impulse triggers vesicle fusion, neurotransmitter release, and receptor binding, generating a new potential in the post-synaptic neuron.

  8. 8

    Common trap

    Naming the wrong surface during depolarisation (saying the outer surface stays positive at the stimulated site) and assuming chemical synapses, being the common type, must also be the faster one — both reverse what NCERT actually states.

  9. 9

    Similar NEET-style question

    A stimulus depolarises site P on an axon. At the adjacent resting site Q, is the outer surface positive or negative, and what happens to it next? (Answer: at rest, site Q's outer surface is positive and inner surface negative — the undisturbed polarised state. Because P has just depolarised, current flows on the inner surface from P to Q and on the outer surface from Q to P to complete the circuit; this reverses Q's polarity, so Q's outer surface becomes negative and inner surface becomes positive, generating a fresh action potential at Q — the same mechanism that carried the impulse from A to B in the main example, applied one site further along.)

What to remember before solving Neural Neuron Impulse Synapse questions

Definition

Myelinated and Unmyelinated Axons

There are two types of axons: myelinated and non-myelinated. The myelinated nerve fibres are enveloped with Schwann cells, which form a myelin sheath around the axon; the gaps between two adjacent myelin sheaths are called nodes of Ranvier. Myelinated nerve fibres are found in spinal and cranial nerves. An unmyelinated nerve fibre is enclosed by a Schwann cell that does not form a myelin sheath around the axon, and this type is commonly found in the autonomous and somatic neural systems.

-- NCERT Class 11 Biology, Chapter 18, p. 232
Definition

Resting Potential

As a result of the Na+/K+ pump maintaining ionic gradients, the outer surface of the axonal membrane possesses a positive charge while its inner surface becomes negatively charged, so it is polarised. The electrical potential difference across the resting plasma membrane is called the resting potential.

-- NCERT Class 11 Biology, Chapter 18, p. 233
Definition

Types of Synapses

A nerve impulse passes from one neuron to another through junctions called synapses. A synapse is formed by the membranes of a pre-synaptic neuron and a post-synaptic neuron, which may or may not be separated by a gap called the synaptic cleft. There are two types: at electrical synapses, the pre- and post-synaptic membranes are very close and current flows directly between neurons, with transmission similar to and always faster than chemical synapses; electrical synapses are rare in our system. At a chemical synapse, the membranes are separated by a fluid-filled synaptic cleft, and transmission uses neurotransmitters.

-- NCERT Class 11 Biology, Chapter 18, p. 234
Key Fact

Neuron Structure and Types

A neuron is a microscopic structure composed of three major parts: cell body, dendrites and axon. The cell body contains cytoplasm with typical organelles and granular Nissl's granules; dendrites are short, repeatedly branching fibres containing Nissl's granules that transmit impulses towards the cell body. The axon is a long fibre whose distal branched end terminates in bulb-like synaptic knobs containing synaptic vesicles with neurotransmitters, transmitting impulses away from the cell body to a synapse or neuromuscular junction. Based on axon/dendrite number, neurons are multipolar (one axon, two or more dendrites; cerebral cortex), bipolar (one axon, one dendrite; retina) or unipolar (one axon only; usually embryonic).

-- NCERT Class 11 Biology, Chapter 18, p. 232
Key Fact

Ionic Basis of the Resting Membrane

Neurons are excitable because their membranes are polarised. Different ion channels on the neural membrane are selectively permeable to different ions. When a neuron is not conducting an impulse, i.e., resting, the axonal membrane is comparatively more permeable to potassium ions (K+) and nearly impermeable to sodium ions (Na+), and is impermeable to negatively charged axoplasmic proteins. So the axoplasm has high K+ and negative protein concentration and low Na+, while the outside fluid has low K+ and high Na+, forming a concentration gradient maintained by the Na+/K+ pump, which transports 3 Na+ outward for every 2 K+ inward.

-- NCERT Class 11 Biology, Chapter 18, p. 232
Key Fact

Generation and Conduction of the Action Potential

When a stimulus is applied at a site on the polarised membrane, that site becomes freely permeable to Na+, causing rapid Na+ influx and reversal of polarity: the outer surface becomes negatively charged and the inner side positively charged, i.e., depolarised. The electrical potential difference across the membrane at that site is called the action potential, which is the nerve impulse. Current flow to the next site reverses its polarity too, generating an action potential there, so the impulse is conducted along the axon. The Na+ permeability rise is brief and quickly followed by a rise in K+ permeability, which restores the resting potential.

-- NCERT Class 11 Biology, Chapter 18, p. 233
Key Fact

Transmission at the Chemical Synapse

The axon terminals contain vesicles filled with neurotransmitters. When an impulse (action potential) arrives at the axon terminal, it stimulates movement of the synaptic vesicles towards the membrane, where they fuse with the plasma membrane and release neurotransmitters into the synaptic cleft. The released neurotransmitters bind to specific receptors on the post-synaptic membrane; this binding opens ion channels allowing entry of ions, which can generate a new potential in the post-synaptic neuron. The new potential developed may be either excitatory or inhibitory.

-- NCERT Class 11 Biology, Chapter 18, p. 234

Where do students lose marks on Neural Neuron Impulse Synapse?

These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.

Trap

Resting membrane ion permeability

Category: Similar Terms

Students swap which ion the resting axonal membrane is more permeable to.

When it triggers

A question asks which ion the resting axonal membrane is more permeable to, tempting students to answer Na+ instead of K+.

How to avoid

At rest, the axonal membrane is comparatively more permeable to K+ and nearly impermeable to Na+, giving high intracellular K+ and low intracellular Na+ (C11_BIO_21_FACT_03).

Trap

Chemical synapses assumed faster

Category: Overthinking

Students overthink synaptic transmission and assume chemical synapses, being more common, are also faster than electrical ones.

When it triggers

A question asks which type of synapse transmits an impulse faster, electrical or chemical.

How to avoid

Impulse transmission across an electrical synapse is always faster than across a chemical synapse, even though electrical synapses are rare in our system (C11_BIO_21_DEF_06).

Mistake

Students mix up which neuron type (multipolar, bipolar, unipolar) has which axon/dendrite count and location.

Root cause: term confusion

Correction

Multipolar neurons have one axon and two or more dendrites (cerebral cortex); bipolar neurons have one axon and one dendrite (retina); unipolar neurons have a cell body with one axon only, usually in the embryonic stage (C11_BIO_21_FACT_01).

Wrong option pattern

Swaps the dendrite/axon count or tissue location among multipolar, bipolar and unipolar neurons

More in Human Physiology: 29 exam traps and mistakes · 2 formulas · 1 question pattern from its other lessons.

Neural Neuron Impulse Synapse questions from past NEET papers

1 question from NEET 2022. Answers verified against NTA official keys.

All 71 past-paper questions from Human Physiology →

Sources

NCERT refs: Class 11 Biology Chapter 18, p.232

Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.

Report an error · Every fix is public: corrections log

Test yourself on this topic with real past-paper questions:

Practice this topic →