BiologyICSE Class 8 · Nervous System

Motor Neuron, Types of Nerves, Cranial and Spinal Nerves and the Central Nervous System (ICSE Class 8)

Updated 17 October 2026 · Fact-checked

A motor neuron has a cell body, dendrites, a myelin-covered axon and axon terminals, and carries impulses from the CNS to muscles. Nerves are sensory, motor or mixed; there are 12 pairs of cranial and 31 pairs of spinal nerves. The CNS is the brain (cerebrum for thought and voluntary action, cerebellum for balance, medulla for involuntary actions) and the spinal cord.

Follow the arrow: dendrites to cell body, along the myelinated axon, to axon terminals, then the muscle.

Illustration drawn for QuestionBuddy.

Understand Motor Neuron, Types of Nerves and the Central Nervous System

Neuron. The structural and functional unit of the nervous system, a cell specialised to carry nerve impulses.

Structure of a motor neuron (carries impulses from the CNS to a muscle).

Cell body (cyton): nucleus and cytoplasm.
Dendrites: short branches that receive impulses and pass them towards the cell body.
Axon: one long fibre carrying impulses away from the cell body.
Myelin sheath: a fatty white covering that insulates and speeds up the impulse; gaps are nodes of Ranvier.
Axon terminals: fine endings that pass the impulse on to a muscle or gland.

Direction: dendrites → cell body → axon → axon terminals. The gap between two neurons is a synapse.

Nerves. A nerve is a bundle of axons in a protective covering.

Nerve	Carries impulses	Example
Sensory	from receptors to the CNS	optic nerve
Motor	from the CNS to muscles or glands	nerves to eye muscles
Mixed	both	spinal nerves

Cranial nerves arise from the brain: 12 pairs. Spinal nerves arise from the spinal cord: 31 pairs, all mixed.

Central nervous system (CNS) = brain + spinal cord. Protection: skull and vertebral column, three membranes (meninges) and cerebrospinal fluid that acts as a shock absorber.

Part	Position	Main functions
Cerebrum	largest, forebrain	thinking, memory, voluntary actions, senses, speech
Cerebellum	back, below the cerebrum	balance, posture, smooth coordination of movement
Medulla oblongata	lowest part of the brain	involuntary actions: heartbeat, breathing, swallowing, coughing
Spinal cord	inside the vertebral column	conducts impulses between brain and body; reflex centre

The cerebrum is split into two hemispheres; the right controls the left side of the body and the left controls the right.
In the cerebrum grey matter (cell bodies) is outside; in the spinal cord it is inside.
Damage to the medulla can be fatal.

Flow chart: voluntary action. Stimulus → sense organ → sensory nerve → brain (cerebrum) → motor nerve → muscle → action.

Draw-and-label checklist: motor neuron. Dendrites, cell body with nucleus, axon with myelin sheath and nodes of Ranvier, axon terminals; arrow showing direction of impulse.

Motor Neuron, Types of Nerves and the Central Nervous System: key facts

Motor neuron: Dendrites receive
Cell body has nucleus
Axon carries away
Myelin sheath insulates
Axon terminals pass on; Direction: dendrite to terminal.
Nerves: Sensory to CNS
Motor from CNS
Mixed both
Cranial 12 pairs
Spinal 31 pairs; 43 pairs in all.
Brain parts: Cerebrum thinking, voluntary
Cerebellum balance
Medulla heartbeat, breathing
Spinal cord conduction, reflex; Think, balance, heartbeat.

Sensory	to CNS
Motor	from CNS
Mixed	both
Cranial	12 pairs
Spinal	31 pairs

Cerebrum	thinking, voluntary
Cerebellum	balance
Medulla	heartbeat, breathing
Spinal cord	conduction, reflex

How to answer Motor Neuron, Types of Nerves and the Central Nervous System questions

Tasks include labelling a motor neuron, defining nerves and giving numbers, assigning a function to a brain part, or predicting the effect of damage to a part.

1For a neuron, follow the impulse: dendrites, cell body, axon, terminals.
2For nerves, give direction relative to the CNS (to or from).
3For damage questions, match the lost function to the part: balance (cerebellum), breathing (medulla), thinking (cerebrum).
4Quote the numbers: 12 pairs cranial, 31 pairs spinal.
5Mention protection: skull, vertebral column, meninges, CSF.

Motor Neuron, Types of Nerves and the Central Nervous System 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.

Cycling

Suppose a boy learning to ride a bicycle wobbles at first but, after weeks of practice, rides smoothly while thinking about something else.

Which parts of the brain are working?

Try it first, then show the solutionSolution

The cerebrum decides to ride and controls the voluntary movements of legs and hands.
The cerebellum coordinates muscles and keeps balance and posture.
The medulla keeps heartbeat and breathing going without thought.

Answer: Cerebrum, cerebellum and medulla share the work.

Memory hook: Think, balance, heartbeat.

A helmet and the CNS

Suppose a cyclist falls and the helmet cracks, but she has only a headache.

Explain how the CNS is protected and how the helmet adds to it.

Try it first, then show the solutionSolution

The skull covers the brain; the vertebral column covers the spinal cord.
Three membranes and the cerebrospinal fluid act as shock absorbers.
The helmet is an extra layer that takes the force before it reaches the skull.

Answer: Bone, meninges and cerebrospinal fluid protect the CNS, and the helmet adds another shock-absorbing layer.

Memory hook: Bone, membranes, fluid.

Tricks to remember Motor Neuron, Types of Nerves and the Central Nervous System

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

D-C-A-T
Dendrites, Cell body, Axon, Terminals: the order of the impulse.
Use it when: Neuron questions.
12 and 31
12 pairs of cranial nerves; 31 pairs of spinal nerves.
Use it when: Numbers.
CCM
Cerebrum thinks, Cerebellum balances, Medulla keeps heartbeat.
Use it when: Brain part functions.

Motor Neuron, Types of Nerves and the Central Nervous System: solved examples

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

Example 1: Which part · 1 mark · MCQBoard level

The part of a motor neuron that carries the impulse away from the cell body is the:
(a) dendrite (b) axon (c) synapse (d) node

Try it first, then show the solutionSolution

The axon carries impulses away from the cell body.

Answer: (b).

Example 2: What is a nerve · 2 marks · Short answerBoard level

Define a nerve and say how a mixed nerve differs from a sensory nerve.

Try it first, then show the solutionSolution

A nerve is a bundle of axons (nerve fibres) wrapped in a protective covering (1 mark).
A sensory nerve carries impulses only to the CNS; a mixed nerve has both sensory and motor fibres and carries impulses in both directions (1 mark).

Answer: A bundle of axons; a mixed nerve has sensory and motor fibres.

Example 3: Loss of function · 3 marks · Short answerHarder board level

Name the brain part and the function that would be affected in each case: (a) a person staggers and cannot walk in a straight line; (b) a person forgets and cannot speak clearly; (c) breathing stops.

Try it first, then show the solutionSolution

(a) Cerebellum: balance and coordination (1 mark).
(b) Cerebrum: memory and speech (1 mark).
(c) Medulla oblongata: control of breathing (1 mark).

Answer: (a) Cerebellum. (b) Cerebrum. (c) Medulla oblongata.

Example 4: Why myelin · 3 marks · Short answerHarder board level

Impulses along an axon with a myelin sheath travel faster than those along one without it. Explain, and say what the nodes of Ranvier are. Why is a motor neuron called a long cell?

Try it first, then show the solutionSolution

The sheath is fatty and insulates the axon, which makes the impulse move faster (1 mark).
Nodes of Ranvier are small gaps in the sheath at regular intervals (1 mark).
The axon can be very long, so the neuron is one of the longest cells in the body (1 mark).

Answer: Myelin insulates and speeds up; nodes are gaps; axons can be very long.

Example 5: Two sides · 4 marks · Case-basedHOTS / competency

A man has a head injury on the left side of the cerebrum and cannot move his right arm, though his hearing and heartbeat are normal. (a) Explain why the right arm is affected. (b) Why are heartbeat and breathing unaffected? (c) Name the type of nerve that would carry the command to the arm.

Try it first, then show the solutionSolution

(a) The left hemisphere controls the right side of the body (1 mark).
(b) The medulla, which controls these involuntary actions, is not damaged (1 mark).
(c) A motor nerve carries the command to the muscles (1 mark).
The sensory nerves of hearing are intact, so he still hears (1 mark).

Answer: Left hemisphere controls right side; medulla fine; motor nerve.

Where students lose marks in Motor Neuron, Types of Nerves and the Central Nervous System

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

Mistake: Axon carries towards the cell body
Dendrites and axon are confused.
Do this: Dendrites receive towards the body; the axon carries away.
Usually costs: 1 mark.
Mistake: Mixing 12 and 31
The numbers are learnt without the part they belong to.
Do this: Cranial 12 pairs (brain); spinal 31 pairs (cord).
Usually costs: 1 mark.
Mistake: Balance given to the cerebrum
Cerebrum is thought to run everything.
Do this: Balance belongs to the cerebellum; heartbeat to the medulla.
Usually costs: 1 mark.
Mistake: Calling brain and spinal cord peripheral
CNS and nerves are mixed.
Do this: CNS is brain plus spinal cord; nerves are outside it.
Usually costs: 1 mark.

Exam tips

Impulse order: dendrites, cell body, axon, terminals.
12 pairs cranial, 31 pairs spinal.
Match every lost function to a brain part.
Mention protection: skull, meninges, cerebrospinal fluid.

Practice questions on Motor Neuron, Types of Nerves and the Central Nervous System

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 · Very short answerBoard level
Name the covering of fat around the axon.
Hint
White insulating sheath.
Answer
The myelin sheath.
Q2 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Damage to the medulla oblongata can be fatal.
Reason (R): The medulla controls involuntary actions such as heartbeat and breathing.
(a) (A) is true and (R) is false.
(b) (A) is false and (R) is true.
(c) Both (A) and (R) are true, and (R) is the correct explanation of (A).
(d) Both (A) and (R) are true, but (R) is not the correct explanation of (A).
Hint
What stops if it fails?
Answer
(c).
Q3 · 3 marks · Short answerHarder board level
Classify each as a sensory, motor or mixed nerve: a nerve from the ear to the brain; a nerve from the brain to the muscles of the tongue; a nerve leaving the spinal cord to the leg. Give the reason for each.
Hint
Direction of the impulses.
Answer
The nerve from the ear to the brain is sensory, as it carries impulses to the CNS. The nerve from the brain to the tongue muscles is motor, as it carries impulses from the CNS to an effector. A nerve leaving the spinal cord to the leg is mixed, as all spinal nerves carry both kinds of fibres.
Q4 · 4 marks · Long answerHarder board level
Why is the surface of the cerebrum folded? Describe where grey matter and white matter lie in the cerebrum and in the spinal cord, and what each contains.
Hint
Area and arrangement.
Answer
Folding increases the surface area so that more nerve cells can fit inside the skull. In the cerebrum the grey matter (cell bodies) forms the outer layer and the white matter (myelinated axons) lies inside; in the spinal cord the grey matter is inside and the white matter outside.
Q5 · 3 marks · Case-basedHOTS / competency
Read and answer.
Suppose a man after a stroke can think and speak well, but his hand shakes when he reaches for a glass and he cannot walk straight.
(a) Which part of the brain is likely affected? (1)
(b) Which function of that part is lost? (1)
(c) Why is his thinking unaffected? (1)
Hint
Coordination.
Answer
(a) Cerebellum. (b) Coordination of movement, posture and balance. (c) Thinking is a function of the cerebrum, which is not damaged.

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 neuron?Show answer
The structural and functional unit of the nervous system.
2.Which part controls balance?Show answer
Cerebellum.
3.Which part controls breathing?Show answer
Medulla oblongata.
4.What do the meninges do?Show answer
Cover and protect the brain and spinal cord.
5.Which side of the body does the left hemisphere control?Show answer
The right side.

Motor Neuron, Types of Nerves and the Central Nervous System: frequently asked questions

How detailed are cranial nerves?

Only the definition and number are required.

Is the sensory neuron structure asked?

The syllabus names the structure of a motor neuron.

What are grey and white matter?

Grey matter consists of cell bodies; white matter of myelinated axons.

Where is the reflex arc covered?

In the next topic.