Nervous System ICSE Class 8: Nerves, Motor Neuron, Brain, Spinal Cord and Reflex Action

What Nervous System is about

Weightage: CISCE publishes no question paper, marking scheme or chapter weightage for Class VIII Biology: your school sets and marks the exam. Nervous System sits in the Theme: Human Body - Endocrine, Circulatory and Nervous Systems theme of the CISCE Curriculum for Upper Primary Classes (VI-VIII).Source: CISCE Curriculum for Upper Primary Classes (VI-VIII), RDCD 2016

Key concepts

The neuron and the motor neuron

The neuron (nerve cell) is the structural and functional unit of the nervous system. It is specialised to carry nerve impulses. Neurons are among the longest cells in the body.

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

• Cell body (cyton): contains the nucleus and cytoplasm; controls the cell.
• Dendrites: short, branched projections from the cell body; they receive impulses from other neurons and carry them towards the cell body.
• Axon: a single, long fibre that carries impulses away from the cell body.
• Myelin sheath: a fatty, white covering around the axon; it insulates the axon and speeds up the impulse.
• Nodes of Ranvier: small gaps in the myelin sheath at regular intervals.
• Axon terminals (nerve endings): fine branches at the end of the axon that pass the impulse to the next neuron or to a muscle or gland.

Direction of the impulse in a neuron: dendrites → cell body → axon → axon terminals.

The tiny gap between the axon terminal of one neuron and the dendrite of the next is a synapse; the impulse crosses it by means of chemicals.

Worked example

Name the parts of a motor neuron that (i) receive impulses (ii) contain the nucleus (iii) speed up the impulse.

Try it first, then show the solutionSolution

1. (i) Dendrites receive impulses.
2. (ii) Cell body (cyton). (iii) Myelin sheath around the axon.

Answer: (i) Dendrites (ii) Cell body (iii) Myelin sheath

Types of nerves; cranial and spinal nerves

A nerve is a bundle of axons (nerve fibres) wrapped in a protective covering, like a cable of many wires.

Types of nerves (by function):

Nerve	Carries impulses	Example
Sensory (afferent)	from sense organs (receptors) to the CNS	optic nerve from the eye to the brain
Motor (efferent)	from the CNS to effectors (muscles, glands)	nerves to the eye muscles
Mixed	both: has sensory and motor fibres	all spinal nerves

Cranial nerves: nerves that arise from the brain. There are 12 pairs.

Spinal nerves: nerves that arise from the spinal cord. There are 31 pairs. All spinal nerves are mixed nerves.

So there are 12 + 31 = 43 pairs of nerves in all.

Worked example

Define cranial and spinal nerves and give the number of each.

Try it first, then show the solutionSolution

1. Cranial nerves arise from the brain: 12 pairs.
2. Spinal nerves arise from the spinal cord: 31 pairs.

Answer: 12 pairs of cranial nerves; 31 pairs of spinal nerves.

The brain: protection and cerebrum

The brain is the main control centre. It lies inside the cranium (skull).

Protection of the CNS:

• Bones: the cranium (skull) for the brain; the vertebral column for the spinal cord.
• Meninges: three membranes covering the brain and spinal cord.
• Cerebrospinal fluid (CSF): a fluid between the membranes and around the brain; it acts as a shock absorber and supplies nutrients.

Cerebrum (forebrain): the largest part of the brain, divided into left and right cerebral hemispheres by a deep groove. Its surface is folded (ridges and grooves), which increases its area. Its outer layer is grey matter (cell bodies); inside is white matter (myelinated axons).

Functions of the cerebrum:

• thinking, reasoning, intelligence, memory, learning and imagination;
• voluntary actions (actions under our will, such as writing or kicking);
• senses: sight, hearing, smell, taste and touch are interpreted here;
• speech and emotions.

The right hemisphere controls the left side of the body, and the left hemisphere controls the right side.

Worked example

Why is the surface of the cerebrum folded?

Try it first, then show the solutionSolution

1. Folding increases the surface area of the cerebrum.
2. More area means more grey matter (nerve cell bodies) can fit inside the skull, allowing more complex thinking.

Answer: To increase its surface area and so the number of neurons.

Cerebellum, medulla oblongata and spinal cord

Cerebellum (hindbrain): a smaller, wrinkled part at the back of the brain, below the cerebrum.

• Maintains balance and posture.
• Coordinates voluntary muscle movements, making them smooth and accurate (walking, cycling, threading a needle).

Medulla oblongata (hindbrain): the lowest part of the brain; it continues down as the spinal cord.

• Controls involuntary actions: heartbeat, breathing, blood pressure, swallowing, coughing, sneezing and vomiting.
• Damage to the medulla can be fatal, because heartbeat and breathing stop.

Spinal cord: a long, cylindrical cord of nerve tissue running from the medulla down through the vertebral column.

• Carries impulses from the body to the brain and from the brain to the body.
• Is the centre for many reflex actions (spinal reflexes).
• In the spinal cord, grey matter is inside (H-shaped) and white matter outside, the opposite of the cerebrum.

Part	Main functions
Cerebrum	thinking, memory, voluntary actions, senses, speech
Cerebellum	balance, posture, coordination of movement
Medulla oblongata	heartbeat, breathing, swallowing, coughing, sneezing
Spinal cord	conducts impulses to and from brain; reflex centre

Worked example

After an accident a person can think and speak clearly but keeps losing balance and walks unsteadily. Which part of the brain is likely injured?

Try it first, then show the solutionSolution

1. Thinking and speech (cerebrum) are normal.
2. Balance and coordination are controlled by the cerebellum, so it is likely injured.

Answer: Cerebellum

Reflex action

A reflex action is a quick, automatic, involuntary response to a stimulus, which happens without conscious thought. Most are controlled by the spinal cord, so they are very fast and protect the body from harm.

Key terms:

• Stimulus: a change in the surroundings that the body detects (heat, light, a pinprick).
• Receptor: the sense organ or nerve ending that detects the stimulus (skin, eye).
• Impulse: the electrical message carried along neurons.
• Effector: the muscle or gland that carries out the response.
• Response: the action produced (pulling the hand away).

Reflex arc: the pathway of a reflex action:

stimulus → receptor → sensory neuron → spinal cord (relay neuron) → motor neuron → effector (muscle) → response

At the same time, a message goes up the spinal cord to the brain, so you feel the pain after your hand has already moved.

Common examples:

• pulling the hand away from a hot object or a sharp pin;
• the knee jerk when the tendon below the kneecap is tapped;
• blinking when something comes suddenly near the eyes;
• the pupil becoming smaller in bright light;
• sneezing and coughing (controlled by the medulla);
• watering of the mouth when food is placed on the tongue.

Reflex action	Voluntary action
automatic, without thinking	under conscious control
very fast	slower
mostly controlled by spinal cord	controlled by cerebrum
e.g. knee jerk	e.g. writing

Worked example

In the reflex of pulling your hand from a hot iron, name the stimulus, receptor, effector and response.

Try it first, then show the solutionSolution

1. Stimulus: heat of the iron. Receptor: heat receptors in the skin of the finger.
2. Effector: muscles of the arm. Response: the hand is pulled away.

Answer: Heat; skin receptors; arm muscles; hand withdrawn

Nervous System in real life

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

Topics in this chapter

• Motor Neuron, Types of Nerves and the Central Nervous System
• Reflex Action, Reflex Arc and Voluntary Action

Nervous System: key facts

Impulse in a neuron

$\text{dendrite}\to \text{cell body}\to \text{axon}\to \text{axon terminal}$

One-way travel.

Reflex arc

$\text{receptor}\to \text{sensory neuron}\to \text{spinal cord}\to \text{motor neuron}\to \text{effector}$

Stimulus in, response out.

Nerve count

$12\text{ pairs cranial}+31\text{ pairs spinal}=43\text{ pairs}$

Spinal nerves are all mixed.

CNS

$\text{CNS}=\text{brain}+\text{spinal cord}$

Protected by bone, meninges and CSF.

Brain parts

$\text{cerebrum: think};\text{cerebellum: balance};\text{medulla: heartbeat, breathing}$

One-word jobs to remember.

Nervous System: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks Nervous System

The question patterns to expect, and the method that works for each.

Tricks to remember Nervous System

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

• SAME: Sensory Afferent, Motor Efferent

  Sensory nerves are afferent (arrive at the CNS); motor nerves are efferent (exit the CNS).

  Use it when: Directions of sensory and motor nerves.

• 12 and 31: a month's last date

  12 months, 31 days: 12 pairs of cranial nerves, 31 pairs of spinal nerves.

  Use it when: Numbers of nerves.

• Cerebrum = Cerebral = Clever

  Cerebrum thinks; cereBELLum keeps you balanced like a bell-ringer; Medulla keeps the Motor running (heart and lungs).

  Use it when: Functions of the brain parts.

• D-C-A-T: Dogs Chase A Tail

  Dendrite, Cell body, Axon, Terminal: the path of an impulse.

  Use it when: Direction of impulse in a neuron.

• SR-SMER

  Stimulus, Receptor, Sensory neuron, (spinal cord), Motor neuron, Effector, Response.

  Use it when: Order of the reflex arc.

Nervous System: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section B style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 6-mark question split into 1 + 2 + 3 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Nervous System

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

• ✗Mistake: Saying the axon carries impulses towards the cell body

  Dendrites carry impulses towards the cell body; the axon carries them away.

  ✓Do this: D-C-A-T order.

  Usually costs: 1 mark.

• ✗Mistake: Mixing up 12 and 31

  Cranial nerves are 12 pairs; spinal nerves 31 pairs.

  Wrong working

  31 pairs of cranial nerves

  Right working

  12 pairs of cranial nerves; 31 pairs of spinal nerves

  ✓Do this: 12 months, 31 days.

  Usually costs: 1 mark.

• ✗Mistake: Saying reflexes are controlled by the cerebrum

  Most reflexes are controlled by the spinal cord; some (sneezing, coughing) by the medulla.

  ✓Do this: Reflex = short cut, no thinking.

  Usually costs: 1 mark.

• ✗Mistake: Confusing receptor and effector

  The receptor detects the stimulus; the effector (muscle or gland) produces the response.

  ✓Do this: Receptor receives; effector effects (does).

  Usually costs: 1 mark each.

• ✗Mistake: Giving balance to the cerebrum or heartbeat to the cerebellum

  Balance is the cerebellum's job; heartbeat and breathing belong to the medulla.

  ✓Do this: Think, balance, heartbeat: cerebrum, cerebellum, medulla.

  Usually costs: 1 mark.

Practice questions on Nervous System

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

1. Q1 · 1 mark · MCQExam level

   The part of the brain that controls balance is the:
   (a) cerebrum
   (b) cerebellum
   (c) medulla oblongata
   (d) spinal cord

   Hint

   Bell-ringer.

   Answer

   (b)

2. Q2 · 1 mark · Very short answerExam level

   Name the fatty covering of an axon.

   Hint

   White insulation.

   Answer

   Myelin sheath.

3. Q3 · 2 marks · Short answerExam level

   Define (i) stimulus (ii) response.

   Hint

   Change; reaction.

   Answer

   (i) A stimulus is a change in the surroundings that is detected by a receptor. (ii) A response is the action produced by an effector because of the stimulus.

4. Q4 · 2 marks · Short answerExam level

   Give two functions of the spinal cord.

   Hint

   Conduct; reflex.

   Answer

   It conducts impulses to and from the brain, and it acts as the centre for reflex actions.

5. Q5 · 1 mark · Assertion-reasonHarder exam level

   Assertion (A): All spinal nerves are mixed nerves.
   Reason (R): Spinal nerves carry both sensory and motor fibres.
   (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 for (A).
   (d) Both (A) and (R) are true, but (R) is not the correct explanation for (A).

   Hint

   Definition of mixed.

   Answer

   (c)

6. Q6 · 2 marks · Short answerHarder exam level

   How many nerves (not pairs) arise from the brain and spinal cord together?

   Hint

   Double each count.

   Answer

   Cranial: 12 × 2 = 24; spinal: 31 × 2 = 62; total 86 nerves (43 pairs).

7. Q7 · 2 marks · Short answerHarder exam level

   Distinguish between sensory and motor nerves (two points).

   Hint

   Direction; connected to.

   Answer

   Sensory nerves carry impulses from receptors (sense organs) to the CNS; motor nerves carry impulses from the CNS to effectors (muscles and glands).

8. Q8 · 2 marks · Short answerHarder exam level

   Identify the stimulus, receptor, effector and response when the pupil of the eye becomes smaller in bright light.

   Hint

   Light and the iris.

   Answer

   Stimulus: bright light. Receptor: the retina of the eye. Effector: muscles of the iris. Response: the pupil becomes smaller.

9. Q9 · 3 marks · Short answerHarder exam level

   Give three functions of the cerebrum.

   Hint

   Think, act, sense.

   Answer

   Thinking, reasoning and memory; control of voluntary actions; interpreting senses such as sight and hearing (also speech and emotions).

10. Q10 · 2 marks · Short answerHarder exam level

    Why is an injury to the medulla oblongata more dangerous than one to the cerebellum?

    Hint

    Vital functions.

    Answer

    The medulla controls vital involuntary actions such as heartbeat and breathing; damage can stop them and cause death. Cerebellum damage affects balance and coordination but not life-supporting functions.

11. Q11 · 2 marks · Short answerHarder exam level

    An impulse travels 0.9 m from the fingertip to the spinal cord and 0.9 m back at 60 m/s. How long does the reflex path take in milliseconds, and how many times faster is it than a reaction time of 0.21 s for catching a falling ruler?

    Hint

    Total distance ÷ speed.

    Answer

    1.8 m ÷ 60 m/s = 0.03 s = 30 ms; 0.21 ÷ 0.03 = 7 times faster.

12. Q12 · 3 marks · Short answerHOTS / competency

    A student says: 'Blinking is always a reflex.' Is this correct? Explain with two situations.

    Hint

    Can you blink on purpose?

    Answer

    Not always. Blinking when an insect suddenly flies at the eye is a reflex: automatic and protective. But we can also blink on purpose, for example when asked to blink three times; that is a voluntary action controlled by the cerebrum.

13. Q13 · 3 marks · Short answerHOTS / competency

    A person cannot smell or taste food after a head injury, though the nose and tongue are healthy. Suggest which part of the nervous system may be damaged and explain.

    Hint

    Receptors are fine; the messages are not reaching or being interpreted.

    Answer

    The receptors work, so the problem is in the sensory (cranial) nerves carrying impulses from the nose and tongue to the brain, or in the part of the cerebrum that interprets smell and taste. Without impulses reaching the cerebrum or being interpreted, no sensation is felt.

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. 1.What does the CNS consist of?Show answer

   Brain and spinal cord.

2. 2.Which nerves carry impulses to the CNS?Show answer

   Sensory nerves.

3. 3.Number of pairs of cranial nerves?Show answer

   12.

4. 4.Function of the myelin sheath?Show answer

   Insulates the axon and speeds up impulses.

5. 5.Largest part of the brain?Show answer

   Cerebrum.

6. 6.Which part controls breathing and heartbeat?Show answer

   Medulla oblongata.

7. 7.What is an effector?Show answer

   A muscle or gland that carries out the response.

8. 8.Give two examples of reflex actions.Show answer

   Knee jerk; withdrawing hand from a hot object (also blinking, sneezing).

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you draw and label a motor neuron?
• ☐Can you state the function of each type of nerve?
• ☐Can you give the numbers of cranial and spinal nerves?
• ☐Can you name the brain part for any given activity?
• ☐Can you write the reflex arc in order?
• ☐Can you give four differences between reflex and voluntary actions?

Nervous System: frequently asked questions