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KS3 Science · Year 8Chapter 6 · The nervous system and reflexes
Year 8 · Chapter 6

The nervous system and reflexes

By the end of this chapter you will be able to

  • Name the parts of the nervous system and say what each does
  • Name the three types of neurone and say what each one carries
  • Describe how a synapse passes a signal from one neurone to the next
  • Trace a reflex arc from receptor to effector, and say why a reflex is quick
  • Measure and calculate a reaction time from ruler drop data

You should already know

  • Cells and their structure (Year 7 Chapter 2)
  • Mean and simple division (Year 8 Chapter 1)

Where this leads

  • Energy transfers in the body and in machines (Year 8 Chapter 11)

National Curriculum for England, Key Stage 3 science, Year 8. Content: 8B8 the human nervous system: receptors, sensory, relay and motor neurones, the brain and spinal cord; the reflex arc as a fast, automatic response; measuring reaction time.

Exercise P  Do you remember?

1
FWhat is the name of the organ that controls the body and is inside the skull? Write the word.
2
FWhat is 8 multiplied by 5? Write the number.
3
FWhat is the name of the organ that the eye uses to detect light? Write the word.
4
FWhat is the name of a protein that speeds up a reaction in a living thing? Write the word.
5
FWhat is 30 divided by 6? Write the number.
6.1

The nervous system

Year 8
8B8: the human nervous system; the brain and spinal cord (central) and the nerves (peripheral)
Video lesson: The nervous systemComing soon
Story. Ruby touches a hot pan and her hand moves away before she even thinks about it. “How did my hand know so quickly?” she asks. Her teacher explains that messages travel along nerves, and the spinal cord and brain are the control centre.

The central nervous system (CNS) is the brain and spinal cord. It receives information and coordinates the response. The peripheral nervous system is the nerves that carry information between the CNS and the rest of the body.

A receptor detects a stimulus, a change in the environment, such as light or heat. An effector is a muscle or gland that produces the response.

brainspinal cordreceptorseffectorssensorymotorcentral nervous system = brain and spinal cord
The central nervous system: brain and spinal cord, with sensory nerves from receptors and motor nerves to effectors.

Key ideas

  • The CNS is the brain and spinal cord.
  • Receptors detect stimuli; effectors (muscles or glands) produce responses.
  • Sensory nerves carry information in; motor nerves carry instructions out.
  • The brain and spinal cord coordinate the response.
Worked example 6.1.1
What is the name of the control centre formed by the brain and spinal cord? Write the words.

Method. Recall the name.

  1. Recall. The brain and spinal cord together are the central nervous system.
    central nervous system
  2. Check. Nerves outside them are the peripheral nervous system.
    peripheral
  3. Write the answer.
    central nervous system

Check. The CNS coordinates the response. ✓

Worked example 6.1.2
What is the name of the part of the body that detects a stimulus? Write the word.

Method. Recall the word.

  1. Recall. A structure that detects a stimulus is a receptor.
    receptor
  2. Check. The receptor sends information to the CNS.
    sends information
  3. Write the answer.
    receptor

Check. Receptors include light receptors in the eye. ✓

Worked example 6.1.3
Name the effector that produces a movement in response. Write the word.

Method. Recall the effector.

  1. Recall. An effector is a muscle or gland that produces a response.
    muscle or gland
  2. Name it. A muscle that moves the hand is an effector.
    muscle
  3. Write the answer.
    muscle

Check. Glands also act as effectors. ✓

Watch out

Watch out: the brain is not the only control centre. The spinal cord alone can control some fast responses.

Exam tip

Exam tip. Name the parts in order: receptor, nerve, CNS, nerve, effector.

Try it (with your teacher, or a grown-up at home). Look at the picture of the central nervous system. Trace the path of a message from a sensor in the skin to a muscle in the arm. Do not test anyone with a sharp object.

Exercise 6A  The nervous system

1
FWhat are the brain and spinal cord together called? Write the words.
2
FWhat are the nerves outside the CNS called? Write the words.
3
FWhich organ is the main control centre of the body? Write the word.
4
FWhich structure detects a stimulus? Write the word.
5
FWhat is the word for a change in the environment that a receptor detects? Write the word.
6
FWhat is the name for the muscle or gland that produces the response? Write the word.
7
FIs the spinal cord part of the central nervous system? Write yes or no.
8
FIs the eye a receptor? Write yes or no.
9
FIs a gland an example of an effector? Write yes or no.
10
FWhich part of the nervous system carries information from the brain to a muscle? Write the words.
11
PSAli says the brain is the only part that controls a response. Is Ali right? Write yes or no.
12
PSWorking scientifically: a pupil wants to show that the eye is a receptor. Name the stimulus the eye detects. Write the word.
13
RExplain why a message from a sensory nerve must reach the CNS before a response is made. Write one sentence.
6.2

Neurones and receptors

Year 8
8B8: sensory, relay and motor neurones; the receptors for sight, hearing, touch and smell
Video lesson: Neurones and receptorsComing soon
Story. Omar hears a car horn and turns his head at once. “Which parts did the signal pass through?” he asks. His teacher says, “The ear detected the sound. A sensory neurone carried the message, and a motor neurone carried the instruction to the muscles in your neck.”

A neurone is a nerve cell that carries electrical signals, called impulses. There are three types, with three jobs.

Receptors are found in sense organs. Each sense has its own receptors.

The table gives the three types of neurone.

NeuroneJob
sensory neuronecarries impulses from a receptor to the central nervous system
relay neuronelies in the brain or spinal cord and links neurones
motor neuronecarries impulses from the central nervous system to an effector

The table gives the sense organs, the stimuli they detect, and the receptors in each.

Sense organStimulusReceptor
eyelightlight receptors in the retina
earsoundhearing receptors in the cochlea
skintouch, pressure, heatreceptors in the skin
nosesmellsreceptors in the nasal cavity

Key ideas

  • A sensory neurone carries impulses from a receptor to the CNS.
  • A relay neurone lies in the CNS and links neurones.
  • A motor neurone carries impulses from the CNS to an effector.
  • Each sense organ has receptors for its own stimulus.
Worked example 6.2.1
Which type of neurone carries impulses from a receptor to the CNS? Write the words.

Method. Recall the type.

  1. Recall. A sensory neurone carries impulses from a receptor to the CNS.
    sensory neurone
  2. Check. The motor neurone carries impulses the other way.
    other way
  3. Write the answer.
    sensory neurone

Check. Sensory neurones are at the start of the pathway. ✓

Worked example 6.2.2
Which neurone lies in the brain or spinal cord and links the neurones? Write the words.

Method. Recall the type.

  1. Recall. A relay neurone lies in the CNS and links the sensory and motor neurones.
    relay neurone
  2. Check. It is found only in the CNS.
    only in the CNS
  3. Write the answer.
    relay neurone

Check. A relay neurone is in the middle of the pathway. ✓

Worked example 6.2.3
Which sense organ detects sound? Write the word.

Method. Recall the sense organ.

  1. Recall. The ear has receptors that detect sound.
    ear
  2. Check. Light is detected by the eye, not the ear.
    not the ear
  3. Write the answer.
    ear

Check. The receptors in the ear are for hearing. ✓

Watch out

Watch out: a neurone is a cell, not an organ. Impulses travel along a neurone, and the neurones of many cells form a nerve.

Exam tip

Exam tip. Name the neurone by its job: sensory (receptor to CNS), relay (inside CNS), motor (CNS to effector).

Try it (with your teacher, or a grown-up at home). Look at the receptors table and say which sense organ you would use to detect a smell. Do not sniff or taste any substance; say the answer from the table.

Exercise 6B  Neurones and receptors

1
FWhich neurone carries impulses from a receptor to the CNS? Write the words.
2
FWhich neurone carries impulses from the CNS to an effector? Write the words.
3
FWhich neurone is found only in the brain and spinal cord? Write the words.
4
FWhich sense organ detects light? Write the word.
5
FWhich sense organ detects sound? Write the word.
6
FWhich sense organ has receptors for smell? Write the word.
7
FWhich sense organ has receptors for touch, pressure and heat? Write the word.
8
FWhat is the name of the nerve cell that carries electrical signals called? Write the word.
9
FIs a sensory neurone found in the eye? Write yes or no.
10
PSRuby says the motor neurone carries impulses to the receptor. Is Ruby right? Write yes or no.
11
PSWorking scientifically: name the type of neurone that is in the middle of the pathway, inside the CNS. Write the words.
12
RExplain why a receptor and a sensory neurone must both work for a stimulus to be noticed. Write one sentence.
13
RChallenge: explain why people with an injured sensory neurone may not notice a touch on that part of the body. Write one sentence.
6.3

Synapses

Year 8
8B8: the synapse, a gap between neurones, and how signals pass across it
Video lesson: SynapsesComing soon
Story. Sam is playing a fast game on a tablet, and his fingers respond to what he sees almost at once. “Is every neurone joined to the next one?” he asks. His teacher says, “No. Between neurones is a tiny gap called a synapse, and a chemical message crosses it.”

A synapse is the gap between two neurones. When an impulse reaches the end of a neurone, it causes chemical messengers to be released. These diffuse across the synapse and bind to the next neurone, which starts a new impulse.

Because the message crosses the synapse by diffusion, the signal is passed on only one way, from one neurone to the next.

first neuronenext neuronesynapsechemical messengers diffuse across the gap
A synapse: a gap between two neurones, with chemical messengers crossing it.

Key ideas

  • A synapse is the gap between two neurones.
  • Chemical messengers diffuse across the synapse.
  • The signal passes on to the next neurone, which starts a new impulse.
  • Signals travel in one direction across a synapse.
Worked example 6.3.1
What is the name of the gap between two neurones? Write the word.

Method. Recall the word.

  1. Recall. The gap between two neurones is a synapse.
    synapse
  2. Check. The message crosses it by chemical messengers.
    chemical messengers
  3. Write the answer.
    synapse

Check. Synapses let signals pass to the next neurone. ✓

Worked example 6.3.2
What crosses the synapse to pass a signal on? Write the words.

Method. Recall the message.

  1. Recall. Chemical messengers diffuse across the synapse.
    chemical messengers
  2. Check. These bind to the next neurone and start a new impulse.
    new impulse
  3. Write the answer.
    chemical messengers

Check. The messengers are released by the first neurone. ✓

Worked example 6.3.3
In which direction does a signal pass across a synapse: one way, or both ways? Write the words.

Method. Recall the direction.

  1. Recall. The signal passes from one neurone to the next.
    one neurone to the next
  2. Decide. So the signal passes one way.
    one way
  3. Write the answer.
    one way

Check. The next neurone starts the next impulse. ✓

Watch out

Watch out: the synapse is a gap, not a wire. Chemical messengers cross the gap; the electrical impulse does not jump across it.

Exam tip

Exam tip. Say “chemical messengers diffuse across the synapse” for the exam answer.

Try it (with your teacher, or a grown-up at home). Look at the synapse picture. Explain in your own words why a signal passes one way. Do not take any drug that affects the nervous system; the picture is enough.

Exercise 6C  Synapses

1
FWhat is the gap between two neurones called? Write the word.
2
FWhat is released at a synapse to pass the signal on? Write the words.
3
FDo chemical messengers diffuse across a synapse? Write yes or no.
4
FDoes a synapse pass a signal in one direction only? Write yes or no.
5
FWhat starts a new impulse in the next neurone? Write the words.
6
FWhich kind of substance is the chemical messenger in the synapse? Write the word: protein, sugar, or messenger.
7
FDoes an electrical impulse jump across the synapse? Write yes or no.
8
FName the process that moves the chemical messengers across the gap. Write the word.
9
PSAli says a synapse is a neurone with a gap in it. Is Ali right? Write yes or no.
10
PSWorking scientifically: name one thing that is passed across a synapse to continue a signal. Write the words.
11
RExplain why a synapse means a signal can pass on only one way. Write one sentence.
12
RChallenge: explain why a drug that blocks a synapse could slow down a reaction. Write one sentence.
6.4

The reflex arc

Year 8
8B8: the reflex arc as a fast, automatic response that does not need conscious thought
Video lesson: The reflex arcComing soon
Story. Ruby touches a hot pan and her hand moves away at once. “I did not decide to do that,” she says. Her teacher says, “That is a reflex. The message goes through the spinal cord, so the response is faster than a thought.”

A reflex is a fast, automatic response to a stimulus. It does not need a conscious decision. The pathway is called a reflex arc:

receptor → sensory neurone → relay neurone (in the spinal cord) → motor neurone → effector.

The relay neurone in the spinal cord connects the sensory and motor neurones directly, so the message does not need to reach the brain for the reflex to happen. The brain may be told afterwards.

receptorsensory neuronerelay neurone(spinal cord)motor neuroneeffector (muscle)stimulus, then a fast, automatic response; the brain is not needed for the reflex itself
The reflex arc: receptor, sensory neurone, relay neurone in the spinal cord, motor neurone and effector.

Key ideas

  • A reflex is fast and automatic.
  • The reflex arc goes receptor, sensory, relay, motor, effector.
  • The relay neurone is in the spinal cord, so the brain is not needed for the reflex.
  • Reflexes protect the body from harm.
Worked example 6.4.1
In a reflex arc, which neurone is in the spinal cord? Write the words.

Method. Recall the position.

  1. Recall. The relay neurone lies in the spinal cord.
    relay neurone
  2. Name it. So the neurone is a relay neurone.
    relay neurone
  3. Write the answer.
    relay neurone

Check. Sensory neurones are not in the spinal cord. ✓

Worked example 6.4.2
Put these in order: motor neurone, receptor, effector, sensory neurone. Write the first word.

Method. Follow the arc.

  1. Order. The arc starts at the receptor, then the sensory neurone.
    receptor
  2. First word. So the first word is receptor.
    receptor
  3. Write the answer.
    receptor

Check. The arc ends at the effector. ✓

Worked example 6.4.3
Why is a reflex faster than a conscious response? Write one word: fewer neurones, or more neurones.

Method. Think about the pathway.

  1. Think. A reflex passes through the spinal cord and not the brain.
    not the brain
  2. Decide. So fewer neurones are involved, and the reflex is faster.
    fewer neurones
  3. Write the answer.
    fewer neurones

Check. The shorter pathway is faster. ✓

Watch out

Watch out: a reflex does not need the brain to make the response, but the brain can still be told about the stimulus afterwards.

Exam tip

Exam tip. Name all five steps of the reflex arc in order, and name the relay neurone in the spinal cord.

Try it (with your teacher, or a grown-up at home). Ask a partner to sit with one leg crossed and tap just below the knee with the edge of your hand, gently. Watch what happens. Do not strike anyone hard; stop if it hurts.

Exercise 6D  The reflex arc

1
FWhat is a fast, automatic response to a stimulus called? Write the word.
2
FWhat is the name of the pathway a reflex takes? Write the words.
3
FIn the reflex arc, which neurone comes straight after the receptor? Write the words.
4
FIn a reflex arc, which neurone goes straight to the effector? Write the words.
5
FIs the brain needed for a reflex to happen? Write yes or no.
6
FName the reflex in which the hand moves away from a hot object. Write the word.
7
FName one other reflex that protects the body. Accept any one: blinking, or the knee jerk.
8
FWhich neurone is found only in the CNS? Write the words.
9
FIs a reflex a conscious response? Write yes or no.
10
PSAli says a reflex always needs the brain to make a decision. Is Ali right? Write yes or no.
11
PSWorking scientifically: put the reflex arc in order from the stimulus to the response. Write the first and last step: receptor and effector.
12
RExplain why a reflex protects the body from harm. Write one sentence.
13
RChallenge: explain why the brain may only become aware of a reflex after the response has happened. Write one sentence.
6.5

Measuring reaction time

Year 8
8B8: measuring reaction time with a ruler drop test; calculating a mean and judging a fair test
Video lesson: Measuring reaction timeComing soon
Story. Ali and Ruby test their reaction times with a ruler. Ruby catches it near the 10 cm mark; Ali catches it at 40 cm. “Who is quicker?” Ali asks. Their teacher says, “The higher the mark, the longer the ruler fell before you caught it. The conversion table turns the distance into a time.”

In a ruler drop test, one partner holds a ruler above the other partner’s open thumb and finger, then lets it go. The catcher closes their fingers as soon as they can. The distance the ruler falls, read from the mark where it is caught, is converted to a reaction time using the table.

Repeat the test several times and find the mean distance. A fair test keeps the same hand, the same starting position and the same instructions.

distancethe caught mark on the ruler gives the distancecatch the ruler as fast as you can
A ruler drop test: the distance the ruler falls before it is caught gives the reaction time.

The conversion table gives the reaction time for each distance caught.

Distance caught (cm)Reaction time (s)
100.14
200.20
300.25
400.29

Key ideas

  • Reaction time is the time between the stimulus and the response.
  • A longer fall means a longer reaction time.
  • Repeat the test and find the mean distance before converting.
  • Keep the hand, the start and the instructions the same for a fair test.
Worked example 6.5.1
The three trial distances are 20 cm, 30 cm and 40 cm. What is the mean distance? Write the number of cm.

Method. Add the distances, then divide by 3.

  1. Add. The total is 20 + 30 + 40 = 90 cm.
    90
  2. Divide. Divide 90 by 3.
    90 / 3 = 30
  3. Write the answer.
    30 cm

Check. 30 x 3 = 90. ✓

Worked example 6.5.2
Using the mean distance of 30 cm and the conversion table, what is the reaction time? Write the number of seconds.

Method. Read the table for 30 cm.

  1. Read. The table gives 0.25 s for a distance of 30 cm.
    0.25
  2. Name it. So the reaction time is 0.25 s.
    0.25
  3. Write the answer.
    0.25 s

Check. The reaction time is the mean of the readings converted. ✓

Worked example 6.5.3
Which distance caught gives the longest reaction time in the table? Write the number of cm.

Method. Find the largest time.

  1. Look. The longest time in the table is 0.29 s.
    0.29
  2. Name it. That is for a distance of 40 cm.
    40
  3. Write the answer.
    40 cm

Check. The longest fall gives the longest time. ✓

Watch out

Watch out: reaction time is only one part of the test. Distractions, a different hand, or a different start position make the test unfair.

Exam tip

Exam tip. Find the mean of the distances first, then read the time from the table. Give the answer in seconds.

Try it (with your teacher, or a grown-up at home). With your partner and a grown-up holding the ruler, do the ruler drop test three times with the same hand. Record the mark each time, find the mean distance, and convert it using the table. Do not hold the ruler above anyone without a grown-up.

Exercise 6E  Measuring reaction time

1
FWhat is the time between a stimulus and a response called? Write the words.
2
FIn a ruler drop test, what is the stimulus? Write the word.
3
FUsing the table, what is the reaction time for a distance of 20 cm? Write the number of seconds.
4
FUsing the table, what is the reaction time for a distance of 10 cm? Write the number of seconds.
5
FUsing the table, which distance gives a reaction time of 0.25 s? Write the number of cm.
6
FThree trials are caught at 10 cm, 20 cm and 30 cm. What is the mean distance, in cm? Write the number.
7
FUsing the table, what is the difference in reaction time between 40 cm and 10 cm? Write the number of seconds.
8
PSRuby says that a longer fall means a shorter reaction time. Is Ruby right? Write yes or no.
9
PSWorking scientifically: name one control variable for a fair ruler drop test. Accept any one: the same hand, or the same starting position.
10
RExplain why a pupil should repeat the ruler test several times before finding the mean. Write one sentence.
11
RChallenge: a pupil catches the ruler at 0 cm, and says the reaction time is zero. Explain why this is not a good result. Write one sentence.

Chapter summary

SkillMethod
Nervous systemCNS (brain and spinal cord) coordinates; peripheral nerves link to the body.
NeuronesSensory: receptor to CNS; relay: inside CNS; motor: CNS to effector.
SynapseGap between neurones; chemical messengers diffuse across it, one way.
Reflex arcReceptor, sensory, relay (spinal cord), motor, effector: fast and automatic.
Reaction timeRuler drop test: mean distance, converted to a time with the table.

Can you…?

  • I can name the parts of the nervous system and say what each does
  • I can name the three types of neurone and say what each carries
  • I can describe how a synapse passes a signal from one neurone to the next
  • I can trace a reflex arc from receptor to effector
  • I can calculate a reaction time from ruler drop data

Key terms

central nervous system
the brain and spinal cord
receptor
a cell or organ that detects a stimulus
effector
a muscle or gland that produces a response
neurone
a nerve cell that carries electrical impulses
synapse
the gap between two neurones
reflex
a fast, automatic response to a stimulus
reflex arc
the pathway a reflex takes, through the spinal cord
reaction time
the time between a stimulus and the response

Exercise 6Z  Check-up

1
What are the brain and spinal cord together called? Write the words.
2
Which neurone carries impulses from a receptor to the CNS? Write the words.
3
Which neurone lies inside the CNS and links the other neurones? Write the words.
4
What is the gap between two neurones called? Write the word.
5
What crosses a synapse to pass a signal on? Write the words.
6
Put these in the reflex arc in order, starting with the receptor: sensory neurone, effector, motor neurone. Write the second item.
7
Is the brain needed for a reflex to happen? Write yes or no.
8
Using the table, what is the reaction time for 30 cm? Write the number of seconds.
9
Three trials are caught at 20 cm, 30 cm and 40 cm. What is the mean distance, in cm? Write the number.
10
Explain why a reflex is faster than a conscious response. Write one sentence.

Total 14 marks. Allow about 14 minutes.

Exercise 6X  Practice test questions

1
FName the type of neurone that carries impulses from the CNS to an effector. Write the words.[1 mark]
2
FName the gap between two neurones. Write the word.[1 mark]
3
FName the part of the reflex arc that is inside the spinal cord. Write the words.[1 mark]
4
FWrite the five steps of the reflex arc in order, starting with the receptor. Write the first and last step.[2 marks]
5
RDescribe what happens at a synapse. Write one sentence.[2 marks]
6
FName the sense organ that detects light. Write the word.[1 mark]
7
FUsing the table, what is the reaction time for 40 cm? Write the number of seconds.[1 mark]
8
PSThree trials are caught at 10 cm, 30 cm and 40 cm. Find the mean distance, in cm. Write the number.[2 marks]
9
RExplain why a reflex is a protection. Write one sentence.[2 marks]
10
PSWhich part of the body detects the stimulus in a ruler drop test: the eye or the hand? Write the word.[1 mark]
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