Chapter 1 of 8

All 8 chapters
  1. 1The nervous system, readunderstood
  2. 2Reflexes, readunderstood
  3. 3The eye, readunderstood
  4. 4Hormones, readunderstood
  5. 5Blood sugar and diabetes, readunderstood
  6. 6Temperature and water balance, readunderstood
  7. 7Hormones and the menstrual cycle, readunderstood
  8. 8Plant responses, readunderstood

S4.5.1 Chapter 1

The nervous system

You touch something hot. How does the news get to your brain, and how does your hand get the order to move?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Trace a message from a receptor through the brain to a muscle
  • Name the main parts of the brain and their jobs

Put a finger on this page. At once you know that it is smooth, or rough, or cool. The nervous system does this. It senses what is around you, sends the news to a control centre, and sends orders back to your muscles. The control centre is the central nervous system: the brain and the spinal cord. Nerves join it to the rest of the body.

The head and body from the front with three parts labelled: brain, spinal cord and nerves. The brain is in the head, the spinal cord runs down the middle of the body, and nerves branch from the cord to both sides of the body.brainnervesspinalcord
The brain and the spinal cord make up the central nervous system. Nerves run from them to every part of the body.

A change around you, such as heat, light or pressure, is a stimulus. A receptor is a cell that detects a stimulus. Receptors are in your skin, your eyes, your ears, your tongue and your nose.

A receptor starts an impulse. An impulse is a tiny electrical signal that runs along a nerve cell. Nerve cells carry it to the central nervous system. There the brain or the spinal cord decides what to do, and an impulse goes out to a muscle or a gland. A muscle or a gland that does the job is an effector.

Draw it first

The path of a message in three steps

  1. Three pictures of three boxes joined by arrows, one after another. First the boxes are labelled stimulus, receptor and sensory neurone. Then they are labelled sensory neurone, brain or spinal cord and motor neurone. Last they are labelled motor neurone, effector and response.stimulusreceptorsensory neurone

    1 A stimulus acts on a receptor. The receptor starts an impulse in a sensory neurone.

  2. Three pictures of three boxes joined by arrows, one after another. First the boxes are labelled stimulus, receptor and sensory neurone. Then they are labelled sensory neurone, brain or spinal cord and motor neurone. Last they are labelled motor neurone, effector and response.sensory neuronebrain or spinal cordmotor neurone

    2 The sensory neurone carries the impulse to the brain or spinal cord. There it passes to a motor neurone.

  3. Three pictures of three boxes joined by arrows, one after another. First the boxes are labelled stimulus, receptor and sensory neurone. Then they are labelled sensory neurone, brain or spinal cord and motor neurone. Last they are labelled motor neurone, effector and response.motor neuroneeffectorresponse

    3 The motor neurone carries the impulse to an effector, which makes a response.

Six boxes joined by arrows that point down. The boxes are labelled stimulus (a change), receptor, sensory neurone, central nervous system (brain or spinal cord), motor neurone and effector (muscle or gland).stimulus (a change)receptorsensory neuronecentral nervous system (brainor spinal cord)motor neuroneeffector (muscle or gland)
The whole path in one picture. The response of a muscle is to move. The response of a gland is to release a fluid, such as sweat.

A neurone is a nerve cell. Neurones are the longest cells in the body. One can reach from your spine to your toe. A nerve is a bundle of many long neurone fibres, held together like wires in a cable.

Look at a motor neurone. The dendrites at the top receive the impulse. The cell body holds the nucleus. The axon is a long thin fibre that carries the impulse away. Fatty sheaths wrap the axon in pieces. They work like the plastic round a wire, and they let an impulse travel much faster. At the end, nerve endings pass the impulse on.

A motor neurone, standing up. The labels are dendrites, cell body, axon, nucleus, fatty sheath (myelin), direction of impulse and nerve endings. An arrow beside the axon points down, from the dendrites to the nerve endings.dendritescell bodyaxonnucleusfattysheath(myelin)directionofimpulsenerveendings
A motor neurone. The cell body is at the end where the impulse starts. The impulse runs down the long axon to the nerve endings.

The body has three kinds of neurone. A sensory neurone carries impulses from a receptor to the central nervous system. A relay neurone is short. It joins neurones inside the brain or spinal cord. A motor neurone carries impulses from the central nervous system to an effector.

A sensory neurone, standing up. The labels are dendrites, axon, direction of impulse, nucleus, cell body, fatty sheath (myelin) and nerve endings. The cell body sits on a short branch to one side of the long fibre.dendritesdirectionofimpulseaxonnucleuscell bodyfattysheath(myelin)nerveendings
A sensory neurone. Its cell body sits on a short branch to one side. The impulse passes it and goes on along the fibre towards the spinal cord.
A relay neurone, standing up. The labels are dendrites, cell body, axon, nucleus, fatty sheath (myelin), direction of impulse and nerve endings. It is shorter than the motor neurone.dendritescell bodyaxonnucleusdirectionofimpulsefattysheath(myelin)nerveendings
A relay neurone is short. It lies inside the spinal cord or brain and joins a sensory neurone to a motor neurone.

How much does the fatty sheath matter? Suppose a message must travel 1.2 m. Look at the table, then work out the time for the neurone with the sheath. The speeds are round numbers. Real speeds differ between neurones.

A table with the columns Fibre, Speed and Length. The row sheath has speed 100 m/s and length 1.2 m. The row no sheath has speed 1 m/s and length 1.2 m.FibreSpeedLengthsheath100 m/s1.2 mno sheath1 m/s1.2 m
Two fibres of the same length. The one with a fatty sheath carries the impulse much faster.

Worked example

How long does an impulse take to travel 1.2 m along a fibre with a fatty sheath, at 100 m/s?

  1. Time = distance ÷ speed.
  2. Distance is 1.2 m and speed is 100 m/s, so time = 1.2 ÷ 100 = 0.012 s.
  3. One second is 1 000 ms, so 0.012 s = 12 ms.

Answer: 12 ms

Two neurones in a row do not touch. A tiny gap lies between them, called a synapse. An impulse cannot cross the gap as electricity. Instead the first neurone releases a chemical from small bags called vesicles. The chemical spreads across the gap and fits receptor sites on the next neurone. These are tiny parts of the cell's surface, not the receptor cells of the senses. That starts a new impulse.

The end of one neurone above a gap, with the next neurone below. The labels are first neurone, vesicle, gap, impulse, chemical, receptor site and next neurone. Dots between the two neurones are the chemical crossing the gap.firstneuronevesiclegapimpulsechemicalreceptorsitenextneurone
A synapse. Small bags called vesicles hold the chemical. The gap is drawn far wider than the real one, which is too narrow to see with a school microscope. The chemical crosses it in a tiny fraction of a second.

The chemical is released on one side only, and only the next neurone has receptor sites for it. So a message at a synapse can move in one direction. This keeps the path in order: receptor, then brain, then muscle.

Now the brain. It has three parts that you should know. The cerebrum is the large, folded top part. It is used for thinking, memory and speech, and it receives what the senses report. The cerebellum is at the back, below the cerebrum. It helps with balance and with smooth movement. The medulla joins the brain to the spinal cord. It controls the heart rate and breathing, which carry on without your thinking about them.

The brain from the side, with the spinal cord leaving below. The labels are cerebrum: thinking, memory, speech; cerebellum: balance and movement; medulla: heart rate and breathing; and spinal cord.cerebrum:thinking, memory,speechcerebellum:balance andmovementmedulla: heartrate and breathingspinal cord
The brain from the side. These are three main parts. The brain has more parts than this, and they work together.

Try it

Which parts of the skin have the most receptors?

You need: two pencils with blunt ends, or two blunt sticks, a ruler marked in millimetres, a friend, paper and a pencil to record results.

Be safe: Use blunt ends and touch lightly. Never use a pin or anything sharp.

  1. Close your eyes. Your friend touches your fingertip lightly with one pencil point, or with two points 50 mm apart. They choose one or two at random. You say "one" or "two".
  2. Try again with the points 40 mm apart, then 30, 20, 10, 5, 3 and 2 mm. Mix in single touches. Write down the smallest gap at which you were right every time.
  3. Repeat on your palm and on the back of your forearm.
  4. Swap places, so that your friend is tested too. Make a table of your results.

What you will see: On the fingertip you can feel two points even when they are very close. On the forearm the points must be much further apart, or you feel only one. The fingertip has many more touch receptors in each square millimetre. The numbers in the picture are an example. Yours will differ.

If you cannot try it, this is what happens.
A table with the columns Where and Smallest gap. The fingertip is 3 mm, the palm is 10 mm and the forearm is 40 mm. These are example results.WhereSmallest gapfingertip3 mmpalm10 mmforearm40 mm

A wrong idea many people have

A nerve is a tube with a liquid in it, like a blood vessel.

Why it feels right: Nerves look like thin white threads, a little like blood vessels, and we say that a message "flows" along them.

The test that breaks it: A nerve is a bundle of many long neurone fibres. Each neurone carries a tiny electrical impulse, not a liquid. In a fibre with a fatty sheath the impulse runs at about 100 m/s, far faster than blood moves.

What is true: A nerve is a bundle of neurones. Each one carries an electrical impulse along its axon. At a synapse a chemical carries the message across the gap.

Key words

central nervous system
the brain and the spinal cord, where messages are sorted and orders are sent.
receptor
a cell that detects a stimulus, such as light, heat or pressure.
effector
a muscle or a gland that makes a response.
neurone
a nerve cell. It carries impulses.
synapse
the gap between two neurones, which a chemical crosses.
cerebrum
the large top part of the brain, used for thinking, memory and speech.
cerebellum
the part of the brain at the back that helps with balance and smooth movement.
medulla
the part of the brain that controls heart rate and breathing.

Back to your guess

The answer is B: Nerve cells carry the news to the brain, and other nerve cells carry the order to a muscle.

A receptor in the skin starts a tiny electrical signal in a nerve cell. Nerve cells carry it to the brain or spinal cord. Other nerve cells carry the order to a muscle. Blood is far too slow for this.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. Which list shows the path of a message from a touched finger to a muscle, in the right order?

    Tap your answer.

    1. Not this one. The two kinds of neurone are the wrong way round. A sensory neurone carries the message in. A motor neurone carries the order out. Try another.

    2. Yes, that is right. The receptor detects the stimulus. A sensory neurone takes the impulse in. The brain or spinal cord sorts it. A motor neurone takes the order out to the effector.

    3. Not this one. This path runs backwards. A message starts at a receptor and ends at an effector. Try another.

    See the answer

    receptor, sensory neurone, brain or spinal cord, motor neurone, effector

    The receptor detects the stimulus. A sensory neurone takes the impulse in. The brain or spinal cord sorts it. A motor neurone takes the order out to the effector.

    • If you chose A: The two kinds of neurone are the wrong way round. A sensory neurone carries the message in. A motor neurone carries the order out.
    • If you chose C: This path runs backwards. A message starts at a receptor and ends at an effector.

  2. Which part of a neurone is wrapped in fatty sheaths that make an impulse travel faster?

    Tap your answer.

    1. Not this one. Dendrites are the branched ends that receive impulses. They have no sheath. Try another.

    2. Not this one. The cell body holds the nucleus. The sheaths are along the long fibre, not on the body. Try another.

    3. Yes, that is right. The fatty sheaths wrap the axon in pieces. An impulse jumps along the gaps between the pieces, which is much faster than running along bare fibre.

    4. Not this one. The nerve endings pass the impulse on at the synapse. The sheaths are along the axon. Try another.

    See the answer

    The axon

    The fatty sheaths wrap the axon in pieces. An impulse jumps along the gaps between the pieces, which is much faster than running along bare fibre.

    • If you chose A: Dendrites are the branched ends that receive impulses. They have no sheath.
    • If you chose B: The cell body holds the nucleus. The sheaths are along the long fibre, not on the body.
    • If you chose D: The nerve endings pass the impulse on at the synapse. The sheaths are along the axon.

  3. Which kind of neurone is short, lies inside the spinal cord, and joins a sensory neurone to a motor neurone?

    Tap your answer.

    1. Yes, that is right. A relay neurone stays inside the brain or spinal cord. It receives the impulse from a sensory neurone and passes it on to a motor neurone.

    2. Not this one. A motor neurone carries orders from the central nervous system out to a muscle or gland. It is a long neurone. Try another.

    3. Not this one. A sensory neurone carries impulses in from a receptor. It is long, and it runs from the skin or an organ to the spinal cord. Try another.

    See the answer

    A relay neurone

    A relay neurone stays inside the brain or spinal cord. It receives the impulse from a sensory neurone and passes it on to a motor neurone.

    • If you chose B: A motor neurone carries orders from the central nervous system out to a muscle or gland. It is a long neurone.
    • If you chose C: A sensory neurone carries impulses in from a receptor. It is long, and it runs from the skin or an organ to the spinal cord.

  4. How does a message cross the gap between two neurones?

    Tap your answer.

    1. Not this one. The impulse stops at the end of the first neurone. It cannot cross a gap as a spark. Try another.

    2. Not this one. They stay separate. There is always a gap between them. Try another.

    3. Yes, that is right. The first neurone releases a chemical. It spreads across the synapse and fits receptor sites on the next neurone, which starts a new impulse.

    See the answer

    A chemical crosses the gap and sets off a new impulse.

    The first neurone releases a chemical. It spreads across the synapse and fits receptor sites on the next neurone, which starts a new impulse.

    • If you chose A: The impulse stops at the end of the first neurone. It cannot cross a gap as a spark.
    • If you chose B: They stay separate. There is always a gap between them.

  5. Which part of the brain controls heart rate and breathing?

    Tap your answer.

    1. Yes, that is right. The medulla joins the brain to the spinal cord. It keeps the heart beating and the lungs breathing without any thought.

    2. Not this one. The cerebrum is for thinking, memory and speech. Your heart and breathing carry on even when you sleep and do not think. Try another.

    3. Not this one. The cerebellum helps with balance and smooth movement, not with heart rate and breathing. Try another.

    See the answer

    The medulla

    The medulla joins the brain to the spinal cord. It keeps the heart beating and the lungs breathing without any thought.

    • If you chose B: The cerebrum is for thinking, memory and speech. Your heart and breathing carry on even when you sleep and do not think.
    • If you chose C: The cerebellum helps with balance and smooth movement, not with heart rate and breathing.

  6. A cyclist stays upright on a bike and moves smoothly. Which part of the brain helps most with this?

    Tap your answer.

    1. Not this one. The medulla controls heart rate and breathing. It does not do balance. Try another.

    2. Yes, that is right. The cerebellum takes reports about position and movement and helps the muscles work smoothly together to keep balance.

    3. Not this one. The spinal cord carries impulses between the brain and the body. The balance is worked out in the cerebellum, in the brain. Try another.

    See the answer

    The cerebellum

    The cerebellum takes reports about position and movement and helps the muscles work smoothly together to keep balance.

    • If you chose A: The medulla controls heart rate and breathing. It does not do balance.
    • If you chose C: The spinal cord carries impulses between the brain and the body. The balance is worked out in the cerebellum, in the brain.

  7. An impulse travels 1.5 m along a fibre with a fatty sheath at 100 m/s. How long does it take, in milliseconds?

    See the answer

    15 ms

    Time = distance ÷ speed = 1.5 ÷ 100 = 0.015 s. One second is 1 000 ms, so 0.015 s is 15 ms.

  8. A fibre with a fatty sheath carries impulses at 100 m/s. A fibre without one carries them at 1 m/s. How many times faster is the fibre with the sheath?

    See the answer

    100 times

    100 ÷ 1 = 100, so the fibre with the sheath is 100 times as fast.

Keep it

Copy this into your exercise book.

Stimulus, receptor, sensory neurone, brain or spinal cord, motor neurone, effector, response. A chemical crosses each synapse. Cerebrum thinks, cerebellum balances, medulla keeps heart and breathing going.

Draw: The six boxes of the path with arrows, and a small side view of the brain with three parts named.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

No internet tomorrow? Save the whole book as one file (1.1 MB). It opens on any phone.