Chapter 2 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

S4.5.2 Chapter 2

Reflexes

Why do you pull your hand away from a hot pan before you even feel the pain?

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

  • Draw a reflex arc with sensory, relay and motor neurones
  • Measure your reaction time with a falling ruler

Your hand touches a hot pan. It is away before you feel anything. This is a reflex: a quick, automatic response to a stimulus. You do not choose it, and you do not need to think about it. It protects you.

The path of a reflex is called the reflex arc. It uses the same kinds of neurone you met in the last chapter. The difference is the short route. The impulse does not wait for the thinking part of the brain.

Draw it first

A reflex in three steps

  1. Three pictures of three boxes joined by arrows, one after another. First the boxes are labelled heat on the skin, receptor starts an impulse and sensory neurone. Then they are labelled sensory neurone, relay neurone in the spinal cord and motor neurone. Last they are labelled motor neurone, muscle contracts and hand pulls away.heat on the skinreceptor starts an impulsesensory neurone

    1 Heat acts on a receptor in the skin. 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 heat on the skin, receptor starts an impulse and sensory neurone. Then they are labelled sensory neurone, relay neurone in the spinal cord and motor neurone. Last they are labelled motor neurone, muscle contracts and hand pulls away.sensory neuronerelay neurone in the spinalcordmotor neurone

    2 The sensory neurone carries the impulse to the spinal cord. A relay neurone there passes it on to a motor neurone.

  3. Three pictures of three boxes joined by arrows, one after another. First the boxes are labelled heat on the skin, receptor starts an impulse and sensory neurone. Then they are labelled sensory neurone, relay neurone in the spinal cord and motor neurone. Last they are labelled motor neurone, muscle contracts and hand pulls away.motor neuronemuscle contractshand pulls away

    3 The motor neurone carries the impulse to a muscle in the arm. The muscle contracts and pulls the hand away.

A reflex arc. The labels are stimulus, receptor, sensory neurone, brain, spinal cord, relay neurone, motor neurone and muscle (effector). Heat on the skin acts on the receptor. The impulse runs up the sensory neurone to the spinal cord, across the relay neurone and down the motor neurone to the muscle. The brain is drawn above the cord, and no neurone of the arc touches it. A dotted shape in the middle of the cord marks its central part.sensoryneuronestimulusreceptorbrainspinalcordrelayneuronemotorneuronemuscle(effector)
The reflex arc for a hot object. The brain is drawn, but the path does not go through it. The dotted shape is the central part of the cord, where the relay neurone lies.

Look at the picture. The sensory neurone enters the spinal cord. The relay neurone joins it to the motor neurone. The picture does not show them, but at each join between two neurones there is a synapse. Many reflex arcs have a relay neurone. A few do not. In the knee jerk, the sensory neurone passes the impulse straight to the motor neurone.

The spinal cord is not the only place for a reflex. Blinking and the pupil reflex pass through parts of the brain that you do not control. What they share is the short path. No conscious decision is needed, and the action is over before you could decide.

A table with the columns Action and Reflex. Hand off a flame: yes. Kicking a ball: no. Blinking at dust: yes. Writing a word: no. Pupil in light: yes.ActionReflexhand off a flameyeskicking a ballnoblinking at dustyeswriting a wordnopupil in lightyes
Some actions are reflexes and some are decisions. A reflex is quick, automatic and protective.

At the same moment, other impulses run up the spinal cord to the brain. The brain then tells you about the heat. That is a longer path, so you feel the pain after your hand has already moved. Now you can decide what to do next, such as running cold water over the hand.

A table with the columns Reflex and Decision. Row one: spinal cord, brain. Row two: very fast, slower. Row three: automatic, you choose. Row four: protects you, any job.ReflexDecisionspinal cordbrainvery fastslowerautomaticyou chooseprotects youany job
A reflex and a decision compared. A decision uses the brain, with more neurones and more synapses, so it takes longer.

How much longer? You can measure it. Your reaction time is the time from a stimulus to your response. A person drops a ruler without warning, and you catch it. The further it falls before you catch it, the longer your reaction time.

Gravity makes the ruler speed up as it falls. Starting from rest, it falls a distance d in a time t = √(2d ÷ g). Here g = 9.8 m/s² and d is in metres. Look at the picture, then work out the time.

A 30 cm ruler with a scale of 0, 5, 10, 15, 20, 25 and 30 and the unit cm. A dashed line shows where the fingers waited, ready at 0 cm. Fingers are drawn at the point where they caught it, caught at 20 cm.051015202530cmready at 0 cmcaught at 20 cm
The fingers wait level with 0 cm. The ruler falls, and the fingers catch it at the mark shown. That mark is the distance fallen.

Worked example

A ruler is caught at 20 cm. What was the reaction time, in milliseconds?

  1. The ruler fell 20 cm, which is 0.20 m.
  2. Use t = √(2d ÷ g), with g = 9.8 m/s².
  3. t = √(2 × 0.20 ÷ 9.8) = √0.0408 = 0.202 s.
  4. One second is 1 000 ms, so 0.202 s = 202 ms.

Answer: 202 ms

You do not have to work out the square root each time. Here is a table of the same sum for other distances, and the same results drawn as a graph. The line bends, because the ruler speeds up as it falls.

A table with the columns Fell and Time. 5 cm: 101 ms. 10 cm: 143 ms. 15 cm: 175 ms. 20 cm: 202 ms. 25 cm: 226 ms. 30 cm: 247 ms.FellTime5 cm101 ms10 cm143 ms15 cm175 ms20 cm202 ms25 cm226 ms30 cm247 ms
Time of fall for a ruler that falls 5 cm to 30 cm, to the nearest millisecond.
A line graph. The x axis is distance fallen in cm, from 0 to 30 in steps of 5. The y axis is time in ms, from 0 to 250 in steps of 50. The line starts at 0 and 0, climbs steeply at first, then more slowly, and reaches about 247 ms at 30 cm. It is a curve, not a straight line.051015202530050100150200250distance fallen in cmtime in ms
The time of the fall against the distance fallen. A ruler that falls twice as far takes less than twice as long.

See it three ways

As a picture

A table with the columns Caught at and Reaction time. 5 cm gives 101 ms, 10 cm gives 143 ms, 20 cm gives 202 ms.Caught atReaction time5 cm101 ms10 cm143 ms20 cm202 ms

In words

The farther the ruler falls before you catch it, the longer your reaction time. Twice the distance does not mean twice the time, because the ruler keeps speeding up.

In symbols

t = √(2d ÷ g), with d in metres and g = 9.8 m/s². For d = 0.20 m, t = √(0.40 ÷ 9.8) = 0.202 s, which is 202 ms.

One catch is not enough. You may be ready or you may be distracted. So take several catches and find the mean. Here are five catches from one learner. Find the time for each, then the mean.

A table with the columns Try and Caught at. Try 1: 18 cm. Try 2: 22 cm. Try 3: 20 cm. Try 4: 16 cm. Try 5: 24 cm.TryCaught at118 cm222 cm320 cm416 cm524 cm
Five catches from one learner. Each is the distance the ruler fell.

Worked example

A learner caught the ruler at 18, 22, 20, 16 and 24 cm. What is the mean reaction time, to the nearest millisecond?

  1. Find the time for each catch. The five times are 192 ms, 212 ms, 202 ms, 181 ms and 221 ms.
  2. Add the five times: 192 + 212 + 202 + 181 + 221 = 1 008 ms.
  3. Divide by 5: 1 008 ÷ 5 = 201.6, which is 202 ms to the nearest millisecond.

Answer: 202 ms

Try it

Measure your reaction time

You need: a 30 cm ruler, plastic or wood, a friend, a table or chair, and a clear floor, paper and a pencil.

Be safe: Sit down and keep the floor clear. Hold the ruler low, over a table. Never drop it near anyone's face, and never stand on a chair.

  1. Sit with your forearm resting on a table and your hand over the edge. Your friend holds the ruler by its top end, hanging between your open thumb and finger. The 0 cm end is level with the top of your finger.
  2. Your friend lets go without warning, at a time of their own choosing. Catch the ruler as fast as you can.
  3. Read the mark level with the top of your finger. This is the distance in cm that the ruler fell. Write it down.
  4. Do this five times. Use the table or the formula to turn each distance into a time. Then find the mean.
  5. Swap places. Or try again when you are tired, or while someone talks to you, and compare.

What you will see: Most people get a time between about 150 ms and 250 ms. If the ruler falls right through your fingers, count that go as a miss and try again. Your time changes from one go to the next. It is usually longer when you are tired or distracted. A reflex, such as pulling your hand from heat, takes far less time, because no decision is needed.

If you cannot try it, this is what happens.
A 30 cm ruler with a scale of 0, 5, 10, 15, 20, 25 and 30 and the unit cm. A dashed line shows where the fingers waited, ready at 0 cm. Fingers are drawn where they caught it, caught at 18 cm. This is an example.051015202530cmready at 0 cmcaught at 18 cm

A wrong idea many people have

When your hand pulls away from something hot, your brain decides to move it, very fast.

Why it feels right: The movement is quick, and we think the brain controls everything we do.

The test that breaks it: The hand moves before you feel the pain. The pain is felt in the brain. If the brain had decided first, you would feel it first.

What is true: In a reflex, the order to the muscle comes from the spinal cord. The brain is told at about the same time, but the movement does not wait for it.

Key words

reflex
a quick, automatic response that you do not decide.
reflex arc
the path of a reflex: receptor, sensory neurone, relay neurone, motor neurone, effector.
reaction time
the time from a stimulus to your response.

Back to your guess

The answer is A: The impulse goes to the spinal cord, which sends the order to the muscle without waiting for the brain to decide.

In a reflex the spinal cord passes the impulse straight from a sensory neurone to a motor neurone. The hand moves first. A second message goes up to the brain, so you feel the pain a moment later.

Check yourself

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

  1. What is a reflex?

    Tap your answer.

    1. Not this one. That describes a decision. A reflex is automatic, and it does not wait for you to plan. Try another.

    2. Not this one. A reflex still uses nerve impulses. They follow a short path called the reflex arc. Try another.

    3. Yes, that is right. A reflex is fast and automatic. It uses a short path of neurones, and it often protects you from harm.

    See the answer

    A quick, automatic response to a stimulus that you do not decide.

    A reflex is fast and automatic. It uses a short path of neurones, and it often protects you from harm.

    • If you chose A: That describes a decision. A reflex is automatic, and it does not wait for you to plan.
    • If you chose B: A reflex still uses nerve impulses. They follow a short path called the reflex arc.

  2. Which list shows the parts of a basic reflex arc in the right order?

    Tap your answer.

    1. Yes, that is right. The receptor starts the impulse. The sensory neurone carries it to the spinal cord. The relay neurone passes it to the motor neurone, which carries it to the effector.

    2. Not this one. In a reflex the impulse does not wait for the brain. A relay neurone in the spinal cord passes it from the sensory to the motor neurone. Try another.

    3. Not this one. The neurones are in the wrong order. The sensory neurone comes first, because it carries the impulse in from the receptor. Go to S4.5.1. Try another.

    See the answer

    receptor, sensory neurone, relay neurone, motor neurone, effector

    The receptor starts the impulse. The sensory neurone carries it to the spinal cord. The relay neurone passes it to the motor neurone, which carries it to the effector.

    • If you chose B: In a reflex the impulse does not wait for the brain. A relay neurone in the spinal cord passes it from the sensory to the motor neurone.
    • If you chose C: The neurones are in the wrong order. The sensory neurone comes first, because it carries the impulse in from the receptor. Go to S4.5.1.

  3. Why is a reflex faster than a decision that you think about?

    Tap your answer.

    1. Not this one. Impulses travel at much the same speed along similar neurones. The reflex is faster because its path is shorter. Try another.

    2. Yes, that is right. A reflex uses only a few neurones and two synapses. A decision uses the brain, with many more neurones and synapses, so it takes longer.

    3. Not this one. The muscle works at the same speed. The time saved is in the nerve path, before the muscle starts. Try another.

    See the answer

    The path is shorter, with fewer synapses, and no decision is needed.

    A reflex uses only a few neurones and two synapses. A decision uses the brain, with many more neurones and synapses, so it takes longer.

    • If you chose A: Impulses travel at much the same speed along similar neurones. The reflex is faster because its path is shorter.
    • If you chose C: The muscle works at the same speed. The time saved is in the nerve path, before the muscle starts.

  4. Sipho touches a hot pan and pulls his hand away. A moment later he feels pain. Why does the pain come after the movement?

    Tap your answer.

    1. Not this one. The pain starts in the receptors of the skin, not in the muscle. The muscle moves because of the reflex. Try another.

    2. Yes, that is right. The reflex arc is short. The impulses to the brain have a longer path, so the brain tells him about the heat after the hand has moved.

    3. Not this one. The brain does not switch it off. The pain message simply takes a longer path than the reflex does. Try another.

    See the answer

    Other impulses must travel all the way up to the brain, which takes longer than the reflex.

    The reflex arc is short. The impulses to the brain have a longer path, so the brain tells him about the heat after the hand has moved.

    • If you chose A: The pain starts in the receptors of the skin, not in the muscle. The muscle moves because of the reflex.
    • If you chose C: The brain does not switch it off. The pain message simply takes a longer path than the reflex does.

  5. Which of these is not a reflex?

    Tap your answer.

    1. Yes, that is right. You decide to catch a ball, and you can learn to do it better. A reflex is automatic, and you do not choose it.

    2. Not this one. This is a reflex. It protects the eye and happens without a decision. Try another.

    3. Not this one. This is a reflex. It protects the eye from too much light, and you cannot decide it. Try another.

    See the answer

    Catching a ball that someone throws to you

    You decide to catch a ball, and you can learn to do it better. A reflex is automatic, and you do not choose it.

    • If you chose B: This is a reflex. It protects the eye and happens without a decision.
    • If you chose C: This is a reflex. It protects the eye from too much light, and you cannot decide it.

  6. In the falling ruler test, Mei catches the ruler at a longer distance than Hana. What does this show?

    Tap your answer.

    1. Not this one. The test does not measure a reflex. It measures the time for a decision and a response. Try another.

    2. Not this one. The further the ruler falls, the more time has passed. Mei took longer to catch it. Try another.

    3. Yes, that is right. The ruler speeds up as it falls, so a longer fall means more time has passed. Mei took longer to respond than Hana.

    See the answer

    Mei has a longer reaction time.

    The ruler speeds up as it falls, so a longer fall means more time has passed. Mei took longer to respond than Hana.

    • If you chose A: The test does not measure a reflex. It measures the time for a decision and a response.
    • If you chose B: The further the ruler falls, the more time has passed. Mei took longer to catch it.

  7. From an earlier chapter

    A message crosses a synapse in the reflex arc. What carries it across the gap?

    Tap your answer.

    1. Not this one. The impulse does not cross the gap as a spark. Try another.

    2. Not this one. Blood does not carry the message across a synapse. Try another.

    3. Yes, that is right. The first neurone releases a chemical. It crosses the synapse and sets off an impulse in the next neurone.

    4. Not this one. The neurones stay apart, with a gap between them. Try another.

    See the answer

    A chemical

    The first neurone releases a chemical. It crosses the synapse and sets off an impulse in the next neurone.

    • If you chose A: The impulse does not cross the gap as a spark.
    • If you chose B: Blood does not carry the message across a synapse.
    • If you chose D: The neurones stay apart, with a gap between them.

  8. A ruler is caught at 25 cm. What is the reaction time in milliseconds?

    See the answer

    226 ms

    The ruler fell 0.25 m. t = √(2 × 0.25 ÷ 9.8) = √0.051 = 0.226 s, which is 226 ms.

  9. Suppose a pulling away reflex takes 50 ms. Kofi's reaction time in the ruler test is 200 ms. How many times as long as the reflex is his reaction time?

    See the answer

    4 times

    200 ÷ 50 = 4, so his reaction time is 4 times as long as the reflex.

Keep it

Copy this into your exercise book.

A reflex is quick, automatic and protective. Its path is receptor, sensory neurone, relay neurone, motor neurone, effector. The brain is told later. Reaction time is longer, because the brain decides.

Draw: The reflex arc as a loop through the spinal cord, with the brain drawn to one side.

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.