The nervous system and reflexes

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Big idea: a system for fast messages

Big idea (key concept): Systems. The nervous system is a set of organs that work together as one communication network. No single part could coordinate your body on its own.
Related concept: Interaction. Receptors, neurones, the brain and your muscles all interact. A change in the world outside (or inside) you causes a change in your body within a fraction of a second.
Global context: Identities and relationships. Every touch, sight and sound you experience, and every movement you make, depends on this system. It is part of what makes you who you are and how you connect with other people.
Information travels in a chain: stimulus (a change in the environment) then receptor (detects it) then coordinator (the brain or spinal cord decides what to do) then effector (a muscle or gland) then response.
The central nervous system (CNS) is the brain and spinal cord. The peripheral nervous system is all the nerves that connect the CNS to the rest of the body.
Examples of receptors: light-sensitive cells in the retina, pressure and pain receptors in the skin, sound receptors in the ear. Effectors are muscles (they contract) and glands (they release a substance such as sweat).

Neurones carry the impulses

A neurone is a nerve cell. It carries information as an electrical impulse. There are three types, each adapted to its job.
A sensory neurone carries impulses from a receptor to the CNS. A relay neurone (in the brain or spinal cord) connects sensory neurones to motor neurones. A motor neurone carries impulses from the CNS to an effector.
Neurones have dendrites (branched ends that receive signals), a cell body containing the nucleus, and a long axon that carries the impulse away. Long axons let one cell reach from the spinal cord to a muscle in the foot.
Many long axons are wrapped in a fatty myelin sheath. Myelin acts as an electrical insulator and makes the impulse travel much faster. Fast myelinated neurones can carry impulses at around 100 metres per second.
A nerve is not the same as a neurone. A nerve is a bundle of many neurones (nerve fibres) wrapped together, like a cable made of many wires.

Sensory, relay and motor neurones

The synapse: crossing the gap

Neurones do not touch. Between two neurones there is a tiny gap called a synapse (the synaptic cleft).
An electrical impulse cannot jump across the gap, so the signal is passed on chemically. When an impulse reaches the end of the first neurone, vesicles release a chemical called a neurotransmitter into the gap.
The neurotransmitter diffuses across the gap and binds to receptor molecules on the membrane of the next neurone. This sets off a new electrical impulse in the second neurone.
Synapses make sure messages go one way only. Only the first neurone has vesicles of neurotransmitter, and only the second has the matching receptors.
Crossing a synapse takes a little time. This is one reason why a pathway with fewer synapses is quicker, and why reflex pathways are so short.

A synapse between two neurones

Reflexes: fast and automatic

A reflex action is a rapid, automatic response to a stimulus. You do not decide to do it and you do not think about it first. Examples: pulling your hand off something hot, the pupil getting smaller in bright light, blinking.
A reflex follows a reflex arc: stimulus, receptor, sensory neurone, relay neurone (in the spinal cord), motor neurone, effector, response.
A reflex is fast because the pathway is short, with only a few neurones and synapses, and the impulse does not have to be processed by the conscious part of the brain first. The brain is told about it a moment later, which is when you feel the pain.
A voluntary action, such as choosing to pick up a pen, is different. It involves a decision in the brain and a longer pathway, so it is slower.
Reflexes protect you from harm. Because they are automatic, they work even when you are not paying attention.

A reflex arc after a pin is stepped on

The eye: a receptor organ

The cornea and the lens both bend (refract) light so that it is focused onto the retina. The retina contains the light receptor cells (rods and cones), which turn light into electrical impulses. The optic nerve carries those impulses to the brain.
The iris is a ring of muscle. It controls how much light enters through the pupil, the hole in its centre. This is a reflex. In bright light the circular muscles of the iris contract and the radial muscles relax, so the pupil gets smaller and protects the retina. In dim light the radial muscles contract and the circular muscles relax, so the pupil gets larger and lets in more light.
To focus on a near object, the ciliary muscle contracts, the suspensory ligaments slacken and the lens becomes thicker and more curved. To focus on a distant object the ciliary muscle relaxes, the suspensory ligaments are pulled tight and the lens becomes thinner.
Both the pupil reflex and focusing are examples of the nervous system keeping a receptor working well as conditions change.

A labelled cross-section of the eye

Think like a scientist: how fast is your reaction?

Question: does being distracted slow down a person's reaction time? The ruler-drop test gives a simple measure. One person holds a ruler at the zero mark above the other person's open hand, drops it without warning, and the catch distance is read from the ruler. A longer catch distance means a slower reaction.
The independent variable is whether the person is distracted (for example, answering questions or watching a screen). The dependent variable is the catch distance in cm. Control variables: the same ruler, the same hand, the same starting height, the same person doing the dropping and the same room.
One catch could be luck, so do at least five trials for each condition and calculate a mean. If one result is far from the others (an anomaly), check what happened, repeat it, and say how you dealt with it.
Catch distance can be turned into a time using the fact that the ruler falls with constant acceleration: time = square root of (2 x distance / 9.8), with distance in metres. For example, a catch at 20 cm means a reaction time of about 0.20 s.
Inquiry task 1: plan an investigation into whether practice improves reaction time, and say how you would make the results reliable. Inquiry task 2: a student tests only one person, once with each condition, and concludes that distraction slows everyone down. Evaluate the method and the conclusion, and suggest two improvements.

The ruler-drop reaction-time test

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1.Which two organs make up the central nervous system?
Easy
- AThe eyes and the ears
- BThe brain and the spinal cord
- CThe nerves and the muscles
- DThe heart and the lungs
2.What is a receptor?
Easy
- AA chemical that crosses a synapse
- BA muscle that carries out a response
- CA nerve that joins the brain to the spine
- DA cell or organ that detects a stimulus
3.An effector can be either a muscle or a gland.
Easy
True or false?
4.Put the parts of the nervous pathway in the correct order.
Easy
- Effector
- Stimulus
- Coordinator (CNS)
- Receptor
- Response
5.Which type of neurone carries impulses from a receptor to the central nervous system?
Easy
- ASensory neurone
- BRelay neurone
- CEffector neurone
- DMotor neurone
6.Which type of neurone carries impulses from the CNS to an effector?
Easy
- AReceptor neurone
- BRelay neurone
- CSensory neurone
- DMotor neurone
7.Match each part of the nervous system to its job.
Easy
- Sensory neurone
- Relay neurone
- Motor neurone
- Receptor
- Connects sensory and motor neurones inside the CNS
- Detects a stimulus
- Carries impulses from the CNS to an effector
- Carries impulses from a receptor to the CNS
8.Complete the sentence.
Easy
A nerve impulse is an ____ signal that travels along a neurone.

Plus de questions

9.What is the main job of the myelin sheath around some neurones?
Easy
- AIt carries the impulse across a synapse
- BIt stores energy for the neurone
- CIt makes the impulse travel faster
- DIt detects the stimulus
10.Neurones touch each other directly at a synapse.
Easy
True or false?
11.What crosses the gap at a synapse?
Easy
- AAn electrical spark
- BA blood cell
- CA hormone from the thyroid
- DA neurotransmitter
12.Complete the sentence.
Easy
A reflex is a rapid, ____ response to a stimulus.
13.Which of these is a reflex action?
Easy
- AChoosing to kick a ball
- BDeciding to wave at a friend
- CPulling your hand away from a hot pan
- DWriting your name
14.A reflex action needs you to think about it before it happens.
Easy
True or false?
15.Which part of the eye contains the light receptor cells?
Easy
- AThe lens
- BThe retina
- CThe cornea
- DThe iris
16.Which part of the eye controls the size of the pupil?
Easy
- AThe retina
- BThe cornea
- CThe iris
- DThe optic nerve
17.Match each part of the eye to its job.
Easy
- Cornea
- Lens
- Retina
- Optic nerve
- Carries impulses to the brain
- Contains the receptor cells
- Fine-focuses light onto the retina
- Bends light as it enters the eye
18.What happens to the pupil in bright light?
Easy
- AIt stays the same size
- BIt gets larger
- CIt moves to the side
- DIt gets smaller
19.A student did the ruler-drop test five times and caught the ruler at 14 cm, 18 cm, 16 cm, 20 cm and 12 cm. What is the mean catch distance?
Easy
20.Which of these is a stimulus?
Easy
- AA gland releasing sweat
- BAn impulse in a motor neurone
- CA muscle contracting
- DA sudden loud noise
21.Which list shows the correct order for a response to a stimulus?
Medium
- AEffector, receptor, coordinator, response
- BCoordinator, receptor, response, effector
- CReceptor, effector, coordinator, response
- DReceptor, coordinator, effector, response
22.Why is a reflex response faster than a voluntary response?
Medium
- AThe pathway is short and skips conscious thought
- BReflexes only involve muscles and no neurones
- CReflexes use hormones, which are quicker
- DThe impulses travel at a faster speed in reflexes
23.Why is the axon of a motor neurone to the foot very long?
Medium
- AIt must carry the impulse a long way from the spinal cord to the muscle
- BIt makes the impulse travel more slowly and safely
- CIt contains most of the cell's chloroplasts
- DIt has to store a large amount of neurotransmitter
24.What is the difference between a neurone and a nerve?
Medium
- AThere is no difference; the words mean the same thing
- BA nerve only carries impulses from the CNS to the body
- CA nerve is a single cell and a neurone is a bundle of cells
- DA nerve is a bundle of many neurones, while a neurone is a single cell
25.Why can an impulse cross a synapse in one direction only?
Medium
- AMyelin blocks the impulse from going backwards
- BElectrical impulses can only travel forwards through cells
- CThe second neurone is always much longer than the first
- DNeurotransmitter is released from one side and the receptors are only on the other
26.Put these events at a synapse in the correct order.
Medium
- Neurotransmitter binds to receptor molecules
- An impulse arrives at the end of the first neurone
- Neurotransmitter diffuses across the gap
- A new impulse starts in the second neurone
- Vesicles release neurotransmitter into the gap
27.Which of these are parts of a reflex arc? (select all that apply)
Medium
- AAn effector
- BA relay neurone
- CA receptor
- DA hormone carried in the blood
- EA decision in the cerebral cortex
28.Put the stages of a reflex arc in the correct order.
Medium
- Motor neurone
- Relay neurone in the spinal cord
- Effector
- Receptor in the skin
- Sensory neurone
29.Where is the relay neurone in a typical reflex arc?
Medium
- AIn the spinal cord
- BIn the skin beside the receptor
- CIn the retina
- DInside the muscle
30.A person looks from a distant tree to a book. What happens in the eye?
Medium
- AThe suspensory ligaments tighten and the lens becomes thicker
- BThe ciliary muscle contracts and the lens becomes thinner
- CThe ciliary muscle relaxes and the lens becomes thicker
- DThe ciliary muscle contracts and the lens becomes thicker
31.In dim light, which muscles of the iris contract?
Medium
- AThe radial muscles, making the pupil larger
- BThe circular muscles, making the pupil smaller
- CThe radial muscles, making the pupil smaller
- DThe ciliary muscles, making the lens thinner
32.When you look at a distant object, the lens becomes thinner.
Medium
True or false?
33.The optic nerve carries impulses from the retina to the brain. Which type of neurone does it contain?
Medium
- ASensory neurones
- BMotor neurones
- CRelay neurones only
- DHormone-secreting neurones
34.In one question, a sensory neurone carries impulses at 80 metres per second. How long, in milliseconds, does an impulse take to travel 1.2 metres along it?
Medium
35.In a class experiment, a student did the ruler-drop test five times while answering questions. The catch distances were 22 cm, 26 cm, 24 cm, 28 cm and 25 cm. What is the mean catch distance?
Medium
36.
A student did five ruler-drop trials in each condition.
Trial Resting catch distance (cm) Distracted catch distance (cm)
1 14 22
2 18 26
3 16 24
4 20 28
5 12 25
Which conclusion is best supported by these results?
Medium
- AResting trials were all slower than distracted trials
- BBeing distracted did not change the student's reaction time
- CBeing distracted gave a mean catch distance of 25 cm compared with 16 cm resting, so reaction was slower
- DBeing distracted gave a shorter catch distance, so reaction was faster
37.A student plans to test whether listening to music changes the ruler-drop catch distance. What is the independent variable?
Medium
- AThe catch distance in cm
- BWhether or not music is playing
- CThe length of the ruler
- DThe hand that is used to catch
38.In the music investigation, which of these should be kept the same to make it a fair test?
Medium
- AThe mean of the results
- BThe starting height of the ruler
- CThe catch distance measured
- DWhether music is playing
39.To test the effect of distraction, a student should use the same hand for catching in every trial.
Medium
True or false?
40.Describe the pathway taken by a nerve impulse when a person's hand touches a hot object and pulls away.
Medium
41.
A student compared the hands used to catch the ruler. Five trials each.
Hand Catch distances (cm)
Writing hand 15, 17, 16, 14, 18
Other hand 21, 23, 19, 22, 20
The student claims that the writing hand reacts faster in everyone. Which is the best evaluation?
Hard
- AThe results cannot be used at all, because the two means are both whole numbers
- BThe claim is proven for everyone, because every writing-hand result is shorter than every other-hand result
- CThe means are 16 cm and 21 cm: this supports the claim for this student only, as one person cannot show it for everyone
- DThe claim is false for all people, because the means of the two hands are different numbers
42.In one trial set the catch distances were 18 cm, 17 cm, 19 cm, 41 cm and 18 cm. What is the best way to handle the 41 cm result?
Hard
- AIgnore it without saying anything to anyone, because it looks different
- BChange it to 18 cm quietly so that the whole set of results looks neater
- CCheck what went wrong and repeat; if it is a clear mistake, report 18 cm without it and say so
- DInclude it and report a mean of 22.6 cm without any comment at all
43.
A catch distance can be converted to a reaction time using this table.
Catch distance (cm) Reaction time (s)
10 0.14
15 0.17
20 0.20
25 0.23
30 0.25
Another student's mean catch distance was 15 cm when resting and 25 cm when distracted. By what percentage did the reaction time increase? Give your answer to the nearest whole number.
Hard
44.A person walks from bright sunlight into a dark room. Which sequence explains how the eye responds?
Hard
- AReceptors in the retina detect less light, impulses go to the CNS, radial muscles of the iris contract, the pupil gets larger
- BReceptors in the iris detect less light, impulses go to the CNS, radial muscles relax, the pupil gets larger
- CReceptors in the retina detect less light, impulses go to the CNS, ciliary muscles contract, the lens gets thicker
- DReceptors in the lens detect less light, impulses go to the CNS, circular muscles of the iris contract, the pupil gets smaller
45.A drug blocks the receptor molecules on the membrane of the second neurone at a synapse. What is the most likely effect?
Hard
- ANeurotransmitter is no longer released by the first neurone
- BThe impulse in the first neurone travels more slowly
- CThe second neurone makes more neurotransmitter
- DNeurotransmitter crosses the gap, but no new impulse starts in the second neurone
46.A person has damage to the motor neurone in a reflex arc in their leg, but all other neurones work normally. They step on a pin. What is most likely to happen?
Hard
- AThey feel no pain and the leg does not move
- BThey do not feel pain, but the leg pulls away
- CThey feel pain, but the leg muscle does not pull the foot away
- DThe leg pulls away and the pain is felt more strongly
47.A student wants more reliable results from the ruler-drop test. Which changes would help? (select all that apply)
Hard
- AUse the same starting height for every drop
- BLet the catcher watch the dropper's fingers so the drop can be predicted
- CTest several different people
- DDo more trials in each condition and calculate a mean
- EUse a different ruler for each condition
48.Explain how the eye changes to focus on a near object.
Hard
49.Plan an investigation to find out whether practice improves reaction time using the ruler-drop test. State what you would change, measure and keep the same, and how you would make the results reliable.
Hard
50.In a reflex action the brain must decide the response before the muscle contracts.
Hard
True or false?

Notes de révision

## The big idea - The nervous system is one **system** of interacting parts. It carries fast messages from receptors to effectors (**Systems**, **Interaction**). ## The pathway - **Stimulus**, **receptor**, **coordinator** (CNS: brain and spinal cord), **effector** (muscle or gland), **response**. ## Neurones and synapses - **Sensory** (receptor to CNS), **relay** (inside the CNS), **motor** (CNS to effector). - **Myelin** speeds up impulses. A **nerve** is a bundle of neurones. - At a **synapse**, neurotransmitter diffuses across the gap and binds to receptors. Messages go one way only. ## Reflexes - A **reflex** is rapid and automatic. Path: receptor, sensory neurone, relay neurone, motor neurone, effector. - Short pathway and no conscious decision, so it is fast. ## The eye - Bright light: pupil smaller. Dim light: pupil larger. - Near: ciliary muscle contracts, lens thicker. Far: ciliary muscle relaxes, lens thinner. ## Investigating - Ruler-drop test: longer catch distance means slower reaction. Change one variable, control the rest, repeat and use a mean. Check anomalies.

Stimulus: A change in the environment that is detected by a receptor.
Receptor: A cell or organ that detects a stimulus.
Effector: A muscle or gland that carries out a response.
Central nervous system: The brain and spinal cord, which coordinate responses.
Neurone: A nerve cell that carries electrical impulses.
Sensory neurone: A neurone that carries impulses from a receptor to the CNS.
Relay neurone: A neurone inside the CNS that links sensory and motor neurones.
Motor neurone: A neurone that carries impulses from the CNS to an effector.
Myelin sheath: A fatty insulating layer around some axons that speeds up impulses.
Synapse: The tiny gap between two neurones, crossed by a neurotransmitter.
Neurotransmitter: A chemical released at a synapse that starts an impulse in the next neurone.
Reflex action: A rapid, automatic response that does not involve a conscious decision.
Retina: The layer at the back of the eye that contains the light receptor cells.
Iris: The ring of muscle that controls the size of the pupil.

Cartes mémo

What is the pathway from stimulus to response?

Stimulus, receptor, coordinator (CNS), effector, response.

What makes up the central nervous system?

The brain and the spinal cord.

What are the two types of effector?

Muscles and glands.

What does a sensory neurone do?

Carries impulses from a receptor to the CNS.

What does a motor neurone do?

Carries impulses from the CNS to an effector.

What does a relay neurone do?

Links sensory and motor neurones inside the CNS.

What is the job of the myelin sheath?

It insulates the axon and makes impulses travel faster.

What is the difference between a neurone and a nerve?

A neurone is one cell. A nerve is a bundle of many neurones.

What is a synapse?

The tiny gap between two neurones.

How does a signal cross a synapse?

Neurotransmitter is released, diffuses across the gap and binds to receptors on the next neurone.

Why do synapses work in one direction only?

Neurotransmitter is released only from the first neurone and receptors are only on the second.

What is a reflex action?

A rapid, automatic response to a stimulus.

Why is a reflex fast?

The pathway is short and does not wait for a conscious decision.

Give the parts of a reflex arc in order.

Receptor, sensory neurone, relay neurone, motor neurone, effector.

What happens to the pupil in bright light?

Circular muscles contract and the pupil gets smaller.

How does the lens change to focus on a near object?

The ciliary muscle contracts, the ligaments slacken and the lens becomes thicker.

What does a longer catch distance mean in the ruler-drop test?

A slower reaction time.

How can you make a reaction-time investigation more reliable?

Do more trials, test more people, control variables and calculate a mean.

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Trial	Resting catch distance (cm)	Distracted catch distance (cm)
1	14	22
2	18	26
3	16	24
4	20	28
5	12	25

Hand	Catch distances (cm)
Writing hand	15, 17, 16, 14, 18
Other hand	21, 23, 19, 22, 20

Catch distance (cm)	Reaction time (s)
10	0.14
15	0.17
20	0.20
25	0.23
30	0.25