The nervous system and reflexes
By the end of this chapter you will be able to
- Name the parts of the nervous system and the three types of neurone, and say what each carries
- Trace a reflex arc from the stimulus to the response
- Calculate reaction time from a table, and explain the factors that slow it
- Describe the pupil reflex and how the eye focuses
- [H] Explain how a neurotransmitter passes the impulse across a synapse
You should already know
- Stimulus and response in living things (KS3 Biology)
- Cells and their parts (Chapter 2)
Where this leads
- Hormones and the nervous system compared (Chapter 12)
- Muscle contraction in respiration (Chapter 10)
- Drugs and reaction time (Chapter 14)
GCSE Biology. Content follows the GCSE (9-1) subject content for England (Edexcel and AQA). Content references: Describe the central nervous system and the three types of neurone, Describe the reflex arc, and explain why reflexes are fast and automatic, Explain reaction time and the factors that affect it, and use a ruler-drop test, Describe how the pupil reflex and focusing work in the eye, [H] Describe synapses and the role of neurotransmitters.
Exercise P Do you remember?
Method. Move the point two places to the right.
- Multiply. 0.2 x 100 = 20.0.2 x 100 = 20
- Write the answer. 20
Check. 20 divided by 100 is 0.2. ✓
Method. Recall the organelle.
- Recall. Aerobic respiration releases energy in the mitochondria.releases energy
- Write the answer. mitochondria
Check. Chloroplasts make glucose. ✓
Method. Recall the long part of a neurone.
- Recall. The long fibre that carries impulses away from the cell body is the axon.long fibre
- Write the answer. axon
Check. The cell body is the part with the nucleus. ✓
Method. Recall the term.
- Recall. A fast, automatic response to a stimulus is a reflex.fast, automatic
- Write the answer. reflex
Check. Reflexes protect the body. ✓
Method. Divide 40 by 4.
- Divide. 40 / 4 = 10.40 / 4 = 10
- Write the answer. 10
Check. 10 x 4 = 40. ✓
The nervous system and neurones
The nervous system has two parts. The central nervous system (CNS) is the brain and the spinal cord. The rest of the nerves make up the peripheral nervous system. Messages travel as electrical impulses along neurones (nerve cells). A neurone has a cell body, dendrites that receive impulses, and a long axon that carries them. Many axons are covered by a myelin sheath, which helps the impulse travel faster.
| Neurone | Carries impulses from | Found in |
|---|---|---|
| sensory neurone | a receptor to the CNS | the nerves from the skin and eyes |
| relay neurone | one neurone to another, inside the CNS | the brain and spinal cord |
| motor neurone | the CNS to an effector (muscle or gland) | the nerves to the muscles |
Key ideas
- The CNS is the brain and the spinal cord.
- Sensory neurones carry impulses from receptors to the CNS.
- Relay neurones are inside the CNS, linking one neurone to another.
- Motor neurones carry impulses from the CNS to effectors such as muscles and glands.
Method. Recall the central part.
- Recall. The central nervous system is the brain and the spinal cord.brain and spinal cord
- Name it. So the answer is the central nervous system (CNS).central nervous system
- Write the answer. central nervous system
Check. The nerves outside it are the peripheral nervous system. ✓
Method. Recall the direction of impulses from receptors.
- Recall. Receptors send impulses to the CNS along sensory neurones.sensory neurones
- Name it. So the type is a sensory neurone.sensory neurone
- Write the answer. sensory neurone
Check. Motor neurones carry impulses the other way. ✓
Method. Recall the long part of the neurone.
- Recall. The long fibre carries the impulse away from the cell body.long fibre
- Name it. This is the axon.axon
- Write the answer. axon
Check. Dendrites receive impulses. ✓
Watch out
Watch out: a sensory neurone carries impulses to the CNS, and a motor neurone carries them from the CNS. Do not reverse them.
Exam tip
Exam tip. Name the neurone by where it carries impulses from and to. “Receptor to CNS” is the sensory neurone.
Try it (with your teacher). With your teacher, use a labelled diagram of a neurone. Trace the path of an impulse from a receptor in the skin to a muscle, naming each type of neurone.
Exercise 11A The nervous system and neurones
Method. Recall the name.
- Recall. The brain and the spinal cord make up the central nervous system.central nervous system
- Write the answer. central nervous system
Check. The CNS is abbreviated CNS. ✓
Method. Recall the neurone from receptors.
- Recall. Sensory neurones carry impulses from receptors to the CNS.from receptors
- Write the answer. sensory neurone
Check. Sensory means it senses. ✓
Method. Recall the neurone to effectors.
- Recall. Motor neurones carry impulses from the CNS to effectors.to effectors
- Write the answer. motor neurone
Check. Motor means it causes movement. ✓
Method. Recall the neurone inside the CNS.
- Recall. Relay neurones are inside the brain and spinal cord.inside the brain
- Write the answer. relay neurone
Check. They link sensory and motor neurones. ✓
Method. Recall the receiving part.
- Recall. Dendrites receive impulses from other neurones.receive impulses
- Write the answer. dendrites
Check. The axon carries impulses away. ✓
Method. Recall the covering.
- Recall. The myelin sheath covers many axons and speeds up impulses.speeds up impulses
- Write the answer. myelin sheath
Check. The sheath is made of fatty cells. ✓
Method. Recall the outer part.
- Recall. The nerves outside the CNS make up the peripheral nervous system.outside the CNS
- Write the answer. peripheral nervous system
Check. The CNS is the brain and spinal cord. ✓
Method. Recall the parts of the CNS.
- Recall. The CNS is the brain and the spinal cord.brain and the spinal cord
- Write the answer. So yes.yes
- Write the answer. yes
Check. The spinal cord is protected by the bones of the spine. ✓
Method. The eye is a receptor.
- Think. The eye is a receptor, and its impulses go to the CNS.to the CNS
- Name it. So a sensory neurone carries the impulse.sensory neurone
- Write the answer. sensory neurone
Check. Motor neurones leave the CNS. ✓
Method. Think about the direction of the message.
- Think. The message leaves the CNS and goes to the muscle.leaves the CNS
- Name it. So it is carried by a motor neurone.motor neurone
- Write the answer. motor neurone
Check. The muscle is an effector. ✓
Method. Think about the sheath.
- Think. The sheath acts as an insulator around the axon.insulator
- Conclude. So the impulse can jump between gaps in the sheath, which is faster.jump
- Write the answer. Accept any sentence that says the myelin sheath insulates the axon so the impulse jumps along it.
Check. Speed matters for quick reflexes. ✓
Method. Link the neurone to the features of a cell.
- Cell. A neurone has a cell body with a nucleus, like other cells.cell body with a nucleus
- Adapted. It is also adapted for carrying impulses, with long dendrites and an axon.carrying impulses
- Write the answer. Accept any sentence that says it has a cell body with a nucleus and is adapted to carry impulses along its length.
Check. Structure fits the job of carrying messages. ✓
Reflex arcs
A reflex arc is the pathway for a reflex action. A stimulus is detected by a receptor. A sensory neurone carries the impulse to the spinal cord, where a relay neurone passes it on. A motor neurone carries the impulse to an effector (a muscle or gland), which produces the response.
Reflexes are fast because the impulse goes to the spinal cord and does not travel to the brain. They are automatic, so they protect the body without needing thought.
Key ideas
- A reflex arc: stimulus, receptor, sensory neurone, relay neurone, motor neurone, effector, response.
- The impulse goes to the spinal cord, not the brain, so the reflex is fast.
- Reflexes are automatic and protect the body.
- Effectors are muscles or glands.
Method. Recall the structure that detects stimuli.
- Recall. A receptor detects the stimulus.detects the stimulus
- Name it. So the part is the receptor.receptor
- Write the answer. receptor
Check. Receptors in the skin detect heat. ✓
Method. Recall the neurone inside the spinal cord.
- Recall. The relay neurone is inside the spinal cord.inside the spinal cord
- Name it. So it is the relay neurone.relay neurone
- Write the answer. relay neurone
Check. The relay neurone links the sensory and motor neurones. ✓
Method. Follow the impulse from the stimulus.
- Start. The receptor detects the stimulus first.receptor
- Next. The sensory neurone carries the impulse.sensory neurone
- Then. The motor neurone carries it on to the effector.motor neurone, effector
- Write the answer. receptor, sensory neurone, motor neurone, effector
Check. The effector gives the response. ✓
Watch out
Watch out: a reflex does not need the brain to make the decision. The brain is told afterwards, so the response happens first.
Exam tip
Exam tip. When you trace a reflex arc, name each step in order from the stimulus to the response. The spinal cord must be named as the place where the impulse is relayed.
Try it (with your teacher). With your teacher, sit with one leg crossed and the knee relaxed. A tap just below the kneecap makes the lower leg jump. Describe the reflex arc for this response, naming each part.
Exercise 11B Reflex arcs
Method. Recall the detector.
- Recall. The receptor detects the stimulus.detects the stimulus
- Write the answer. receptor
Check. Receptors can be in the skin or eyes. ✓
Method. Recall the neurone from the receptor.
- Recall. The sensory neurone carries the impulse from the receptor to the CNS.from the receptor
- Write the answer. sensory neurone
Check. It is the first neurone of the arc. ✓
Method. Recall the neurone inside the CNS.
- Recall. The relay neurone is inside the spinal cord.inside the spinal cord
- Write the answer. relay neurone
Check. It links the sensory and motor neurones. ✓
Method. Recall the last neurone.
- Recall. The motor neurone carries the impulse out to the effector.out to the effector
- Write the answer. motor neurone
Check. It is the last neurone of the arc. ✓
Method. Recall the term.
- Recall. An effector is the muscle or gland that produces the response.produces the response
- Write the answer. effector
Check. Effectors respond to impulses from motor neurones. ✓
Method. Recall the path of the impulse.
- Recall. In a reflex the impulse is relayed in the spinal cord, not the brain.in the spinal cord
- Write the answer. So no.no
- Write the answer. no
Check. This is why reflexes are fast. ✓
Method. Recall where the relay neurone is.
- Recall. The relay neurone sits in the spinal cord.sits in the spinal cord
- Write the answer. spinal cord
Check. The spinal cord is part of the CNS. ✓
Method. Follow the impulse from the stimulus.
- Start. The stimulus comes first.stimulus
- Next. The sensory neurone carries the impulse to the spinal cord.sensory neurone
- Then. The relay neurone passes it on, and the effector responds.relay neurone, effector
- Write the answer. stimulus, sensory neurone, relay neurone, effector
Check. The response comes last. ✓
Method. Compare the two pathways.
- Reflex. A reflex uses the spinal cord and does not go to the brain.spinal cord
- Conscious. A conscious response sends the impulse up to the brain and back, which takes longer.takes longer
- Write the answer. Accept any sentence that says the impulse does not travel to the brain, so there are fewer steps.
Check. Fewer steps means a faster response. ✓
Method. Think about the harm.
- Think. A hot object could burn the skin if the hand stayed still.burn the skin
- Conclude. A fast automatic response moves the hand away before the damage is done.before the damage
- Write the answer. Accept any sentence that says a fast, automatic response can move the body away from harm before damage is done.
Check. Reflexes are protective. ✓
Method. Find the muscle that moves the lower leg.
- Think. The muscle at the front of the thigh moves the lower leg.front of the thigh
- Name it. So the effector is the thigh muscle.thigh muscle
- Write the answer. Accept the muscle in the thigh, or the quadriceps.
Check. The effector is the muscle that gives the response. ✓
Method. Link the speed to the protection.
- Speed. Relaying in the spinal cord needs fewer steps than the brain.fewer steps
- Protect. A fast response protects the body from harm before the brain has time to decide.before the brain
- Conclude. So the spinal cord is used for speed, not thought.speed, not thought
- Write the answer. Accept any answer that says the brain is slower, and the reflex needs a fast response to protect the body.
Check. Speed and protection are the reasons. ✓
Reaction time and the ruler-drop practical
Reaction time is the time from a stimulus to the response. It is affected by factors such as tiredness, distraction, drugs, alcohol and age. It is made up of the time for the impulse to travel to the brain and back, and the time for the brain to decide.
In the ruler-drop test, a ruler is dropped and caught. The distance it falls is read from the ruler. The reaction time is then read from a table of distance and time, because the time depends on the distance the ruler falls.
Safety. Use only the ruler test. Do not give anyone a drink, a drug or alcohol for this practical. The factor tested is distraction, for example a noise or a question.
| Distance fallen by the ruler (cm) | Reaction time (s) |
|---|---|
| 5 | 0.10 |
| 20 | 0.20 |
| 45 | 0.30 |
| 80 | 0.40 |
| Condition (invented practice data) | Distance fallen (cm) | Reaction time (s) |
|---|---|---|
| no distraction | 20 | 0.20 |
| with a distracting noise | 45 | 0.30 |
| tired (after no sleep) | 80 | 0.40 |
Key ideas
- Reaction time is the time from stimulus to response.
- Factors include tiredness, distraction, drugs and alcohol.
- The ruler test: the distance fallen is read, and the time is found from a table.
- The factor tested is changed on purpose; the others are kept the same.
Method. Read the table for 20 cm.
- Find 20 cm. Find 20 cm in the first column of the table.20 cm
- Read. The reaction time for 20 cm is 0.20 s.0.20
- Write the answer. 0.20
Check. A longer fall takes a longer time. ✓
Method. Read the table for 80 cm.
- Find 80 cm. Find 80 cm in the table.80 cm
- Read. The reaction time for 80 cm is 0.40 s.0.40
- Write the answer. 0.40
Check. The reaction time doubles from 0.20 s. ✓
Method. Subtract the two reaction times.
- Read. The times are 0.30 s with the noise and 0.20 s with no distraction.0.30 and 0.20
- Subtract. The difference is 0.30 - 0.20 = 0.10 s.0.30 - 0.20 = 0.10
- Write the answer. 0.10
Check. 0.20 + 0.10 = 0.30. ✓
Watch out
Watch out: the reaction time is read from the table using the distance fallen, not from the distance you would expect. A longer fall gives a longer time.
Exam tip
Exam tip. When you compare reaction times, write both values and the difference, with the unit.
Try it (with your teacher). With your teacher and a partner, drop a ruler without warning, and catch it. Repeat five times with no distraction, then five times with a distraction such as counting aloud. Record the distance each time, and find the mean. Do not use any drink, drug or alcohol.
Exercise 11C Reaction time and the ruler-drop practical
Method. Find the variable changed on purpose.
- Find it. The factor tested, such as distraction, is changed on purpose.changed on purpose
- Write the answer. distraction (or the factor tested)
Check. The distance fallen is the dependent variable. ✓
Method. Find what is measured.
- Find it. The distance the ruler falls before it is caught is measured.distance the ruler falls
- Write the answer. distance fallen
Check. The reaction time is found from it. ✓
Method. Read the table for 5 cm.
- Read. The reaction time for 5 cm is 0.10 s.0.10
- Write the answer. 0.10
Check. The shortest fall gives the shortest time. ✓
Method. Read the table for 45 cm.
- Read. The reaction time for 45 cm is 0.30 s.0.30
- Write the answer. 0.30
Check. Longer than 20 cm. ✓
Method. Recall the factors that slow reaction time.
- Recall. Tiredness, distraction, drugs and alcohol slow reaction time.slow reaction time
- Choose. So tiredness slows reaction time.tiredness
- Write the answer. tiredness
Check. Being warm is not a usual factor in this list. ✓
Method. Recall the factor.
- Recall. Distraction, such as a noise, slows reaction time.a noise
- Write the answer. distraction
Check. Distraction is tested in the practical. ✓
Method. Recall the factors.
- Recall. Alcohol and some drugs slow the brain’s response.slow the brain
- Choose one. Any one of these is a correct answer.any one
- Write the answer. Accept any one of: alcohol, or a drug that sedates the brain.
Check. Never test this with a drink or a drug. ✓
Method. Read the table for the mean fall.
- Mean. The mean distance is 20 cm.20 cm
- Read. The reaction time for 20 cm is 0.20 s.0.20
- Write the answer. 0.20
Check. The mean is used before the time is read. ✓
Method. Subtract the two reaction times.
- Read. The times are 0.40 s when tired and 0.20 s with no distraction.0.40 and 0.20
- Subtract. The difference is 0.40 - 0.20 = 0.20 s.0.40 - 0.20 = 0.20
- Write the answer. 0.20
Check. 0.20 + 0.20 = 0.40. ✓
Method. Think about the repeats.
- Think. A single drop can be affected by chance.affected by chance
- Conclude. Repeats show the spread of results and give a more reliable mean.more reliable mean
- Write the answer. Accept any sentence that says repeats show the spread of results and give a more reliable mean.
Check. Repeats make the results more reliable. ✓
Method. Think about what must stay the same.
- Think. The ruler, the catcher and the starting position must be kept the same.kept the same
- Name one. So the same ruler is a control variable.same ruler
- Write the answer. Accept any one: the same ruler, the same catcher, or the same starting position.
Check. Only the factor being tested should change. ✓
Method. Describe the stages of reaction time.
- Travel. The impulse must travel to the brain and then to the muscle.travel
- Decide. The brain must also process the information and decide the response, which takes time.decide the response
- Write the answer. Accept any answer that says the brain must also decide what to do, which takes time after the impulse arrives.
Check. Reaction time includes processing as well as travel. ✓
The eye: focusing and the pupil reflex
The pupil reflex controls how much light enters the eye. In bright light, the circular muscles in the iris contract, so the pupil gets smaller. In dim light, the radial muscles contract, so the pupil gets larger. The light reaches the retina, where the receptors (light-sensitive cells) detect it.
The eye focuses by changing the shape of the lens. For near objects, the ciliary muscles contract, and the lens becomes fatter. For far objects, the ciliary muscles relax, and the lens becomes thinner.
Key ideas
- Bright light: the pupil gets smaller (circular muscles in the iris contract).
- Dim light: the pupil gets larger (radial muscles contract).
- The retina contains the light-sensitive receptors.
- For near objects the lens becomes fatter; for far objects it becomes thinner.
Method. Recall the coloured part of the eye.
- Recall. The iris is the coloured muscle that controls the pupil.coloured muscle
- Name it. So the part is the iris.iris
- Write the answer. iris
Check. The pupil is the hole in the middle. ✓
Method. Recall the pupil reflex.
- Recall. In bright light the circular muscles contract, so less light enters.less light enters
- Name it. So the pupil gets smaller.smaller
- Write the answer. smaller
Check. In dim light the pupil gets larger. ✓
Method. Recall the layer at the back of the eye.
- Recall. The light-sensitive receptors are at the back of the eye.at the back
- Name it. This layer is the retina.retina
- Write the answer. retina
Check. The lens focuses light on the retina. ✓
Watch out
Watch out: the pupil is the hole, not the coloured part. The iris is the coloured part, and it controls the size of the pupil.
Exam tip
Exam tip. When you describe the pupil reflex, say which muscles contract and what happens to the pupil in each light.
Try it (with your teacher). With your teacher and a partner, sit in a dim room for a minute. Then look at a bright window and watch the pupils of your partner. Describe what happens and name the reflex.
Exercise 11D The eye: focusing and the pupil reflex
Method. Recall the coloured muscle.
- Recall. The iris is the coloured muscle that changes the size of the pupil.changes the size
- Write the answer. iris
Check. The pupil is the hole in the iris. ✓
Method. Recall the pupil reflex.
- Recall. The circular muscles in the iris contract in bright light, so the pupil gets smaller.gets smaller
- Write the answer. gets smaller
Check. Less light then enters the eye. ✓
Method. Recall the pupil reflex in dim light.
- Recall. The radial muscles contract in dim light, so the pupil gets larger.gets larger
- Write the answer. gets larger
Check. More light then enters the eye. ✓
Method. Recall the layer.
- Recall. The retina contains the light-sensitive receptors.light-sensitive receptors
- Write the answer. retina
Check. The lens focuses light onto the retina. ✓
Method. Recall the focusing structure.
- Recall. The lens changes its shape to focus light on the retina.changes its shape
- Write the answer. lens
Check. The cornea also bends light, but does not change shape. ✓
Method. Recall the focusing for near objects.
- Recall. For near objects the ciliary muscles contract, and the lens becomes fatter.becomes fatter
- Write the answer. fatter
Check. A fatter lens bends light more. ✓
Method. Recall the focusing for far objects.
- Recall. For far objects the ciliary muscles relax, and the lens becomes thinner.relax
- Write the answer. relax
Check. A thinner lens bends light less. ✓
Method. Recall what a reflex is.
- Recall. The pupil reflex is an automatic response to light.automatic
- Write the answer. So it is not conscious.no
- Write the answer. no
Check. You cannot easily stop it. ✓
Method. Think about the hole.
- Think. The light enters through the hole in the middle of the iris, which is the pupil.hole in the middle
- Name it. So light enters through the pupil.pupil
- Write the answer. pupil
Check. The iris is the coloured ring around the pupil. ✓
Method. Think about too much light.
- Think. Too much light entering could dazzle or damage the receptors.dazzle
- Conclude. A small pupil limits the light, protecting the retina.protecting the retina
- Write the answer. Accept any sentence that says it limits the amount of light entering, so the retina is not damaged.
Check. Small pupil means less light. ✓
Method. Describe each change.
- Near. For a near object the lens becomes fatter.fatter
- Far. For a far object the lens becomes thinner.thinner
- Write the answer. Accept any sentence that says the lens becomes fatter for near and thinner for far.
Check. The change in shape focuses the image. ✓
Method. Link the reflex to the image.
- Adjust. The pupil adjusts the amount of light entering the eye automatically.adjusts the amount
- Image. Good light levels on the retina keep the image clear in both bright and dim conditions.keep the image clear
- Write the answer. Accept any answer that says it adjusts the amount of light entering the eye automatically, and keeps the image clear in different light.
Check. It works without thought. ✓
Synapses and neurotransmitters [H]
[H] Higher tier only. This section is higher-tier content. It describes the synapse as a chemical junction between neurones.
[H] A synapse is the junction between two neurones, with a small gap between them. When an impulse reaches the end of the first neurone, it releases a neurotransmitter. The neurotransmitter diffuses across the gap and binds to receptors on the second neurone, which triggers a new impulse. The transmission is one way: from the first neurone to the second.
Key ideas
- [H] A synapse is a gap between two neurones.
- [H] A neurotransmitter is released from the first neurone and diffuses across the gap.
- [H] The neurotransmitter binds to receptors on the second neurone, which starts a new impulse.
- [H] Synapses allow impulses to pass in one direction only.
Method. Recall the junction.
- Recall. The junction between two neurones is called a synapse.a synapse
- Name it. So the gap is the synapse.synapse
- Write the answer. synapse
Check. The gap is very small. ✓
Method. Recall the messenger.
- Recall. The chemical that crosses the synapse is a neurotransmitter.crosses the synapse
- Name it. So the chemical is a neurotransmitter.neurotransmitter
- Write the answer. neurotransmitter
Check. Neurotransmitters are released from the first neurone. ✓
Method. Think about where the neurotransmitter is released.
- Think. The neurotransmitter is released only from the end of the first neurone.released only from the first
- Conclude. So the impulse crosses the synapse in one direction only.one direction only
- Write the answer. The neurotransmitter is released only from the first neurone, so the impulse passes in one direction only.
Check. The receptors are only on the second neurone. ✓
Watch out
Watch out: the neurotransmitter does not carry the impulse across the gap as an electrical signal. It diffuses across the gap, and then a new impulse is started.
Exam tip
Exam tip. [H] Say that the neurotransmitter diffuses across the synapse and binds to receptors. Do not say it “carries the impulse”.
Try it (with your teacher). With your teacher, look at a labelled diagram of a synapse. Label the pre-synaptic neurone, the post-synaptic neurone, the gap, and the neurotransmitter, and draw an arrow to show the direction.
Exercise 11E [H] Synapses and neurotransmitters
Method. Recall the junction.
- Recall. The junction between two neurones is a synapse.junction
- Write the answer. synapse
Check. Synapses are very small gaps. ✓
Method. Recall the chemical.
- Recall. A neurotransmitter is released and diffuses across the synapse.diffuses across
- Write the answer. neurotransmitter
Check. Neurotransmitters trigger a new impulse. ✓
Method. Recall the movement.
- Recall. The neurotransmitter diffuses across the small gap.diffuses across
- Write the answer. diffuse
Check. Diffusion is passive. ✓
Method. Recall the binding site.
- Recall. The neurotransmitter binds to receptors on the second neurone.binds to receptors
- Write the answer. receptors
Check. Binding starts a new impulse. ✓
Method. Recall the direction.
- Recall. The neurotransmitter is released only from the first neurone.released only from the first
- Name it. So transmission is one way.one way
- Write the answer. one way
Check. The impulse passes from the first neurone to the second. ✓
Method. Recall where it is released.
- Recall. The neurotransmitter is released from the end of the first neurone.end of the first neurone
- Write the answer. the end of the first neurone
Check. Receptors are on the second neurone. ✓
Method. Recall the movement across the gap.
- Recall. The neurotransmitter moves across the gap by diffusion.moves across the gap
- Write the answer. diffusion
Check. Diffusion moves particles down a gradient. ✓
Method. Think about the neurones.
- Think. Neurones do not touch, so an electrical signal cannot cross directly.do not touch
- Conclude. So a chemical passes the message across the gap to the next neurone.chemical
- Write the answer. Accept any sentence that says the gap lets the impulse be passed on, through a chemical, to the next neurone.
Check. The chemical bridges the gap. ✓
Method. Think about what happens when the chemical binds.
- Think. The neurotransmitter binds to the receptors.binds to the receptors
- Conclude. This binding starts a new impulse in the second neurone.new impulse
- Write the answer. Accept any sentence that says the receptors bind the neurotransmitter, which starts a new impulse.
Check. Binding is the trigger for the next impulse. ✓
Method. Link binding to the impulse.
- Block. If the receptors are blocked, the neurotransmitter cannot bind to them.cannot bind
- Conclude. So no new impulse starts in the second neurone, and the response is slowed.slowed
- Write the answer. Accept any answer that says the neurotransmitter cannot bind, so no new impulse is started in the second neurone.
Check. Receptors are the target of many drugs. ✓
Chapter summary
| Skill | Method |
|---|---|
| Neurones | Sensory (receptor to CNS), relay (inside CNS), motor (CNS to effector). |
| Reflex arc | Stimulus, receptor, sensory neurone, relay neurone in the spinal cord, motor neurone, effector, response. |
| Reaction time | Time from stimulus to response; read from a table using the distance fallen in the ruler test. |
| Factors slowing reaction | Tiredness, distraction, drugs and alcohol. |
| Pupil reflex | Bright light: pupil smaller; dim light: pupil larger. |
| Focusing | Near: lens fatter; far: lens thinner (ciliary muscles). |
| [H] Synapse | A gap between neurones; a neurotransmitter diffuses across it and binds to receptors. |
Can you…?
- I can name the parts of the nervous system and the three types of neurone
- I can trace a reflex arc from the stimulus to the response
- I can calculate a reaction time from a table and explain the factors that slow it
- I can describe the pupil reflex and how the eye focuses
- [H] I can describe a synapse and the role of a neurotransmitter
Key terms
- central nervous system
- the brain and the spinal cord
- neurone
- a nerve cell that carries electrical impulses
- reflex arc
- the pathway of a reflex action from receptor to effector
- receptor
- a cell or structure that detects a stimulus
- effector
- a muscle or gland that produces a response
- reaction time
- the time from a stimulus to the response
- iris
- the coloured muscle that controls the size of the pupil
- retina
- the layer at the back of the eye that contains light-sensitive receptors
- [H] synapse
- the junction between two neurones
- [H] neurotransmitter
- a chemical released at a synapse that diffuses across it
Exercise 11Z Check-up
Method. Recall the name.
- Recall. The brain and spinal cord make up the central nervous system.central nervous system
- Write the answer. central nervous system
Check. It is abbreviated CNS. ✓
Method. Recall the neurone.
- Recall. Sensory neurones carry impulses from receptors to the CNS.from receptors
- Write the answer. sensory neurone
Check. Motor neurones go the other way. ✓
Method. Recall the neurone inside the CNS.
- Recall. Relay neurones are inside the CNS.inside the CNS
- Write the answer. relay neurone
Check. They link other neurones. ✓
Method. Recall the term.
- Recall. The effector is the muscle or gland that gives the response.gives the response
- Write the answer. effector
Check. The effector follows the motor neurone. ✓
Method. Recall the first step of the arc.
- Recall. A reflex arc starts with a stimulus, which the receptor detects.starts with a stimulus
- Write the answer. stimulus
Check. The receptor detects the stimulus. ✓
Method. Read the table.
- Read. The reaction time for 20 cm is 0.20 s.0.20
- Write the answer. 0.20
Check. The table is the key to this test. ✓
Method. Recall the factor.
- Recall. Distraction slows reaction time.slows reaction time
- Write the answer. distraction
Check. Tiredness also slows reaction time. ✓
Method. Recall the pupil reflex.
- Recall. In bright light the pupil gets smaller.gets smaller
- Write the answer. smaller
Check. Dim light makes the pupil larger. ✓
Method. Recall the layer.
- Recall. The retina contains the light-sensitive receptors.light-sensitive receptors
- Write the answer. retina
Check. The lens focuses light onto it. ✓
Method. Compare the pathways.
- Reflex. A reflex is relayed in the spinal cord and does not go to the brain.spinal cord
- Conclude. So the response happens with fewer steps and is faster.fewer steps
- Write the answer. Accept any sentence that says the impulse does not go to the brain, so there are fewer steps.
Check. Fewer steps mean a faster response. ✓
Total 24 marks. Allow about 24 minutes.
Exercise 11X Exam-style questions
Method. Recall the neurone.
- Recall. Motor neurones carry impulses from the CNS to effectors.from the CNS
- Write the answer. motor neurone
Check. Sensory neurones go the other way. ✓
Method. Recall the muscle.
- Recall. The iris changes the size of the pupil.changes the size of the pupil
- Write the answer. iris
Check. The pupil is the hole. ✓
Method. Name the factor, then the test.
- Factor. Distraction slows reaction time.distraction
- Test. A noise is made while the ruler is dropped, to test the effect of distraction.noise is made
- Write the answer. Accept any answer that names distraction, and says a noise is made while the ruler is dropped.
Check. Keep the other conditions the same. ✓
Method. List the parts in order.
- Start. The receptor detects the stimulus.receptor
- Middle. The sensory neurone carries the impulse to the spinal cord, where the relay neurone passes it on.relay neurone
- End. The motor neurone carries the impulse to the effector, which gives the response.effector
- Write the answer. Accept any three of: receptor; sensory neurone; relay neurone in the spinal cord; motor neurone; effector.
Check. Each part passes the impulse to the next. ✓
Method. Read each time, then subtract.
- Tired. The reaction time for 80 cm is 0.40 s.0.40
- No distraction. The reaction time for 20 cm is 0.20 s.0.20
- Difference. The difference is 0.40 - 0.20 = 0.20 s.0.20
- Write the answer. Accept 0.40, 0.20 and 0.20.
Check. 0.20 + 0.20 = 0.40. ✓
Method. Describe each light condition.
- Bright. In bright light the circular muscles in the iris contract, so the pupil gets smaller.circular muscles
- Dim. In dim light the radial muscles contract, so the pupil gets larger.radial muscles
- Write the answer. Accept any answer that says circular muscles in the iris contract in bright light so the pupil gets smaller, and radial muscles contract in dim light so the pupil gets larger.
Check. Two sets of muscles work in opposition. ✓
Method. Describe each case.
- Near. For near objects the ciliary muscles contract and the lens becomes fatter.becomes fatter
- Far. For far objects the ciliary muscles relax and the lens becomes thinner.becomes thinner
- Write the answer. Accept any answer that says the ciliary muscles contract and the lens becomes fatter for near objects, and relax and the lens becomes thinner for far objects.
Check. The change in shape focuses the light. ✓
Method. Find the value that does not fit, then the mean without it.
- Anomaly. The value 45 cm is much larger than the other four results.45 cm
- Mean. Without it, the four values 20, 20, 20 and 20 give a mean of 20 cm.20 cm
- Reason. The anomaly is only removed if there is a stated reason, such as a mistake in the drop.stated reason
- Write the answer. Accept 45 cm as the anomaly and 20 cm as the mean.
Check. Check the reason before removing a result. ✓
Method. Compare speed and duration.
- Nervous. The nervous system sends electrical impulses along neurones, which is fast and brief.fast and brief
- Hormones. Hormones travel in the blood, which is slower and the effect lasts longer.slower and lasts longer
- Write the answer. Accept any answer that says the nervous system acts quickly and briefly, while hormones act more slowly and for longer.
Check. Speed and duration are the two points. ✓
Method. Follow the chemical across the gap.
- Release. The neurotransmitter is released from the first neurone.released
- Diffuse. It diffuses across the synapse.diffuses across
- Bind. It binds to receptors on the second neurone, starting a new impulse.new impulse
- Write the answer. Accept any three of: released from the first neurone; diffuses across the gap; binds to receptors on the second neurone; starts a new impulse.
Check. Each point follows from the one before. ✓