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Nervous system and reflexes

Trace reflex pathways and explain fast responses.

Start here

The key idea

The nervous system detects stimuli and coordinates responses. Reflexes are rapid, automatic responses that reduce harm.

Reflex arcTrace the impulse from stimulus to a protective response.
Reflex arcTrace the impulse from stimulus to a protective response.receptorsensoryCNSmusclefast, automatic response
Revision notes

The bit that matters

Learn the process in clean chunks. If a sentence explains a cause, make sure you can say the effect too.

1

The nervous system

The nervous system lets humans react to their surroundings and coordinate behaviour.Receptors detect stimuli and send electrical impulses along sensory neurones to the central nervous system, which is the brain and spinal cord.The central nervous system processes the information and sends impulses along motor neurones to effectors, which are muscles or glands that carry out a response.

2

The reflex arc

Reflex actions are rapid and automatic and do not involve the conscious part of the brain, which helps protect the body from harm.In a reflex arc an impulse passes from a receptor along a sensory neurone to a relay neurone in the spinal cord, then along a motor neurone to an effector.Because the brain is not involved the response is very fast.

3

Synapses

A synapse is the tiny gap between two neurones.When an impulse reaches the end of a neurone it triggers the release of chemicals called neurotransmitters, which diffuse across the gap and bind to receptors on the next neurone.This sets off a new electrical impulse in the next neurone, allowing the signal to continue.

4

The brain

The brain is made of billions of interconnected neurones and controls complex behaviour.The cerebral cortex is concerned with consciousness, intelligence, memory and language, the cerebellum coordinates muscular activity and balance, and the medulla controls unconscious activities such as heart rate and breathing.Studying the brain is difficult because it is complex, delicate and easily damaged.

Key terms

Definitions to learn

Stimulus

A change in the environment that is detected by a receptor.

Receptor

A cell or organ that detects a stimulus.

Central nervous system

The brain and spinal cord that coordinate responses.

Effector

A muscle or gland that carries out a response.

Synapse

The gap between two neurones across which neurotransmitters diffuse.

Reflex action

A fast automatic response that does not involve the conscious brain.

Worked example

Trace the pathway when a person touches a hot object.

1

A receptor detects the heat stimulus.

2

An impulse travels along a sensory neurone to the CNS.

3

It passes through a relay neurone and then a motor neurone.

4

An effector muscle contracts.

Final answer

The hand withdraws rapidly through a reflex arc.

Exam habit

Trace the reflex arc in full: receptor → sensory neurone → relay neurone → motor neurone → effector.Omitting the relay neurone is the most common error. Name both the effector type (muscle or gland) and the response.

Watch out

A reflex is automatic, not necessarily conscious.

Examiner tips

How to score full marks

  • 1Always give the pathway in the correct order: stimulus, receptor, sensory neurone, central nervous system, motor neurone, effector, response.
  • 2When explaining synapses, state that the signal becomes chemical then electrical again, naming the neurotransmitter diffusing across the gap.
  • 3Stress that reflexes are automatic and do not involve conscious thought, which is why they are fast and protective.
Practice questions

Try these yourself

Open each answer only after you have explained the full biological process.

1What is an effector?
Mark scheme
  1. 1.Define its role in a response.
A muscle or gland that carries out a response.
2Why are reflexes useful?
Mark scheme
  1. 1.Link speed to protection.
They are rapid and automatic, reducing the chance of injury.
3What happens at a synapse?
Mark scheme
  1. 1.Mention chemicals and diffusion.
A chemical neurotransmitter diffuses across the gap and triggers an impulse in the next neurone.
4Name the two organs that make up the central nervous system.[1 mark]
Mark scheme
  1. 1.Recall the coordinating organs.
Brain and spinal cord (1)
5What is an effector?[1 mark]
Mark scheme
  1. 1.State what carries out the response.
A muscle or gland that carries out a response (1)
6Describe how an impulse crosses a synapse.[3 marks]
Mark scheme
  1. 1.Mention neurotransmitter, diffusion and a new impulse.
A chemical called a neurotransmitter is released (1); it diffuses across the gap (1); and triggers a new electrical impulse in the next neurone (1)
7Explain why a reflex action is faster than a conscious response.[3 marks]
Mark scheme
  1. 1.Compare the pathway with one involving the brain.
A reflex does not involve the conscious part of the brain (1); the impulse passes through a relay neurone in the spinal cord (1); so the pathway is shorter and the response is automatic and faster (1)
8A person touches a hot pan and immediately pulls their hand away before they feel pain. Describe the full reflex arc involved in this response.[5 marks]
Mark scheme
  1. 1.Name each structure in order.
  2. 2.Explain why this protects the body.
Heat is the stimulus detected by a receptor in the skin (1); an impulse passes along a sensory neurone to the spinal cord (1); it passes through a relay neurone to a motor neurone (1); the impulse reaches an effector muscle which contracts to move the hand away (1); the response is automatic and fast so it protects the body from damage (1)
9State the names of the three types of neurone involved in a spinal reflex arc and state the role of each.[3 marks]
Mark scheme
  1. 1.Name sensory, relay and motor neurones.
  2. 2.Give the function of each.
Sensory neurone carries impulses from receptor to spinal cord (1); relay neurone connects sensory to motor neurone within the spinal cord (1); motor neurone carries impulses from spinal cord to effector (1).
10Explain why some drugs that affect neurotransmitters at synapses can alter behaviour and mood.[4 marks]
Mark scheme
  1. 1.Link neurotransmitters to transmission of nerve impulses.
  2. 2.Explain how changing their concentration affects signalling.
Neurotransmitters transmit signals between neurones at synapses (1); drugs can mimic neurotransmitters and bind to the same receptors, triggering extra impulses (1); or block receptors, preventing normal neurotransmitter binding and reducing signalling (1); changing the pattern of impulses in the brain alters mood, perception or behaviour (1).
11Describe the function of the cerebral cortex, the cerebellum and the medulla oblongata.[3 marks]
Mark scheme
  1. 1.Give one function for each brain region.
Cerebral cortex: controls consciousness, memory, language and intelligence (1); cerebellum: coordinates muscular movement and balance (1); medulla oblongata: controls unconscious activities such as heart rate and breathing rate (1).
12Explain why scientists find it difficult to map the exact functions of different areas of the brain.[4 marks]
Mark scheme
  1. 1.Link to the complexity, delicacy and individual variation of the brain.
The brain contains billions of neurones with highly complex interconnections (1); it is delicate and easily damaged so experiments are difficult to perform ethically (1); brain function can be studied after injury but the results are variable between individuals (1); current scanning technology does not have high enough resolution to map individual synaptic connections (1).
13A student accidentally touches a drawing pinpin and withdraws their finger. They become aware of the pain approximately 0.5 seconds later. Explain this time difference in terms of the nervous system pathways involved.[4 marks]
Mark scheme
  1. 1.Distinguish the reflex arc pathway from the conscious pain pathway.
  2. 2.Explain why the conscious pathway takes longer.
The withdrawal reflex uses a short pathway through the spinal cord only and does not involve the brain, so it occurs very rapidly (1); the pain sensation travels along a separate pathway via sensory neurones up the spinal cord to the cerebral cortex of the brain (1); this longer pathway with more synapses to cross takes additional time (1); hence the student moves their finger before consciously feeling the pain (1).
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