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AQA GCSE Biology

The nervous system and reflexes — AQA GCSE Biology revision

Free revision notes, key terms, common exam traps and 5 practice questions with answers. About 4 minutes to read.

Control basics

  • Homeostasis keeps internal conditions (temperature, blood glucose, water) stable for enzymes and cells.
  • Control systems need a receptor, a coordination centre and an effector.

Reflex arc

  • Stimulus → receptor → sensory neurone → relay neurone (spinal cord) → motor neurone → effector → response.
  • Reflexes are automatic and fast because the conscious brain is not involved.
  • Synapse: the impulse crosses as a chemical, which slightly slows transmission.

Key terms

Receptor
Cell that detects a stimulus.
Effector
A muscle or gland that carries out the response.
Synapse
Gap between neurones crossed by chemicals.
Stimulus
A change in the environment that is detected and produces a response.
Central nervous system
The brain and spinal cord, where information is processed.
Coordination centre
The brain, spinal cord or pancreas — processes information and organises a response.

Common exam traps

  • Leaving out 'relay neurone' in reflex arc sequences.
  • Saying the brain controls reflexes — the point is that it doesn't.
  • Thinking the brain decides reflex responses — the spinal cord/relay neurone does, which is why they're so fast.

Practice questions with answers

  1. 1. Put the reflex arc in order.

    Answer: Stimulus → receptor → sensory neurone → relay neurone → motor neurone → effector → response

    Learn this sequence word for word.

  2. 2. Why are reflex actions fast?

    • • They involve fewer synapses and skip conscious brain processing
    • • The impulse travels as light
    • • They use hormones
    • • They only happen in the hand

    Answer: They involve fewer synapses and skip conscious brain processing

    Speed protects the body from harm.

  3. 3. Explain how an impulse crosses a synapse.

    Answer: A chemical is released, diffuses across the gap and triggers an impulse in the next neurone.

    Chemical + diffusion + new impulse.

  4. 4. Which part of the eye detects light?

    • • Cornea
    • • Retina
    • • Lens
    • • Iris

    Answer: Retina

    The retina contains light-sensitive receptor cells that turn light into nerve impulses.

  5. 5. State the function of the myelin sheath around some neurones.

    Answer: It insulates the axon and speeds up the electrical impulse.

    Myelinated neurones transmit signals faster than unmyelinated ones.

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