The human nervous system — AQA GCSE Combined Science: Trilogy
How neurones, the CNS and reflex arcs give fast, automatic responses, and the required practical on reaction time.
Revision notes
3 short notes, in the order of the specification. Each one in short:
The nervous system enables humans to react to their surroundings and to coordinate their behaviour. Receptors detect a stimulus, and neurones carry information as electrical impulses to the central nervous system (CNS), which is the brain and spinal cord. The CNS coordinates the response of effectors: muscles contracting or glands secreting hormones.
A reflex action is an automatic and rapid response that does not involve the conscious part of the brain. In a reflex arc, an impulse passes from a receptor along a sensory neurone to the CNS, through a relay neurone, then along a motor neurone to an effector. Reflexes protect the body from harm.
Required practical: reaction time
PracticalReaction time is the time taken to respond to a stimulus. In this required practical, a partner drops a ruler without warning and the volunteer catches it as fast as possible: a shorter distance fallen means a faster reaction. You change one factor, such as caffeine or practice, keep the other variables the same, and repeat to calculate a mean.
Exam questions
3 exam-style questions (9 marks), each with its mark scheme.
Answer the questionsFlashcards
8 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of homeostasis and response on one page, so you can see where this subtopic fits.
Open the mind mapPractical sheet
Method, variables, risks and a results table, with questions to answer.
Print it
Free PDFs to print or save.
Quick check questions
What is the CNS?
Show answer
The central nervous system: the brain and spinal cord.
Put these in order: motor neurone, sensory neurone, relay neurone.
Show answer
Sensory neurone, relay neurone, motor neurone.
Why are reflex actions important?
Show answer
They are automatic and rapid, protecting the body from harm without involving the conscious part of the brain.
Frequently asked questions
What is a reflex arc?
A reflex arc is the pathway of an automatic, rapid response. A receptor detects a stimulus, a sensory neurone carries the impulse to the CNS, a relay neurone passes it on, and a motor neurone carries it to an effector, a muscle or gland, which responds. It does not involve the conscious part of the brain.
Why are reflexes faster than normal responses?
Reflexes are faster because the reflex arc does not go through the conscious part of the brain. The impulse passes from the sensory neurone, through a relay neurone, to the motor neurone, so the response happens before you are aware of it. This protects the body, for example pulling your hand away from a hot object.
How does a nerve impulse cross the synapse?
A nerve impulse crosses a synapse using chemicals. A synapse is a gap between two neurones. When an impulse reaches the end of one neurone, chemicals diffuse across the gap and start an impulse in the next neurone. This is how an impulse passes from a sensory neurone to a relay neurone, and on to a motor neurone.
Written and checked against the AQA GCSE Combined Science (8464) specification · Updated October 2026