Neurones and reflexes — Edexcel GCSE Combined Science
Sensory receptors, the three types of neurone, the structure of neurones, synapses and neurotransmitters, and how a reflex arc works.
Revision notes
5 short notes, in the order of the specification. Each one in short:
A nervous pathway carries information from a sensory receptor to an effector. A receptor detects a stimulus and produces an electrical impulse. A sensory neurone carries it to the central nervous system, a relay neurone passes it on, and a motor neurone carries it to an effector, a muscle or gland, which produces a response.
Neurones are cells specialised to carry electrical impulses quickly over long distances. The axon is a long fibre that carries impulses away from the cell body, and the dendron carries impulses towards it. The myelin sheath is a fatty layer that insulates the neurone and speeds up the transmission of impulses.
A synapse is a gap between two neurones. The electrical impulse cannot cross the gap, so the first neurone releases a chemical called a neurotransmitter. The neurotransmitter diffuses across the synapse and binds to receptors on the next neurone, which triggers a new electrical impulse. The impulse itself never crosses the gap.
A reflex is a fast, automatic response to a stimulus that does not involve conscious thought, and it protects the body from harm. It follows a reflex arc: receptor, sensory neurone, relay neurone, motor neurone and effector. It is fast because the impulses do not have to travel to the conscious part of the brain first.
Reaction time is the time between a stimulus and a response. It can be measured with the ruler-drop test: one person drops a ruler without warning and the other catches it between thumb and finger. The shorter the distance the ruler falls, the faster the reaction. Repeat several times and calculate a mean.
Exam questions
7 exam-style questions (19 marks), each with its mark scheme.
Answer the questionsFlashcards
12 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of cells and control on one page, so you can see where this subtopic fits.
Open the mind mapPrint it
Free PDFs to print or save.
Quick check questions
Name the three types of neurone in a reflex arc, in order.
Show answer
Sensory neurone, relay neurone, motor neurone.
What does the myelin sheath do?
Show answer
It insulates the neurone and speeds up the transmission of impulses.
What is a synapse?
Show answer
A gap between two neurones.
How does an impulse cross a synapse?
Show answer
A neurotransmitter is released, diffuses across the gap and triggers a new impulse in the next neurone.
Why are reflexes important?
Show answer
They are fast and automatic, so they protect the body from harm.
Frequently asked questions
What is a reflex arc?
A reflex arc is the pathway followed in a reflex, a fast, automatic response that does not involve conscious thought. A receptor detects a stimulus, a sensory neurone carries impulses to the CNS, a relay neurone passes them to a motor neurone, and the motor neurone carries them to an effector, which responds.
Why are reflexes faster than normal responses?
Reflexes are faster because the impulses do not have to travel to the conscious part of the brain before the response. The pathway goes from the sensory neurone through a relay neurone in the CNS straight to the motor neurone, so the effector responds automatically. This speed protects the body from harm.
How does a nerve impulse cross the synapse?
A nerve impulse cannot cross the synapse as an electrical impulse. Instead, when it reaches the end of the first neurone, a chemical called a neurotransmitter is released into the gap. The neurotransmitter diffuses across and binds to receptors on the next neurone, which triggers a new electrical impulse in that neurone.
What are the three types of neurone?
The three types of neurone are sensory, relay and motor neurones. Sensory neurones carry impulses from a receptor to the central nervous system. Relay neurones, found in the CNS, connect sensory neurones to motor neurones. Motor neurones carry impulses from the CNS to an effector, such as a muscle or a gland.
How do you measure reaction time with a ruler?
Use the ruler-drop test. One person holds a ruler vertically with the zero level with the top of the other person's open thumb and finger, then drops it without warning. The other person catches it, and the distance it fell is recorded. A shorter distance means a faster reaction. Repeat and calculate a mean.
Written and checked against the Edexcel GCSE Combined Science (1SC0) specification · Updated October 2026