Investigating reaction times

Cells and control · Neurones and reflexes · note 5 of 5

Spec 2.13, 2.14Download all Neurones and reflexes notes (PDF)Download all notes (PDF)7 pages

Investigating reaction timesSpec 2.13, 2.14

In short

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.

The suggested practicals ask you to investigate human responses to external stimuli, reaction times and the speed of transmission of electrical impulses in the nervous system.

Exam tip:

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. They drop it without warning and the other person catches it. Record the distance it fell. A shorter distance means a faster reaction time.

Independent variable: the factor you change, such as hand used, practice or a distraction.
Control variables: the same person dropping the ruler, the same ruler, the same starting position and the same person catching.
Repeat several times and calculate a mean. This reduces the effect of random errors and lets you spot anomalous results.
A reaction time is the time between a stimulus and a response.

Typical results: practice usually makes the response quicker, and distractions usually make it slower.

Mean distance

A student catches a ruler five times. The distances it fell were 20, 17, 15, 14 and 12 cm. Calculate the mean.

add the results: 20 + 17 + 15 + 14 + 12 = 78
divide by the number of results: 78 ÷ 5

Answer: 15.6 cm

Maths skill:

If you plot reaction time against a variable on a scatter diagram, look for a correlation. Points sloping down show a negative correlation: as one variable rises, the other falls.

Quick check

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?
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It insulates the neurone and speeds up the transmission of impulses.
What is a synapse?
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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.

Written and checked against the Edexcel GCSE Combined Science (1SC0) specification · Updated October 2026

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.

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