Year 9 · Science · Biological sciences
The nervous system and reaction time
Students learn how the nervous system detects and responds to stimuli, compare reflexes with voluntary responses, and measure their reaction time with a falling ruler.
Awaiting educator review. This material was drafted by our content team with AI assistance and is waiting for review by a qualified educator. Please check it suits your child before using it. Created 10 October 2026.
Learning goals
Learners will
- Describe the pathway of a nerve impulse from receptor to effector.
- Distinguish between reflex actions and voluntary responses.
- Measure and analyse reaction time using repeated trials.
Success looks like
- I can sequence stimulus, receptor, sensory neuron, central nervous system, motor neuron, effector and response.
- I can explain why a reflex is faster than a voluntary response.
- I can convert a ruler-drop distance to a reaction time using a table and calculate an average.
The big idea
The nervous system lets your body detect changes and respond quickly. It includes the central nervous system (brain and spinal cord) and nerves that reach every part of the body.
A response follows a pathway: stimulus → receptor → sensory neuron → central nervous system → motor neuron → effector → response. For example, seeing a ball coming (stimulus detected by the eyes) leads to the arm muscles (effectors) moving to catch it.
Some responses are reflexes. They are automatic and very fast because the pathway is short and often passes through the spinal cord without needing a conscious decision in the brain. Pulling your hand away from a hot pan is a reflex.
Catching a falling ruler is a voluntary response: your brain must process what you see and decide to close your fingers. This takes longer than a reflex.
We can measure reaction time with a ruler. Because gravity makes objects fall faster and faster, the distance the ruler falls before you catch it tells you how long you took. Use this table (calculated from the physics of falling objects):
- 5 cm ≈ 0.10 s; 10 cm ≈ 0.14 s; 15 cm ≈ 0.17 s; 20 cm ≈ 0.20 s; 25 cm ≈ 0.23 s; 30 cm ≈ 0.25 s
Reaction times vary between trials, so scientists repeat measurements and calculate an average. Results that are very different from the others (outliers) should be investigated rather than ignored.
Reaction time matters in everyday life. It is one reason drivers must leave a safe gap behind the car in front, and why fatigue and distractions such as phones increase crash risk.
Worked examples
The ruler-drop test
How do you measure your reaction time with a ruler?
- Sit with your forearm resting on a table and your hand over the edge.
- Your partner holds the ruler vertically so the 0 cm mark is level with the top of your open thumb and finger.
- Without warning, your partner lets go. Catch the ruler as quickly as you can.
- Read the mark level with the top of your thumb. That is the distance fallen.
- Repeat at least five times and convert each distance to a time using the table.
Answer: Record the distance, repeat, convert to time and average.
Calculating an average
Catches were 18 cm, 22 cm and 20 cm. What is the average distance and the approximate reaction time?
- Add: 18 + 22 + 20 = 60 cm.
- Divide by 3: 60 ÷ 3 = 20 cm.
- From the table, 20 cm ≈ 0.20 s.
Answer: Average 20 cm, so a reaction time of about 0.20 s.
Practice check
Have a go, then check your answers. Each answer comes with an explanation.
Answer guide for parents
Which part of the body is an effector?
A muscle in the arm — Effectors are muscles or glands that carry out a response.
Why is a reflex faster than catching a ruler?
The reflex pathway is shorter and does not need a conscious decision — Reflexes bypass conscious decision-making, so the pathway is shorter.
Which is the correct order?
Stimulus → receptor → sensory neuron → CNS → motor neuron → effector — Information travels from the stimulus, through the CNS, to the effector.
Why should you repeat the ruler test several times?
To reduce the effect of random variation and get a reliable average — Averaging repeated trials makes results more reliable.
Which is a reflex action?
Blinking when something flies towards your eye — Blinking to protect the eye is automatic.
Catches were 14 cm, 15 cm and 16 cm. What is the average distance in cm? (number)
15 — (14 + 15 + 16) ÷ 3 = 45 ÷ 3 = 15 cm.
Using the table, what is the reaction time in seconds for a 10 cm catch? (decimal)
0.14 — The table shows 10 cm ≈ 0.14 s.
The brain and spinal cord together are called the central ______ system.
nervous — They make up the central nervous system.
Watch out for
- Some think all responses are controlled by conscious thought. Many reflexes are coordinated through the spinal cord.
- Students may assume a longer catch distance means a faster reaction. A longer distance means a slower reaction.
- Some think the nervous system and hormones are the same. Hormones (endocrine system) travel in the blood and usually act more slowly.
Tips for parents
- Make sure the person dropping the ruler varies the timing so the catcher cannot predict it.
- Try the test with the dominant and non-dominant hand and compare averages.
- Safety: use a plastic ruler, not a metal one, and keep fingers clear of the table edge.
Go further
- Test whether a distraction (such as answering simple maths questions aloud) changes reaction time.
- Use the formula d = ½gt² (with g ≈ 9.8 m/s²) to check the table values yourself.
- Research how the endocrine system controls blood sugar using insulin.