Teacher sheet · Ages 15–16 · about 45–50 minutes
Neurons, action potentials and reaction time
Lesson page: echoesof.science/school/brain/ages-15-16/ · Pupil worksheet: echoesof.science/school/brain/ages-15-16/print/
Learning aims
- Describe neuron structure, grey and white matter and the divisions of the nervous system.
- Explain the resting and action potential with values and ion movements.
- Explain saltatory conduction and synaptic transmission.
- Calculate reaction time from free fall and evaluate the data with uncertainty.
Lesson plan
| Time | Activity |
|---|---|
| 5 min | Starter: 86 billion neurons, 16 W. Is the brain 'expensive'? Compare with a light bulb. |
| 10 min | Read the text; sketch an action potential graph. |
| 20 min | Reaction-time investigation. |
| 5 min | Calculations: t = √(2d/g), E = V/d, E = Pt. |
| 5 min | Quiz or action potential game. |
Materials for the experiment
- A 30 cm or 50 cm ruler per pair
- Results table
- Calculator
- Optional: a metre rule to measure the eye–brain–hand path roughly
Safety: Drop the ruler straight down over a short distance and keep faces clear. Use rulers without sharp edges. The 'sound' catcher keeps eyes closed only while seated and still.
Common misconceptions
| Pupils may think… | Actually… |
|---|---|
| A bigger stimulus gives a bigger action potential. | Action potentials are all-or-nothing; a stronger stimulus does not make one larger. |
| Nerve signals are electric currents flowing like in a wire at near light speed. | They are waves of membrane depolarisation; unmyelinated they move at about 1 m/s. |
| Resting potential has one exact value. | It varies between neurons; textbooks quote values from about −70 to −90 mV. |
Worksheet answers
- At rest about −70 mV (Na⁺ outside, K⁺ inside); at threshold about −55 mV Na⁺ channels open, Na⁺ enters, potential rises to about +40 mV; K⁺ leaves and the membrane repolarises.
- Myelin insulates the axon so the signal is regenerated only at the nodes of Ranvier (saltatory conduction); white matter is mostly myelinated axons.
- Answers depend on data; compare the difference between means with the uncertainties (half the range).
- For example a path of about 1.5 m at 1 m/s would take about 1.5 s, but the measured reaction time is about 0.2 s, so the signal must travel much faster along most of the path.
Quiz answers
- A ruler is caught after falling 0.20 m. What is the reaction time? (g = 9.80 m/s²) About 0.20 s. t = √(2 × 0.20 / 9.80) ≈ 0.20 s.[9]
- What is the typical threshold potential for an action potential? About −55 mV. The membrane must reach about −55 mV to fire.[3]
- Which ion flows into the neuron during depolarisation? Na⁺. Na⁺ channels open and Na⁺ enters, making the inside positive.[3]
- 90 mV across an 8 nm membrane gives a field of about… 1.1 × 10⁷ V/m. E = 0.090 V / 8 × 10⁻⁹ m ≈ 1.1 × 10⁷ V/m = 11 MV/m.[4]
- Why does myelin speed up nerve signals? The signal is regenerated only at the nodes of Ranvier. Saltatory conduction jumps between nodes.[3][4]
- The brain uses about 16 W. How much energy is that in one day (86 400 s)? About 1.4 MJ. E = P × t = 16 × 86 400 ≈ 1.38 × 10⁶ J.[5]
Going further
Calculate the minimum average signal speed needed if your path is 1.5 m and half of your reaction time is spent conducting signals. What does this suggest about the axons involved?
Sources used in this lesson
- [2] Anatomy and Physiology, 12.1 Basic Structure and Function of the Nervous System (OpenStax, Rice University): reliability High
- [3] Biology for AP Courses, 26.2 How Neurons Communicate (OpenStax, Rice University): reliability High
- [4] College Physics 2e, 20.7 Nerve Conduction – Electrocardiograms (OpenStax, Rice University): reliability High
- [5] College Physics 2e, 7.8 Work, Energy, and Power in Humans (OpenStax, Rice University): reliability High
- [6] How Many Neurons Are in the Brain? (BrainFacts.org, Society for Neuroscience): reliability High
- [7] Reaction Time (Neuroscience for Kids) (Eric H. Chudler, University of Washington): reliability Medium-high
- [8] Reaction Time (outreach activity) (Neuroscience Program, Michigan State University): reliability Medium-high
- [9] College Physics, 2.7 Falling Objects (OpenStax, Rice University): reliability High
Full list with links and a note on each source's reliability: echoesof.science/school/brain/sources/
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