How Fast Are Neurons?

Ever wondered how quickly a nerve impulse travels? In this experiment, you’ll measure conduction velocity—the speed at which spikes race along an axon—using the earthworm’s giant fibers as your model.

Devices and Accessories3

About the Experiment

What Will You Learn?

How to calculate conduction velocity from dual-channel recordings.
How to set up a two-electrode neurophysiology rig on an earthworm.
How signal speed varies across organisms and conditions.

Background

The nervous system is fast, but not instantaneous. To discover how fast, we need both the distance a spike travels and the time it takes to get there. Earthworms are perfect for this: their long bodies house three giant axons that carry large, easy-to-record spikes.

Experiment

Measuring Conduction Velocity in an Earthworm
Materials
- Live night-crawler earthworm
- 10% ethanol solution (anesthetic)
- SpikerBox with three pin electrodes
- Balsa wood, cork, or Styrofoam base
- Ruler (to measure electrode spacing)
- Plastic probe or coffee stirrer
Setup
- Anesthetize the worm in 10% ethanol for 3–4 minutes. Do not exceed this—alcohol can kill the worm.
- Place the worm dorsal-side up on your base and insert the three pins just off the midline in the back half of the worm: Channel 1 nearest the tail, then Channel 2, then ground. Space Channels 1 and 2 about 2.6 cm apart and note the exact distance.
Recording
- Tap the posterior end with the plastic probe to elicit a single spike that travels along the lateral giant axons (a tap at the head end fires the median giant axon instead).
- Observe the spike first on Channel 1 and moments later on Channel 2.
- Record several trials for a robust average.

Results & Analysis

Measure the latency between identical spikes on Channels 1 and 2. Divide the electrode spacing by this time difference to calculate conduction velocity:

Velocity = Distance / Time

Does speed vary between taps or between worms? Compare your numbers to other classes—or even to mammalian nerve-conduction data—and discuss why they differ.

Sketch the worm’s escape pathway: touch receptors near the tail, the giant axons running along the nerve cord toward the head, and the muscles that make the worm shorten. Mark where Channel 1 and Channel 2 sit and the distance between them.

NGSS alignment

Performance expectations

MS-LS1-8 Gather and synthesize information that sensory receptors respond to stimuli by sending messages to the brain for immediate behavior or storage as memories.

A touch to the worm's tail sends a message along its nerve cord.
HS-LS1-2 Develop and use a model to illustrate the hierarchical organization of interacting systems that provide specific functions within multicellular organisms.

Sketch the escape pathway from the tail's touch receptors along the giant axons to the muscles, and mark the two recording channels.

Science and engineering practices

2 Developing and Using Models

Sketch the worm's escape pathway and mark Channel 1, Channel 2 and the distance between them.
3 Planning and Carrying Out Investigations

Set up a two-channel recording on an earthworm and record several trials.
4 Analyzing and Interpreting Data

Measure the delay between the same spike on Channel 1 and Channel 2.
5 Using Mathematics and Computational Thinking

Calculate conduction velocity as distance divided by time; compare taps and worms.
6 Constructing Explanations and Designing Solutions

Compare your speed with mammalian nerve-conduction data and explain why they differ.

Crosscutting concepts

Structure and Function

Three giant axons carry large, fast spikes.
Scale, Proportion, and Quantity

Speed is a ratio of distance (centimetres) to time (milliseconds).

Disciplinary core ideas

LS1.A Structure and Function

Giant axons are structures specialized to carry fast signals.
LS1.D Information Processing

A touch sets off a signal that travels through the nervous system.

Backyard Brains mapping using the book's method. Not in the book.