Experiment: Somatotopy

Touch different barbs on a cockroach leg and watch each one produce its own spike pattern. Discover how the nervous system maps where a stimulus occurs.

Devices and Accessories2

About the Experiment

What Will You Learn?

How neurons encode touch from different body parts.
Why spikes from one neuron are identical in size and shape.
The principle of somatotopy—mapping the body in the brain.

Background

Each sensory neuron fires a stereotyped spike, but spikes from neighbouring neurons can differ slightly. By probing individual barbs, you will see that each barb has its own neuron, with its own spike shape on your recording. The cockroach’s nervous system knows where it was touched by which neuron fired, just as our own somatosensory cortex does.

Experiment

Stimulating Individual Barbs
Procedure
1. Mount a cockroach leg and connect two close pins to your SpikerBox.
2. With a toothpick, tap one barb until you hear a clear burst of spikes. Record the waveform.
3. Test several other barbs. Note that spikes from one barb are identical each time, while spikes from different barbs vary in height or shape.
Observation Consistent waveforms from a single barb show that one neuron is firing. Different waveforms reveal activity from other neurons—your first glimpse of a sensory map.
Understanding Somatotopy
Switch between barbs and compare spike patterns. Each barb’s neuron reports a specific location on the leg, letting the cockroach’s nervous system know where it was touched. Humans use the same idea: signals from skin receptors travel to the primary somatosensory cortex, arranged as a body map with hands and lips occupying the largest area.

What Did You Learn?

You confirmed that neurons code location with identity, not spike size, and that somatotopy allows a nervous system—cockroach or human—to tell whether a stimulus hit a finger, foot or face. How might other animals use similar maps?

Sketch the leg, number the barbs you tapped, and draw each one’s waveform beside it: your first sensory map. Then draw the path each barb’s message takes up the leg nerve to the cockroach’s thoracic ganglion (its “mini-brain” for that leg), and mark where your pins recorded.

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.

Each barb's sensory neuron tells the nervous system where the leg was touched.
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 leg's barbs with their waveforms and the path up the leg nerve to the thoracic ganglion: a first sensory map.

Science and engineering practices

2 Developing and Using Models

Sketch the leg, number the barbs, draw each one's waveform, and add the path each message takes up the leg nerve to the thoracic ganglion.
3 Planning and Carrying Out Investigations

Tap individual barbs and record the spikes from each.
4 Analyzing and Interpreting Data

Compare waveforms: identical from one barb, different between barbs.

Crosscutting concepts

Patterns

Spikes from one barb are identical each time.
Structure and Function

Each neuron's place on the leg tells the nervous system where the touch landed.

Disciplinary core ideas

LS1.D Information Processing

Sensory neurons code location for the nervous system.
LS1.A Structure and Function

Specialized sensory neurons in each barb.

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