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?