Think of a single nerve fiber (axon) like a single microscopic copper wire. When it fires, it generates a tiny, all-or-nothing electrical spike called an Action Potential.

However, a whole nerve is like a thick Ethernet cable—it is a bundle containing thousands of these individual wires.

An Evoked Compound Action Potential (ECAP) is the summed electrical activity of that entire bundle of wires firing at the exact same time, which we can measure using an external recording setup.

Deconstructing the Name

To remember it easily, break the term into its three parts:

• Evoked: The signal did not happen spontaneously. We artificially triggered the nerve to fire by injecting a pulse of electricity through a stimulating contact (like an outer ring on an electrode lead).
• Compound: It is an aggregate. You aren't seeing one fiber fire; you are seeing the overlapping sum of hundreds or thousands of fibers reacting together.
• Action Potential: The transient electrical voltage change propagating along the nerve.

Measuring an ECAP: Subtracting the stimulus artifact to isolate the biological signal.. Source: ResearchGate

The Physics of the Signal: Speed and Distance

If you look at an ECAP on an oscilloscope, it doesn't look like a single, perfectly sharp spike. Instead, it forms a complex wave with multiple peaks and valleys. This happens because the "cable" contains wires of varying thicknesses, which conduct electricity at different speeds:

• Thick fibers (A-fibers): These are heavily insulated (myelinated) and conduct signals incredibly fast. Their signals arrive at the recording contact first, creating the initial voltage peak.
• Thin fibers (C-fibers): These lack heavy insulation and conduct much slower. They arrive later, creating a delayed peak.

As the signal travels further down the nerve, the fast signals pull further ahead of the slow signals. This causes the ECAP wave to stretch out over time—a concept called dispersion.

Experiment with this simulator to see how fiber recruitment and distance change the overall shape of the measured voltage:

The Engineering Challenge: The Artifact

When designing a system to measure an ECAP, the hardest part isn't the biology—it's the physics of the hardware.

When you fire a stimulating pulse to evoke the response, that initial jolt of electricity is massive compared to the tiny biological signal. This creates a Stimulus Artifact—a huge voltage spike that spreads through the surrounding tissue and can completely blind the recording amplifier. Capturing a clean ECAP requires precise amplifier blanking, differential recording, and signal processing to mathematically subtract that artifact and reveal the true neural tissue response underneath.

Closed loop with ECAP