To understand an extracellular single-unit spike, it helps to think of it like trying to listen to a specific person talking in a crowded room by holding a microphone just outside their personal space.

In neuroscience, a "spike" is an action potential—the electrical signal a neuron uses to communicate. "Extracellular" means we are measuring it from the outside of the cell, and "single-unit" means we are isolating the electrical chatter of just one specific neuron.

Here is a breakdown of how it works, what it looks like, and why we do it.

1. The Setup: Eavesdropping on a Neuron

To record this signal, scientists use a microelectrode (a microscopic wire or silicon probe) that is carefully lowered into brain tissue.

Recording from just outside the cell membrane. Source: Neupsy Key

Crucially, the electrode does not puncture the cell. It sits in the extracellular fluid right next to the neuron's membrane. Because it's on the outside, it measures the voltage difference between that tiny spot next to the neuron and a distant "ground" reference electrode elsewhere in the body.

2. The Waveform: Why it Looks "Upside Down"

If you've seen a classic textbook diagram of an action potential, it usually spikes upward (positive). However, that is an intracellular recording (taken from inside the cell). An extracellular spike usually looks flipped.

Intracellular (inside) vs. Extracellular (outside) waveforms. Source: Frontiers

Here is why this happens:

1. The Dip (Depolarization): When a neuron fires, it opens channels in its membrane. Positively charged sodium ions ($Na^+$) rush into the cell.
2. The Vacuum: Because positive charge just rushed inside, the fluid immediately outside the cell suddenly loses its positive charge.
3. The Measurement: Your electrode sitting outside registers this sudden drop in local positive charge as a negative voltage dip.
4. The Rebound (Repolarization): Milliseconds later, the cell pumps positively charged potassium ions ($K^+$) back out to reset itself, creating a positive peak on the extracellular electrode before settling back to baseline.

Intracellular vs. Extracellular Recordings

3. Spike Sorting: The Cocktail Party Problem

Because the electrode is sitting in open fluid, it often picks up the electrical spikes from several neighboring neurons at once. To study a "single unit" (one neuron), scientists have to isolate its specific voice from the background noise.