Electrophysiology: measuring the electrical activity of neurons

Electrophysiology is the study of the electrical properties of living cells and tissues, and the techniques used to record and manipulate them. In neuroscience, electrophysiology measures voltages and currents produced by neurons, from single ion channels to whole networks, using tools such as patch clamp pipettes, microelectrode arrays and scalp EEG.

Key takeaways

• Neurons signal with electrical events, chiefly action potentials, and electrophysiology records them with millisecond precision.
• Methods range from intracellular (patch clamp) to extracellular (microelectrode arrays) to non-invasive (EEG).
• Hodgkin and Huxley’s 1952 model of the nerve impulse and Neher and Sakmann’s 1976 single-channel recordings are foundational milestones.
• Microelectrode arrays are the workhorse for cultured neurons and brain organoids because they record many sites over long periods.
• In biocomputing, electrophysiology is both the input channel (stimulation) and the output channel (recording).

What does electrophysiology measure? #

Every neuron maintains a voltage across its membrane, created by ion pumps and selective ion channels. When channels open, ions flow, and the voltage changes. The most important event is the action potential, a brief spike lasting roughly a millisecond that travels along the axon and triggers communication at synapses.

Hodgkin and Huxley described the ionic currents behind the nerve impulse in a 1952 paper in The Journal of Physiology, producing a mathematical model that still underpins computational neuroscience. Electrophysiology records several kinds of signal:

• Membrane potential: the voltage inside a cell relative to outside.
• Ionic currents: flows through channels, measured when voltage is held constant.
• Spikes: action potentials detected inside or outside cells.
• Local field potentials (LFP): slower extracellular signals reflecting summed synaptic and other currents from many cells.
• EEG and ECoG: field signals recorded from the scalp or brain surface.

Buzsáki, Anastassiou and Koch reviewed how these extracellular signals arise in Nature Reviews Neuroscience (2012), emphasizing that LFP and EEG reflect a mix of current sources rather than spikes alone.

What are the main electrophysiology techniques? #

Intracellular vs extracellular recording #

Intracellular methods give the richest view of one cell, including small sub-threshold changes, but they are invasive and typically last minutes to hours. Patch clamp, for which Neher and Sakmann reported single-channel currents in Nature in 1976, is the gold standard.

Extracellular methods sit outside cells. Signals are smaller, and each electrode may pick up several neurons, which is why spike sorting is needed to assign spikes to individual cells. The advantage is scale and longevity: a microelectrode array can record from many sites in the same culture for weeks, and a high-density microelectrode array can sample thousands of electrodes.

How are raw electrophysiology signals processed? #

An extracellular electrode records a single voltage trace that mixes several signals at once. Processing separates them by frequency and shape:

• Filtering: a high-pass or band-pass filter (commonly with a lower cutoff of a few hundred hertz) isolates the fast spike band, while low-pass filtering keeps the slower local field potential.
• Spike detection: threshold crossings in the spike band, often set relative to the estimated noise level on each channel, mark candidate action potentials.
• Spike sorting: waveform shapes and their pattern across nearby electrodes are clustered to assign spikes to putative single neurons.
• Event and rhythm analysis: spike trains are analyzed for firing rates and bursts, and field potentials for oscillations.

Each step involves parameter choices that affect the result, which is why reporting processing settings alongside data is part of good practice.

How is electrophysiology used with brain organoids? #

For brain organoids, electrophysiology answers a basic question: are these neurons functional and connected? A typical workflow looks like this:

1. Place an organoid or a slice of one on an MEA, or attach electrodes to its surface.
2. Record spontaneous activity to measure firing rates and synchronized network bursts.
3. Deliver electrical stimulation and record evoked responses.
4. Optionally apply drugs to test which receptors and channels contribute.
5. Patch clamp selected neurons to characterize their properties in detail.

Recording the same organoid over months reveals how activity matures, which is central to studies of development and disease. Our article on microelectrode arrays explained goes deeper into the hardware.

Why does electrophysiology matter for biocomputing? #

Organoid intelligence and other forms of biocomputing depend on two-way communication with living neurons. Electrophysiology provides both directions:

• Input: electrical stimulation encodes information, such as the position of a ball in a game.
• Output: recorded activity is decoded into actions or answers.
• Feedback: in closed-loop stimulation, recordings drive the next stimulus within milliseconds.

The DishBrain study by Kagan et al. (Neuron, 2022) is an example: neurons on an MEA received stimulation encoding a Pong-like game, and their recorded activity controlled the paddle. Every part of that loop is electrophysiology.

What limits performance #

• Electrode count and density: how many neurons can be observed and addressed.
• Signal-to-noise ratio: how reliably spikes are detected.
• Stimulation artifacts: stimulation pulses can briefly saturate recordings, hiding early responses.
• Latency: the time from recording to new stimulus in a closed loop.
• Data volume: high-density systems generate large data streams that must be processed and stored.

What are the limitations of electrophysiology? #

• Sampling bias: extracellular electrodes favor active, nearby, large neurons and may miss quiet cells.
• Surface access: planar arrays record mainly from cells in contact with the chip, which is a constraint for three-dimensional organoids.
• Invasiveness: intracellular methods damage cells over time.
• Interpretation: field potentials mix many sources, so mapping them to specific cells is difficult.
• Standardization: differing hardware and file formats complicate comparison, which is why standards such as Neurodata Without Borders matter.

Electrophysiology is the foundation on which every biocomputing experiment is built. At Organoid Programming, our runtime and experiment ledger are designed around that reality, treating each recording and stimulus as data that should be timestamped, reproducible and shareable. For the bigger picture, see what is organoid intelligence.

Frequently asked questions

What is electrophysiology in simple terms?

Electrophysiology is the measurement of electrical signals produced by living cells. Neurons communicate with brief voltage changes called action potentials, and electrophysiology uses electrodes placed inside, on or near cells to record those signals or to stimulate cells with electrical currents. It is the main way scientists observe what neurons are doing in real time.

What is the difference between intracellular and extracellular recording?

Intracellular recording, such as whole-cell patch clamp, places an electrode in direct electrical contact with a cell’s interior, measuring its membrane voltage or currents in detail, including small sub-threshold changes. Extracellular recording places electrodes outside cells and detects the much smaller voltage changes produced in surrounding fluid, mainly spikes and field potentials, often from many neurons at once.

What is patch clamp?

Patch clamp is a technique in which a fine glass pipette forms a tight seal with a small patch of cell membrane, allowing very small currents, even through single ion channels, to be measured. Erwin Neher and Bert Sakmann reported single-channel recordings in Nature in 1976 and later received the Nobel Prize in Physiology or Medicine for the technique.

What electrophysiology method is used for brain organoids?

Microelectrode arrays are the most common method for organoids because they record extracellularly from many sites at once, without damaging cells, and can be used repeatedly over weeks or months. Patch clamp is used to characterize individual neurons in detail, and calcium imaging is often used alongside electrical recording as an optical proxy for activity.

Why is electrophysiology important for biocomputing?

In biocomputing, electrophysiology is the input-output interface. Electrical stimulation sends information into living neurons, and recordings read out their responses. Every task, from playing a simple game to classifying signals, depends on how accurately and quickly activity can be recorded, interpreted and fed back. Electrode quality and timing therefore set limits on what a living computer can do.

Sources

1. Hodgkin & Huxley (1952). A quantitative description of membrane current and its application to conduction and excitation in nerve. The Journal of Physiology.
2. Neher & Sakmann (1976). Single-channel currents recorded from membrane of denervated frog muscle fibres. Nature.
3. Buzsáki, Anastassiou & Koch (2012). The origin of extracellular fields and currents: EEG, ECoG, LFP and spikes. Nature Reviews Neuroscience.
4. Kagan et al. (2022). In vitro neurons learn and exhibit sentience when embodied in a simulated game-world. Neuron.