Primer: Electrophysiology

These posts, tagged “Primer,” are posted for two reasons: 1). to help me get better at teaching non-scientists about science-related topics; and 2). to help non-scientists learn more about things they otherwise would not.  So, while I realize most people won’t read these, I’m going to write them anyway, partially for my own benefit, but mostly for yours.

It’s been awhile since I posted one of these, but as I’m working on radically different science than I have in years past, and people ask me “what I do,” I figured I should take the time to explain, to some degree.

Wikipedia defines “electrophysiology” in the following way:

Electrophysiology (from Greek ἥλεκτρον, ēlektron, “amber” [see the etymology of “electron”]; φύσις, physis, “nature, origin”; and -λογία, -logia) is the study of the electrical properties of biological cells and tissues. It involves measurements of voltage change or electric current on a wide variety of scales from single ion channel proteins to whole organs like the heart. In neuroscience, it includes measurements of the electrical activity of neurons, and particularly action potential activity.

So, in the most general sense, I’m “listening to neurons talk to each other,” and occasionally, “interrupting their ‘conversations'” in various ways.  When I talk about “conversations,” I’m referring to the act of neurotransmission, whereby one neuron sends a chemical signal across a synapse to another neuron, resulting in the propagation of that signal (an action potential), or sometimes the inhibition of another signal.

As I talked about in a previous primer, in order for an action potential to occur, various ion channels in the membrane of a neuron must open, allowing sodium (Na+) from outside the cell to come in, and potassium (K+) to go out.  Other ions will play roles as well, including chloride (Cl-) and calcium (Ca2+).

Using electrophysiology, it is possible to measure the movement of these ions across a cell membrane using relatively simple principles of physics.  Specifically, [V=IR], or [voltage = current X resistance].  If you hold two of the terms of this equation constant, it is possible to determine the third term.  Effectively, we do this using a “patch pipette,” a small, sharp, glass tube that has a wire electrode running through it.  If you know the resistance of the pipette, and you hold the electrode at a constant voltage, you can measure the current across the membrane of a cell (i.e. the flow of ions).

In short, this diagram describes the actual process of making this measurement, using a technique called “patch clamp“:

Looking through a microscope (like the one pictured above), you move one of these glass electrode pipettes to be just touching the membrane of a cell.  You have to be very careful so you don’t puncture the cell, thus damaging the cell membrane to the point where you can’t make accurate measurements.  You then apply a small amount of suction using a syringe to actually suck some of the cell membrane inside the pipette.  Once you have a strong seal formed (typically termed a “gigaseal”), you can apply a brief, large amount of suction with your syringe to rupture the membrane of the cell, where now, the inside of the cell is being exchanged with whatever you put on the inside of the pipette.  The internal solution of a pipette is usually something like potassium, basically trying to recreate what the inside of a cell would be, aside from all the organelles, however you can add compounds or drugs to manipulate the actions of channels you are trying to study.  Typically, though, you apply drugs to the outside of the cell, as well.

So, a real-world example of how this technique is used would be in my study of NMDA channels.  The NMDA receptor is a sodium channel and is very important in neurotransmission, but especially in memory.  When I have a cell “patched” like in the diagram above, I can apply the drug, NMDA, to the cell and see a large sodium current on my computer screen, kinda like this one.

So, over time, when a drug like NMDA or this “Blocker” is applied, you can see a change in the current (measured in “picoamps”) across the membrane of the cell.  In this case, we would read these data such that NMDA opens its channel and sodium ions flood inward, then that current is reduced by the “Blocker” that was applied for a few seconds, and then once the application of the “Blocker” was stopped and NMDA alone was applied to the cell, the inward sodium current increased again.

These traces allow you to get information about how channels are opening, what ions are flowing in what direction, and to what degree drugs like this “Blocker” are affecting channels.  It is work like this, for example, that led to characterization of benzodiazepines and barbiturates, drugs that interact with the GABA receptor, a chloride channel.  Without these techniques, it is difficult to know how a drug is affecting a channel at the cellular level.  Just about every cell in your body has channels of some kind, as they are very important for maintaining the function of that cell.  Neurons are just highly specialized to require ions more than some other cells do, though heart cells are also studied in this way, among others.

Effectively, these techniques allow you to determine how a cell works.