Brief Intro to Electrophysiology

Electrophysiology can be an intimidating subject for those who wish to embark on that journey. I have written this brief introduction to electrophysiology to help those, who like me where getting their feet wet in electrophysiological neuroimaging. I will talk about the scales of different electrophysiology measures from micro to macroscopic. I will briefly touch on the neurobiology and physics related to these methods. However this is intended for someone who knows very little about neuroscience. I am currently doing a project that involves analysing electrophysiological data, specifically magnetoencephalography (Don’t worry you will understand what this complicated word means by the end of the article). So I hope you trust me as you guide through this introduction.

Electrophysiology is the measure measurement of electrical or magnetic signals from the brain. These signals can vary in the way they look, on a macroscopic level they can look like a wave and on a microscopic level they can look like a sudden spike. Macroscopic level is usually taken from outside of the head, you may have seen these strange caps used by neuroscientists. These measure magnetic wave or field potentials from the scalp. The reason they look like waves is because they are taking in all the signals from billions of neurons. Lots of neurons together sometimes dance to a rhythm and when they do we can hear it from the scalp. Microscopic on the other hand is usually taken within a single neuron (Intracellular) or around a single neuron (Extracellular). These measure the electrical or magnetic activity in or around the neuron. These look like spikes rather than waves because neuron’s signals are actually binary in nature. They either fire or they don’t and there is no in between.

To get a neuron to fire, which is called an action potential, you must change what is inside and outside a neuron. Neurons have a lot of different gates on them on the parts that receive signals from other neurons. Neurons don’t communicate with each other using electrical impulses they use slower chemical signals. These slower chemical signals like glutamate (This will be important later) bind to the gates to allow them to open. This changes the atoms that are inside and outside of the cell. These are special atoms that have a charge, these are called ions. When these gates open and these ions come in it can change the charge of the neuron. Then when it reaches a certain threshold it will fire an electrical signal from one side of the neuron to another (Neurons are very long). Electrophysiology does not measure action potentials direction instead they measure the charge that is around the neuron receiving the signal.

The closer the measurement tool is to the cell (Even inside the cell) the better we can see these changes in charge, when we look at a bigger scale they become fuzzy. When taking these measurements from nearby the cells or in the cell (Remember neuron is a cell), you are dealing with a small amount of neurons. But as you get further away, to a place like the scalp, you are measuring a much larger amount of neurons. These neurons signals are summed together to create a rhythm, such as a gamma rhythm. But rhythms can cancel each other out and signals from inside the brain are very weak. So to measure on a larger scale we need a particular type of neuron that can sync up and and are aligned to create a measurable signal. Luckily their is a neuron that does the job, it’s called a pyramidal cell, this is because it’s cell body looks like a pyramid. They are generally inline with eachother so if they create a rhythm together it’s strong enough to pick it up. they are excitatory cells and use glutamate to communicate.

Electroencephalography (EEG) and Magnetoencephalography (MEG) are two methods to measure this macro neuron rhythms. It may seem like they are measuring the same thing but using 2 different fields, in a way they are. EEG takes the macroscopic field potential on the scalp and MEG takes the magnetic field from the scalp. However the orientation of the neurons matter to whether they can see if, because they are blind to one orientation. Hold your hand out in a thumbs up position you thumb represents the electric current and your other fingers the magnetic field. If you put a sensor above you thumb MEG won’t see it and viseversa. This makes them in some ways complementary.

There is a lot more to learn about electrophysiology and we haven’t even begun on the cool things we can find out by using these methods (Check out my current projects). But I hope this gave you a good intro into what the method is all about.