Back to Basics Series: Behind the EEG

What is behind the EEG?  We all know that the EEG is the electrical activity generated by the brain’s neurons.

But really, what’s behind that?

Let’s break it down:

First up - what is a neuron?

Simply put:  it’s a nerve cell - the fundamental unit of the brain and spinal cord.  Neurons receive sensory input from our world and they send motor commands to our body.   

They are essentially our information messengers.

How do they communicate with each other?

They send chemical signals (called neurotransmitters) across a tiny gap between them (called a synapse) and the message is received by the neighboring neuron through receptors.   This electrical event that causes the first neuron to release the neurotransmitter is called an Action Potential.

So, what are we actually recording?

This coordinated communication between neurons across all the different brain regions.  Essentially, the collective electrical activity of many neurons firing together produces these brain waves.

Why the different frequencies?

Groups of neurons with similar patterns produce different frequencies - for example, we know alpha is primarily generated in the occipital lobe when we are awake; beta in the frontal lobe, and so forth.

This synchronization of neuronal activity is what is behind the brain waves (also called neuronal oscillations).  These oscillations have various functions and correlate with different behavioral states - awake, drowsy, asleep and the different stages of sleep, etc.

The EEG instrument and the recording electrodes work together to amplify and display this activity.

So we have talked about what happens behind the normal brain waves in an EEG; but what about when something goes wrong?

Intense firing due to an over-excited neuron creates the spike.  When a large number of neurons fire almost simultaneously, it generates a strong electrical signal that appears as a spike on the EEG tracing.  

This large electrical signal is picked up by the recording electrodes and displays what often then is interpreted as an abnormal EEG.

And as we know - frequent spikes cause clinical changes in a patient resulting in a seizure.

Prepping for your Board Exam?

Key words to remember:  Synapse, Neurotransmitter, Action Potential, and Oscillations as they relate to frequency.

Be sure to click here to follow along for more in our Back to Basics series. 

Next up:  Neurotransmitters - What they are and How they function.

Roya Tompkins, MS, REEG/EPT, RPSGT

Resources:

Johns Hopkins Medicine

Oxford dictionary

National Institute of Health