Let’s get excited!
About neurons, specifically. I think it’s clear by my blogging persona that neurons are pretty important to me and without them, I would not be able to do neuromechanics research. Before I dive into what neuromechanists do and share interesting developments in the field, I want my readers to understand why EEG is used to obtain data. Other neuroimaging techniques are used, but I am going to stick with EEG.
Before diving into EEG, however, let’s zoom in a little more and understand what these systems record: electrical activity generated by the brain. More specifically, the electrical activity that we want to read in EEG are the collective signals generated by several neurons in various areas of the brain. And by several, I mean there’s upwards of 85 billion neurons firing off and communicating with each other through these electrical impulses! BECAUSE THEY ARE EXCITED! So, get excited and buckle up as I introduce you to my favorite cells.
They’re not anxious – they’re one of the two types of cells in the nervous system: neurons and glial cells! Neurons are the anatomical building block of the nervous system, and they are excitable. Basically this means that these cells are capable of producing action potentials and nerve impulses by conducting electrical impulses along their plasma membrane. They are comprised of a cell body (soma) and processes called dendrites and axons. Dendrites are short processes extending from the cell body that function as signal receptors and transmit information to the cell. Axons (may be short or long, insulated or not) transmit signals away from the cell.
Neurons may look different from one another based on their function and location. For example, motor neurons are categorized as multipolar neurons because they have several dendrites and an axon. You would expect this because motor functions are complex which means input from several sources would need to get to a motor neuron. The two other categories of neurons include bipolar (one main axon on each pole of the cell) and unipolar (with on peripheral and central axon).
Glial cells are non-excitable, support cells for neurons that are also responsible for protection, myelination, among other functions. These cells are also crucial to the nervous system, but I won’t focus on them today.
The neuron’s main function is to pass on information from point A to point B. They are messengers in the command center (our brain), and they pass on messages through electrical impulses known as action potentials. Neurons are in a polarized state which means that the net charge on the inside is negative (resting membrane potential) and the charge on the outside is positive. This polarization is maintained by a Na+/K+ pump that pumps out positive sodium ions and lets in positive potassium ions. This occurs against their concentration gradient – think of this as trying to get water to flow uphill.
If a stimulus is strong enough, the cell membrane opens its Na+ ion channels to allow an influx of positive ions that depolarize the cell by making the inside less negative. Following this, K+ ions leave the cell along their concentration gradient (the outside becomes negative and positive ions like to go where it’s negative). This is induces the action potential and as it propagates through the cell, the membrane potential can spike up to +40 mV. This spike happens in a very short period, called the absolute refractory period, thanks to the opening of the K+ ion channels and closing of the Na+ channels. This period is called the relative refractory period. The Na+/K+ pump then pushes out 3 Na+ ions and brings in 2 K+ ions, repolarizing the cell by making it more negative on the inside and positive on the outside.
So, this is the excitatory behavior mentioned before, but what happens after the action potential occurs? Well, neurons come into close contact at what we call a synapse – or, what I like to call the transit station. Synapses form a conducting pathway to communicate with postsynaptic cells and they can be excitatory (depolarize receiving cell to start another action potential) or inhibitory (hyperpolarize receiving cell to prevent excitation). Two types of synapses exist:
- Chemical – flow of neurotransmitters from one cell to another by release of vesicles from a terminal and binding to receptors on postsynaptic cell
- Electrical – tunnels between neurons called connexons that allow for a rapid, bidirectional flow of ions
As I mentioned before, axons may be insulated or not – the difference is the presence of fatty myelin sheaths. Myelin sheaths are insulatory and prevent the loss of ions into the extracellular environment, but they are exposed on the ends (nodes of Ranvier). Ion exchange is possible at these nodes, so the action potential “jumps” rapidly from one node to the other in what is called saltatory conduction. Thus, myelinated axons allow for quicker depolarization and unmyelinated axons have a lower conduction velocity.
What does this all mean?
Synaptic signalling between neurons causes the change in voltage across the cell membrane. Where there is a change in voltage or electrical potential, an electrical field is generated. With the vast amount of neurons in the brain, these small changes in voltage add up to result in a stronger electrical field – one that can be picked up by EEG electrodes.
Hopefully, you have a better understanding of what neurons are and why neuromechanists might be interested in them. We are fundamentally trying to understand how the brain communicates with the body and if we intercept the messengers, we might have a better chance of translating that information.
Now that you are equipped with all this exciting neuron knowledge, stay tuned for a breakdown of EEG in my upcoming post! Last thing: What did the neuron say to the glial cell? “Thanks for the support!”