Neuroscience Fundamentals - Module 2: The action Potential
Why is the action potential important?
One of the most important things to understand about the nervous system is the action potential.
This is because all of the functions of the nervous system depend on its ability to send and receive signals - in order for information to reach the brain from the body, a signal must be sent from that specific body part through the nerves, into the spinal cord, and up to the brain. Signals need to move through the brain for it to do its job of processing information and creating responses to the environment. Then those signals need to move back out from the brain into the body in order to control the body’s actions.
The way that all of this happens is through a very specific process - the action potential.
So, understanding this process is the first building block of a broad comprehension of nervous system function.
What is an action potential?
At the simplest level, an action potential is the process by which a neuron sends a signal along its length and transmits that signal to other nerve cells.
To introduce more scientific terms: An action potential occurs when the electric potential at the cell membrane rapidly rises and falls. This process results in the release of neurotransmitter from the end of the neuron, which then triggers the beginning of a new action potential in the next cell in the connecting chain of neurons. Basically, an electric charge is moving down the line of cells.
If this is still confusing, don’t worry. It usually is until broken down into better detail. We’ll get to that in a moment. But first, I want to introduce some key terms that will make this discussion easier. A short glossary is listed below. I have also linked helpful explainers for any concepts within the definition that are likely to require a refresher.
Additionally - if you are unfamiliar with the basic anatomy of a neuron, please reference that information in Module 1.
Glossary:
Ion: an atom or molecule that has a positive or negative electric charge due to the loss or gain of one or more electrons
Depolarization: the transition from a relatively negative charge to a more positive charge at the cell membrane
Membrane potential: the difference between the electric potential on the inside versus the outside of the cell (this can also be referred to as the voltage across the membrane)
Threshold potential: the local membrane potential that must be reached in order to trigger an action potential
So, now that we have some common terms, let’s get to the main point - how does an action potential actually happen?
Steps of an action potential:
I’m going to break this into two options - a basic overview, and a more in-depth explanation. This is because the minutia of the action potential can be overwhelming if the process as a whole is new to you, and you only NEED a broad scale understanding to grasp the gist of how the nervous system works in later modules. But, I’m a geek for details so I hope at least someone else following along will be, too.
Basic overview:
There are a few basic steps to the action potential
1.) The neuron receives a signal
Neurotransmitters are are released into the synapse where they can be picked up by the dendrites of the neuron. The presence of neurotransmitter molecules at this part of the cell causes a local change in the electric charge. If this change is big enough, it will trigger a process that generates the action potential that will move down the cell.
2.) The action potential begins
Once the action potential begins, there is a movement of ions across the cell membrane that causes a rapid positive change in electric potential. This event immediately causes two events:
a repeat of the movement of ions to cause a positive electric potential in the section of cell membrane immediately next to the affected area
the movement of ions in the first section that causes the charge to move in the opposite direction so that the charge across the membrane becomes very negative.
The combination of these cause-and-effect events results in an electric signal (current) moving very quickly from the beginning to end of the cell.
Once the signal reaches the end of the cell, it triggers the release of neurotransmitter molecules from the end of the cell into the space between it and the subsequent neuron.
This process repeats over and over again until the signal has reached its final location.
You may enjoy this video from 2 minute neuroscience overviewing the action potential as well.
Advanced explanation (for the nerds):
If you are interested in dissecting the minute details of neuroscience, this section is for you as it adds some additional information that will help you to break down the chemistry and physics of what is going on within the cell during the action potential. But this part of the module is optional for understanding later segments of neuroscience fundamentals.
Before reading this section, make sure you understand the basics listed above including the linked video on the basics of the action potential.
If all of that makes decent sense to you, then read on!
How do neurotransmitters change membrane potential?
Neurotransmitters can selectively bind to specific receptors embedded in the cell membrane. When this happens, the shape of the receptor protein changes in such a way that creates a path for ions to move across the cell membrane. These changes tend to be small, but when enough neurotransmitter molecules are bound to receptors within a short period of time, the change becomes large enough that the action potential can begin. This video breaks this process down with helpful visual aids, and introduces some basic categories of receptor types.
How does reaching the threshold potential trigger the action potential process?
The cell membrane is embedded with proteins called voltage gated ion channels. These are proteins that change their shape in some way in response to electrical gradients. When the potential across the cell is positive, they will have one shape. When the membrane potential becomes negative, the protein shape changes. These shape changes serve to either open or close the channels to ion movement. There are different kinds of voltage gated ion channels that utilize various mechanisms for shape-change and allow different ions to move through them when open.
This video offers an overview of this process and how it facilitates specific ion movement during the action potential. If you would like to see a more anatomically accurate animation of one of these ion channels, here is a video detailing voltage gated sodium channels, and here is a breakdown of voltage gated potassium channel activity.
How does the action potential cause the release of neurotransmitters?
This happens pretty much the same way as the action potential itself occurs, with very minimal differences. Neurotransmitters are stored inside of the cell in bubble-like structures called vesicles. These vesicles are made up of small sections of cell membrane that are formed into a sphere, thereby separating their contents from the surrounding interior of the cell. The vesicles storing neurotransmitters are covered in specific proteins involved in transport. They don’t do much until the action potential reaches the section of membrane at the end of the cell. But when the action potential reaches that section, the movement of ions kick-starts a process that results in activation of transport proteins that bind to the ones on the vesicle membrane and move the vesicle to the cell membrane at the terminal button. Then the vesicle membrane merges with the cell membrane, and the contents are released into the space outside of the cell. This is usually the synapse, meaning that postsynaptic cell dendrites are located in the immediate area and the neurotransmitter can bind to receptors there to continue the action potential process in the next cell.
Can I get a more in-depth step by step breakdown of the action potential?
Sure! But I’ll be honest, a lot of people have made great materials on this already so I say “If it ain’t broke, why fix it?”. So with that, go enjoy these video explainers.
Summary
An action potential is the process by which cells in the nervous system send and receive signals. It involves the movement of ions across the cell membrane to generate rapid changes in the membrane potential. It is initiated by neurotransmitters binding to the cell dendrites. It results in the release of neurotransmitters from the terminal button.
I hope you enjoyed this explanation of the action potential! Follow along for more neuroscience fundamentals.