The Action Potential
The action potential is an electrical signal that travels from one neuron to another. There would be no communication between neurons if there were no action potentials. Action potentials demonstrate how specialized neurons are. Neurons are incredible cells because they can communicate both electrically (using action potentials) and chemically (using neurotransmitters)! The way that action potentials are propagated through neurons is a complex process, mediated by electrochemical gradients and electrical potentials.
A chemical gradient is determined by the number of molecules outside a neuron compared to within the neuron. For instance, two of the most important ions involved in action potentials are potassium (K+) and sodium (Na+). Na+ ions are highly concentrated outside of neurons, that is, there are many more Na+ ions located in the environment surrounding a given neuron compared to the inside of the neuron. K+ ions, on the other hand, are more highly concentrated inside a given neuron compared to its outside environment.
Molecules will flow down their chemical gradients: they will move from areas with many molecules to areas with fewer molecules - the more space for a molecule, the better! It's similar to people riding in a crowded subway car: once the destination has been reached, people flow out of the subway car in order to get their personal space!
Using Na+ and K+ as examples, Na+ ions will want to move into the cell while K+ ions will want to move out of the cell, down each of their respective concentration gradients.
In addition to chemical gradients, there is an electrical gradient that contributes to the firing of an action potential. The voltage of a neuron at rest, its resting membrane potential, is -70 mV. This means that, when a neuron is not firing an action potential, it is more negative than its surrounding environment. We also know that neurons have a threshold potential. If this threshold is not reached, an action potential will not occur.
When a neuron is active, calcium ions enter the neuron to serve as a chemical signal for lots of internal functions. So a higher number of calcium ions in the neuron is a good indicator that a neuron is active. Researchers have engineered a set of chemicals that when bound to calcium, give off a fluorescent signal. These chemicals are called calcium indicators.
A great feature of calcium indicators is that we can use them in living and behaving animals. For mice, researchers replace a part of the skull with a window so that a camera can record the light given off by the indicators. So now, researchers can control the sensory input to the animal and record the activity of its neurons at the same time.
The influx of calcium is a trigger for the release of neurotransmitters, which in turn, can signal for an action potential to fire if the neurotransmitters released are significant enough to support the signal.
With all of these pieces in place, we can begin to uncover the basics of how an action potential is generated! The first step of an action potential is to have a sufficient stimulus. A sufficient stimulus will reach the threshold potential. Once this threshold is reached, protein channels for Na+ ions open, and Na+ enters the neuron. This influx of ions leads to the neuron becoming positive very quickly. At a point of extreme positivity, two things happen: the Na+ ion channels close ending the influx of Na+ ions, and channels allowing K+ to flow out of the cell open.
Once the K+ channels open, K+ rapidly flows out of the cell, and the neuron becomes negative again. Even as the neuron reaches its original charge, K+ continues to flow out of the cell. In fact, there is a brief period of time when the neuron becomes even more negative than resting potential! At this point, the K+ channels close, so the neuron doesn’t continue to get more negative.
The neuron is in a weird situation! It is more negative than its resting membrane potential, and the Na+ channels that could bring positive ions into the cell are closed. What to do?! Luckily, neurons have what is called a sodium-potassium pump. This handy tool allows the neuron to get back to its resting membrane potential so that it is ready for another go!