ACTION POTENTIAL: PROPAGATION AND PHASES - GEOGRAFÍA - 2025

The action potential is a short-lived electrical or chemical phenomenon that occurs in the neurons of our brain. It can be said that it is the message that a neuron transmits to other neurons.

The action potential is produced in the cell body (nucleus), also called the soma. It travels through the entire axon (neuron extension, similar to a wire) until it reaches its end, called the terminal button.

The action potentials on a given axon always have the same duration and intensity. If the axon branches into other processes, the action potential divides, but its intensity is not reduced.

When the action potential reaches the terminal buttons of the neuron, they secrete chemicals called neurotransmitters. These substances excite or inhibit the neuron that receives them, being able to generate an action potential in said neuron.

Much of what is known about the action potentials of neurons comes from experiments with giant squid axons. It is easy to study because of its size, as it extends from head to tail. They serve so that the animal can move.

Neuron membrane potential

A. Schematic view of an ideal action potential. B. Real record of an action potential. Source: en: Memenen / CC BY-SA (http://creativecommons.org/licenses/by-sa/3.0/)

Neurons have a different electrical charge inside than outside. This difference is called the membrane potential.

When a neuron is in resting potential, it means that its electrical charge is not altered by excitatory or inhibitory synaptic potentials.

On the other hand, when other potentials influence it, the membrane potential can be reduced. This is known as depolarization.

On the contrary, when the membrane potential increases with respect to its normal potential, a phenomenon called hyperpolarization occurs.

When a very rapid reversal of the membrane potential occurs suddenly, an action potential occurs. This consists of a brief electrical impulse, which is translated into the message that travels through the axon of the neuron. It begins in the cell body, reaching the terminal buttons.

The nerve impulse travels down the axon

Importantly, for an action potential to occur, electrical changes must reach a threshold, called the excitation threshold. It is the value of the membrane potential that must necessarily be reached for the action potential to occur.

Schematic of a chemical synapse

Action potentials and changes in ion levels

Membrane permeability of a neuron during an action potential. The resting state (1), sodium and potassium ions cannot pass through the membrane, and the neuron has a negative charge inside. Depolarization (2) of the neuron activates the sodium channel, allowing sodium ions to pass through the membrane of the neuron. Repolarization (3), where sodium channels close and potassium channels open, potassium ions cross the membrane. In the refractory period (4), the membrane potential returns to the resting state as the potassium channels close. Source: Membrane Permeability of a Neuron During an Action Potential.pdf and Action potential, CThompson02

Under normal conditions, the neuron is ready to receive sodium (Na +) inside. However, its membrane is not very permeable to this ion.

In addition, the well-known “sodium-potassium transporters” have a protein found in the cell membrane that is responsible for removing sodium ions from it and introducing potassium ions into it. Specifically, for every 3 sodium ions it extracts, it introduces two potassium ions.

These transporters keep sodium levels low within the cell. If the permeability of the cell were to increase and more sodium entered it suddenly, the membrane potential would change radically. Apparently, this is what triggers an action potential.

Specifically, the permeability of the membrane to sodium would be increased, these entering the neuron. While, at the same time, this would allow potassium ions to leave the cell.

How do these changes in permeability occur?

Cells have embedded in their membrane numerous proteins called ion channels. These have openings through which ions can enter or leave cells, although they are not always open. Channels are closed or opened according to certain events.

There are multiple types of ion channels, and each is usually specialized to conduct exclusively certain types of ions.

For example, an open sodium channel can pass more than 100 million ions per second.

How are action potentials produced?

Neurons transmit information electrochemically. This means that chemicals produce electrical signals.

These chemicals have an electrical charge, which is why they are called ions. The most important in the nervous system are sodium and potassium, which have a positive charge. In addition to calcium (2 positive charges) and chlorine (one negative charge).

Changes in membrane potential

The first step for an action potential to occur is a change in the cell's membrane potential. This change must exceed the excitation threshold.

Specifically, there is a reduction in the membrane potential, which is called depolarization.

Opening of sodium channels

As a consequence, sodium channels embedded in the membrane open, allowing sodium to enter the neuron massively. These are driven by diffusion forces and electrostatic pressure.

Since sodium ions are positively charged, they cause a rapid change in membrane potential.

Potassium channel opening

The axon membrane has both sodium and potassium channels. However, the latter are opened later, because they are less sensitive. That is, they need a higher level of depolarization to open up and that is why they open later.

Closure of sodium channels

There comes a time when the action potential reaches its maximum value. From this period on, the sodium channels are blocked and closed.

They can no longer open again until the membrane reaches its resting potential again. As a consequence, no more sodium can enter the neuron.

Potassium channel closure

However, the potassium channels remain open. This allows potassium ions to flow through the cell.

Due to diffusion and electrostatic pressure, as the interior of the axon is positively charged, the potassium ions are pushed out of the cell. Thus, the membrane potential recovers its usual value. Little by little, the potassium channels are closing.

This exit of cations causes the membrane potential to recover its normal value. When this happens, the potassium channels begin to close again.

As soon as the membrane potential reaches its normal value, the potassium channels close completely. Somewhat later, the sodium channels are reactivated in preparation for another depolarization to open them.

Finally, the sodium-potassium transporters secrete the sodium that had entered and recover the potassium that had previously left.

How is information spread through the axon?

Parts of a neuron. Source: No machine-readable author provided. NickGorton ~ commonswiki assumed (based on copyright claims)

The axon consists of a part of the neuron, a cable-like extension of the neuron. They can be too long to allow neurons that are physically far apart to connect and send information to each other.

The action potential propagates along the axon and reaches the terminal buttons to send messages to the next cell. If we measured the intensity of the action potential from in different areas of the axon, we would find that its intensity remains the same in all areas.

All or nothing law

This occurs because axonal conduction follows a fundamental law: the law of all or nothing. That is, an action potential is given or not. Once it begins, it travels throughout the axon to its end, always maintaining the same size, it does not increase or decrease. Furthermore, if an axon branches out, the action potential divides, but it maintains its size.

Action potentials start at the end of the axon that is attached to the neuron's soma. They usually travel in only one direction.

Action and behavior potentials

You may be wondering at this point: if the action potential is an all-or-nothing process, how do certain behaviors such as muscle contraction occur that can vary between different levels of intensity? This happens by the law of frequency.

Law of frequency

What happens is that a single action potential does not directly provide information. Instead, the information is determined by the discharge frequency or firing rate of an axon. That is, the frequency at which action potentials occur. This is known as the "law of frequency."

Thus, a high frequency of action potentials would lead to a very intense muscle contraction.

The same is true of perception. For example, a very bright visual stimulus, to be captured, must produce a high "firing rate" in the axons attached to the eyes. In this way, the frequency of action potentials reflects the intensity of a physical stimulus.

Therefore, the law of all or nothing is complemented by the law of frequency.

Other forms of information exchange

Action potentials are not the only classes of electrical signals that occur in neurons. For example, sending information across a synapse produces a small electrical impulse in the membrane of the neuron that receives the data.

Scheme of a synapse. Source: Thomas Splettstoesser (www.scistyle.com)

Sometimes a slight depolarization that is too weak to produce an action potential can slightly alter the membrane potential.

However, this alteration gradually decreases as it travels through the axon. In this type of information transmission, neither the sodium nor the potassium channels open or close.

Thus, the axon acts like a submarine cable. As the signal is transmitted through it, its amplitude decreases. This is known as downward conduction, and it occurs due to the characteristics of the axon.

Action potentials and myelin

The axons of almost all mammals are covered in myelin. That is, they have segments surrounded by a substance that allows nerve conduction, making it faster. The myelin coils around the axon without letting the extracellular fluid reach it.

Myelin is produced in the central nervous system by cells called oligodendrocytes. While, in the peripheral nervous system, it is produced by Schwann cells.

The myelin segments, known as myelin sheaths, are divided from each other by bare areas of the axon. These areas are called Ranvier's nodules and they are in contact with the extracellular fluid.

The action potential is transmitted differently in an unmyelinated axon (which is not covered in myelin) than in a myelinated one.

The action potential can travel through the myelin-covered axonal membrane by the properties of the wire. The axon in this way, conducts the electrical change from the place where the action potential occurs to the next node of Ranvier.

This change tapers off slightly, but is strong enough to cause an action potential in the next node. This potential is then triggered or repeated in each node of Ranvier, transporting itself throughout the myelinated area to the next node.

This kind of conduction of action potentials is called saltatory conduction. Its name comes from the Latin “saltare”, which means “to dance”. The concept is because the impulse seems to jump from node to node.

Advantages of saltatory conduction for transmitting action potentials

This type of driving has its advantages. First of all, to save energy. Sodium-potassium transporters spend a lot of energy pulling excess sodium from inside the axon during action potentials.

These sodium-potassium transporters are located in areas of the axon that are not covered by myelin. However, in a myelinated axon, sodium can only enter the nodes of Ranvier. Because of this, much less sodium enters, and because of this, less sodium must be pumped out, so the sodium-potassium transporters have to work less.

Another benefit of myelin is speed. An action potential is conducted faster in a myelinated axon, since the impulse "jumps" from one node to another, without having to pass through the entire axon.

This increase in speed causes animals to think and react faster. Other living beings, such as squid, have axons without myelin that gain speed due to an increase in their size. Squid axons have a large diameter (about 500 µm), which allows them to travel faster (about 35 meters per second).

However, at the same speed the action potentials travel in the axons of cats, although these have a diameter of only 6 µm. What happens is that these axons do contain myelin.

A myelinated axon can conduct action potentials at a speed of about 432 kilometers per hour, with a diameter of 20 µm.

References

1. Action Potentials. (sf). Retrieved on March 5, 2017, from Hyperphysics, Georgia State University: hyperphysics.phy-astr.gsu.edu.
2. Carlson, NR (2006). Physiology of behavior 8th Ed. Madrid: Pearson.
3. Chudler, E. (nd). Lights, Camera, Action Potential. Retrieved on March 5, 2017, from the University of Washington: faculty.washington.edu.
4. Stages of the Action Potential. (sf). Retrieved on March 5, 2017, from Boundless: boundless.com.