Trigger zone of the axon (A complete guide)

In this post we will talk about the axon activation zone, we will describe what an axon is, how it is shaped and how it communicates electrical signals.

Trigger zone of axon

The trigger zone is a region near the axon hillock that activates and initiates the propagation of the action potential of the cell through voltage gated channels inside the cell.

Neurons are the nerve cells thanks to which we are able to think, feel, make decisions and, even more, to be aware.

However, although the concept of “neuron” is well known even beyond laboratories and university classrooms, the truth is that to understand what our mental life is like it is not enough to know that in our head there are tiny cells that send nerve impulses to each other.

You also have to understand that there are different parts of neurons, responsible for carrying out different tasks. Axons are one of these components.

What is an axon?

A neuronal axon is a kind of sleeve or “arm” that comes out of and goes to a position away from the middle of the neuron. This tiny structure’s shape gives us hints about what its purpose is.

Basically, the function of axons is to make the electrical signals go to another position in the body that passes through neurons.

Therefore, the axon is a form of conduit through which nerve impulses travel at full speed; it serves as a medium of communication between the neuron’s central part (which is called the neuronal soma or neuron’s body and is where the DNA nucleus is) and another part of the nervous system to which this electrical stimulation must penetrate.

There is either a portion of the nerve fiber at the end of the axons that stretches when the electrical signal is transmitted to it, or there is a synaptic distance between neurons, which is the level at which these nerve cells, usually by chemical signals, interact with each other.

In other words, the electrical impulse at the tip of the axons is generally converted into a pattern of release of chemical particles through the synaptic space that enter the other neuron.

The size of the axons

If something characterizes the human body, it is by its complexity and by the vast variety of components that work together to make it work well. This implies that their size depends on the type of neuron to which it belongs, and on its position and function, in the case of neuronal axons.

What happens in our nervous system, after all, has a decisive influence on our chances of survival, and that is why evolution has ensured that there are many specialized nerve cells of various shapes and configurations in our species.

Depending on their function, the length of the axons of neurons can differ greatly. For example, there are often neurons with axons shorter than one millimeter in the gray matter regions of the brain, whereas there are other axons outside the central nervous system that are more than a span long, despite being very small.

In short, axons are so short in many cases that the distance between their tip and the body of the neuron is microscopic, and in other cases, they may be several centimeters long without intermediaries to penetrate distant regions.

As for the size of human axons, they typically have a diameter of between one and 20 micrometers (thousandths of a millimeter). This is not, however, a universal rule which applies to all animals that have nerve cells.

For instance, axons may be up to a millimeter thick in some invertebrate animals, such as squid, making it easy to see with the naked eye.

This is because the stronger the axon, the faster the electrical impulse passes through it, and this is an essential skill to make the siphon through which they eject water work well in the case of squid since they have to contract a large portion of muscle tissue at a time in order to be able to avoid jet propulsion easily.

The formation of nerves

Axons are, as we have shown, not only present in the brain. They are spread across the body, like what happens with neuronal bodies: through internal organs, arms and legs, etc.

In fact, a nerve is primarily a collection of axons that is so dense that without the need for a microscope we can see it directly.

What we see when we discover a nerve in a piece of meat is nothing more and nothing less than several axons clustered together in a bundle, combined with other auxiliary nerve cells.

Myelin sheaths

The axons are often not alone but accompanied by elements known as myelin sheaths that bind to their surface to the point of appearing to be an inseparable neuron portion.

Myelin is a fatty material that works, although not exactly, on axons much as a rubber insulator along an electrical wire would do.

In short, the myelin sheaths, distributed around the axon to produce a shape similar to a string of sausages, isolate the inside of the axons from the outside of the axons, so that the electrical signal from the walls is not lost and travels much faster. The defense they provide is targeted at both the neuron itself and the electrical signal transmitted by it.

Action potential: basic definition and characteristics

The action potential is understood as the electric signal or discharge occurring from the set to the set of changes encountered by the neuronal membrane due to electrical differences and the interaction between the neuron’s external and internal setting.

It is a single electrical wave that will be transmitted through the cell membrane until the end of the axon is reached, causing the release of neurotransmitters or ions into the postsynaptic neuron membrane, creating another potential for action in it that will ultimately bring some sort of order or information to some area of the body in the long run.

It begins in the axonal cone, near to the soma, where it is possible to detect a large number of sodium channels.

The capacity for action has the specificity of following the so-called rule of all or nothing. That is, either it happens or it doesn’t happen, and there are no intermediate possibilities.

Despite this, the presence of excitatory or inhibitory potentials that promote or impede it may affect whether the potential appears or not.

A message that is more or less painful (for example, the sense of pain in the face of a poke or a stab will be different) will not produce changes in the strength of the signal, but will only cause action potentials to be carried out more often. Both action potentials will have the same charge, and their sum can only be varied.

In addition to this, and in relation to the above, it should also be remembered that the potential for action cannot be introduced, because they have a short refractory duration during which that portion of the neuron is unable to activate another potential.

Finally, it illustrates the fact that at a particular point in the neuron the action potential is produced and must be generated along any of the points that follow it, not being able to return the electrical signal back.

Phases of the action potential

The action potential is produced by a series of stages, ranging from the initial resting situation to the sending of the electrical signal and ultimately the return to the initial state.

1. Resting potential

This first move assumes a basal state in which no adjustments have yet been made that lead to the potential for intervention. This is a moment when its base electrical charge, the membrane, is at -70mV.

Any tiny depolarizations and electrical variations can enter the membrane during this time, but they are not enough to activate the potential for action.

2. Depolarization

This second step (or the first phase of the potential itself) of stimulation causes an electrical change of sufficient excitatory strength to occur in the neuron membrane (which must cause at least a change of up to -65 mV and up to -40 mV in some neurons) so that the axon cone sodium channels are opened in such a way that sodium ions (positively charged) join massively.

In turn, the sodium/potassium pumps (which, by exchanging three sodium ions for two potassium ions in such a way that more positive ions are expelled than join, usually keep the interior of the cell stable) stop functioning.

This will trigger the membrane’s charge to shift in such a way that it exceeds 30mV. What is known as depolarization is this transition.

After that, the membrane potassium channels begin to open, which, because they are still a positive ion and are entering en masse, will be repelled and will start leaving the cell. This will trigger the slowing of depolarization, as positive ions are lost.

This is why the electrical charge would be 40 mV at most. The sodium channels are closed for a brief period of time and are inactivated (which prevents summative depolarizations). A wave that cannot go back has been created.

3. Repolarization

It ceases being able to reach the neuron because the sodium channels have been blocked, although the fact that the potassium channels remain open allows it to continue to be expelled. That is why the potential is increasingly negative, as is the membrane.

4. Hyperpolarization

The electrical charge of the membrane becomes steadily negative to the point of hyperpolarization as more and more potassium comes out: they hit a degree of negative charge that exceeds that of rest also. The potassium channels are closed at this time, and the sodium channels are activated (without opening).

This implies that the electric charge stops dropping and that a new potential could potentially occur, but the fact that it is hyperpolarized indicates that the amount of charge required for an action potential is far higher than normal. It also reactivates the sodium / potassium pump.

5. Resting potential

The sodium / potassium pump reactivation allows a positive charge to reach the cell little by little, which will eventually lead it to return to its basal state, the resting potential (-70mV).

6. Action potential and neurotransmitter release

From the axon cone to the end of the axon, this complex bioelectric process will be generated in such a way that the electrical signal will advance to the terminal buttons.

These buttons have calcium channels that open when the potential enters them, allowing their material to be released and expelled into the synaptic space by the vesicles that contain neurotransmitters.

Therefore, it is the potential for action that produces the neurotransmitters to be produced, becoming the key source of nervous information transmission in our body.

FAQS: Trigger zone of axon

What are zonal triggers?

A trigger zone in neuroscience and neurology is a region of the body or cell in which a specific form of stimulus induces a particular type of response.

What is the function of a neuron’s trigger zone?

The trigger zone is a region near the axon hillock that opens and initiates the propagation of the action potential of the cell through voltage gated channels inside the cell.

What is the difference between axon collaterals and axon terminals?

The collaterals of the axon are the main branches, while the axon terminals are tiny ends of each branch.

What structures are located on the axon?

Neurons have a structure called the myelin sheath, made up of support cells – Schwann cells – located in the axon. It contains a white, fatty substance that helps isolate and protect axons and increases the transmission of nerve impulses.

What is the trigger zone of a neuron?

The trigger zone is where the chemically operated gate area meets the voltage regulated gate area, normally at the junction of the axon and cell body, the hillock axon.

References

Kole, M.; Stuart, G.J. (2012). Signal processing in the axon initial segment. Neuron 73 (2): 235-247

Squire, Larry (2013). Fundamental neuroscience (4th ed.). Amsterdam: Elsevier/Academic Press. pp. 61–65. ISBN 978-0-12-385-870-2.

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