What are the parts and functions of a neuron?

Such is the importance of these structures in the relationship of the human being with the environment and the responses that are necessary in it, that it is estimated that there are more than 100 billion neurons in our nervous system.

• It may interest you: “The 16 types of neurons (functions and characteristics)”

Despite its relevant role and its high specialization, few people know the specific functions and parts of the neuron. Without this unique and fascinating cell, human thought itself would be impossible, and for this reason, here we tell you all the secrets about it.

The parts of the neuron: an entity of a cellular nature

First of all, we must not lose sight of the fact that, as complex as it may seem, the neuron is just another cell in the human body. For this reason, we have to delimit this concept a bit in itself before delving into neuronal morphological terminologies. For a structure to be considered a cellular entity in itself, it must meet certain requirements. These are the following:

• Individuality: that is, all cells must have some type of membrane that isolates them from the outside and maintains their shape and potential.
• Containing an internal aqueous medium, the cytosol, which accounts for the majority of the cell volume and houses the organelles.
• That contains both DNA (the molecule that stores vital information) and RNA (the molecule that is responsible for translating this information into proteins).
• That it has both enzymes and other proteins, which together produce a series of reactions that give rise to cellular metabolism.

As we can see, the concept of a cell is somewhat stricter than one might initially think. Therefore, pathogenic agents such as viruses are considered acellular as they do not meet these requirements, which makes them, in a theoretical framework, non-living microorganisms.

Now yes, we are going to immerse ourselves in the world of these nerve cells as unique as they are fascinating . We begin by relating the parts of the neuron to its body, the soma.

1. Soma

The soma is the cell body of the neuron, which contains the nucleus and various organelles, which are floating in the cytoplasm. These are the following:

• Nissl bodies : clusters of the rough endoplasmic reticulum (RER) with spirally arranged ribosomes attached. Its function is protein synthesis.
• Smooth endoplasmic reticulum (REL) : It differs from the rough one because it does not have associated ribosomes. It has detoxifying function and lipid synthesis.
• Golgi apparatus : the “postman” of the cell, as it is responsible for labeling proteins and lipids synthesized in it so that they are sent where they are needed.
• A cytoskeleton , made up of filaments and fibrils, which gives rise to the neuronal shape.
• Mitochondria : the famous organelles in charge of energy production within the cell. They are literally power plants of tiny size.

As we can see, the soma of the neuron closely resembles that of any other non-specific cell type. Nissl bodies are the only neuron-specific organelle of note, something that makes sense if we turn to their function. The proteins secreted by these bodies are essential for the transmission of nerve impulses, the main job of this nerve cell.

As an additional interesting fact, studies have revealed that the size of the soma can be conditioned, in part, by social interactions. In a species of cichlid fish (Haplochromis burtoni) it was seen that the size of the soma of some immunoreactive neurons decreased if the individual in question lived with animals of the same species that were older, more aggressive and had a better sexual predisposition. Incredible true?

2. Dendrites

Dendrites are another of the most characteristic parts of the neuron . They are cytoplasmic extensions of the soma, mainly dedicated to the reception of stimuli. They often appear as branches or points extending out from the cell body, giving the neuron that characteristic and striking “worm with tentacles” shape.

To keep ourselves in a simple terminological plane, we will limit ourselves to saying that the dendrites act as an input channel for signals coming from the outside to the soma, since they have chemoreceptors capable of reacting to neurotransmitters, biomolecules synthesized by other neurons.

It is very interesting to know that various studies correlate post-traumatic stress disorders (PTSD) with the morphology of these ramifications. As a neurohormonal response to stress, the dendrites of neurons in the medial prefrontal cortex contract, thus showing less branching. Incredible as it sounds, this correlates with decreased performance on tests of working memory and executive function.

3. The axon

The axon is a thin extension or “tail” of the cell body . Due to its length, sometimes greater than a meter (yes, a meter, because for example in the peripheral system each nerve fiber is an extension of a single cell), this morphological component makes the neuron the largest cell in the human body both in volume and surface.

It is a complex extension, since the cell membrane that surrounds it receives its own name, the axolemma. This membrane is very important in maintaining the membrane potential, as it contains ion channels through which ions can be transported in and out of the cell efficiently. The axolemma, in turn, is surrounded by Schwann cells in the peripheral nervous system, which act as electrical insulators as they contain myelin (a lipoprotein material).

In general, it can be said that the axon is divided into three sections: the axon hillock, the initial segment of the axon, and the rest of the axon body. It is in this “rest” where the Ranvier nodes are present, interruptions that occur along the axon, which give it this “beaded necklace” shape. As in any biological structure, it is essential to emphasize that these interruptions do not occur by chance, since they allow the nervous impulse to be transmitted more quickly, in a saltatory manner and minimizing the possibility of error.

Axon growth requires complex assembly machinery, and various studies collect this process in a simple and didactic way. After all, the axon grows just like a car is made:

• The presence of construction material is required, that is, the elements previously named: “gasoline”.
• Cyclic filaments produced in the growth cone are necessary: ​​“the engine”.
• Connections are needed between the axon hillock filaments and the growth substrate: “the clutch”.
• And lastly, mechanisms are essential to translate external signals that determine the directionality of the axon: “the steering wheel”.

All these concepts, initially strange and convoluted, do nothing more than put into perspective the extremely complex cellular processes that take place so that axonal growth can occur.

Conclusions

The different parts of the neuron aim to produce synaptic contact, that is, the transmission of nerve impulses between two neuronal cells. We can say, in a simple way, that the dendrites are responsible for collecting these signals, which are translated into an action potential that produces a release of new neurotransmitters from the axon to the dendrites of other neurons.

The action potential travels at an average speed of between 2 and 200 kilometers per hour, a figure that is not negligible. As we have seen in this space, the unique structure of the neuron allows nerve impulses to travel throughout our body , allowing us to respond and adapt to any environmental variable surprisingly quickly.

Bibliographic references

• Aguilar Pardo, D.R. (2012). Parts of the Neuron. Biology.
• Barnes, A. P., & Polleux, F. (2009). Establishment of axon-dendrite polarity in developing neurons. Annual review of neuroscience, 32.
• Barrera, JAT Neural Networks. University of Guadalajara Available at: http://www.cucei.udg.mx/sites/default/files/pdf/toral_barrera_jamie_areli.pdf [Accessed October 2016].
• Davis, M. R., & Fernald, R. D. (1990). Social control of neuronal soma size. Journal of neurobiology, 21(8), 1180-1188.
• Giolli, R. A., & Towns, L. C. (1980). A review of axon collateralization in the mammalian visual system. Brain, Behavior and Evolution, 17(5), 364-390.
• Goldberg, J. L. (2003). How does an axon grow?. Genes & development, 17(8), 941-958.
• Gulledge, A. T., & Bravo, J. J. (2016). Neuron morphology influences axon initial segment plasticity. Eneuro, 3(1).
• The Neuron, Shippenburg University. Collected on July 30 at http://webspace.ship.edu/cgboer/genesp/neuronas.html
• Zegarra-Valdivia, J.A., & Chino-Vilca, B.N. (2019). Neurobiology review of post-traumatic stress disorder. Mexican Journal of Neuroscience, 20(1), 21-28.