Decussation of the pyramids: parts and characteristics

Our nervous system is made up of a large number of fibers and bundles that run throughout the body. Our senses, perceptions, thoughts and emotions are governed by this system. Also our ability to move. There are multiple bundles that govern the latter, being especially relevant for voluntary movement those that are part of the pyramidal system.

But if we look from where they originate to where they arrive, we will see a detail that may seem peculiar: at a certain point Specifically, most of the nerve fibers cross from the hemisphere where they originate to the opposite side of the body. This fact is due to the decussation of the pyramids., which we will discuss in this article.

• Related article: "Parts of the human brain (and functions)"

From one hemibody to the other

The pyramidal system is called the system or set of motor-type nerve pathways that go from the cerebral cortex to the motor neurons of the anterior horn of the spinal cord, where they will connect with the motoneurons that will end up causing movement.

This system names itself by the type of neurons that configure it, and generally send information regarding voluntary motor control. One of the main nerve bundles of this system is the corticospinal, which is linked to the precise control of movement and muscle contraction. But the fibers of this system do not remain in a single hemisphere. There comes a point where most motor fibers from one part of the brain cross to the opposite half of the body.

• Related article: "Parts of the Nervous System: functions and anatomical structures"

Crossing the nerve pathways: pyramidal decussation

We call pyramidal decussation crossing made by the pyramidal fibers, passing the nerve fibers from the left side of the brain to the right hemibody and those from the right side to the left. This therefore implies that the part of the brain that controls our right part is the left hemisphere, being damage to the left hemisphere which could lead to paralysis and other conditions on the right side of the body.

However, despite the fact that most nerve fibers cross to the contralateral hemibody, between 15 and 20% of nerve fibers do not go through decussation, continuing to function ipsilaterally (that is, the nerve pathway continues from the brain to its destination in the same hemibody).

From this decusation arise two large bundles of neurons, the anterior corticospinal (which is ipsilateral) and the lateral corticospinal (configured by the majority of nerve fibers that decussate). The lateral corticospinal is associated with the fine movement of the most distal parts of the body, such as the fingers, allowing skills such as writing or manipulating objects. The ventral or anterior, although it does not decussate in the pyramidal decussation of the medulla oblongata, to a great extent it ends up doing so within the spinal cord itself, reducing the percentage of fibers that remain ipsilateral to around 2%. It is responsible for the proximal areas of the extremities, trunk and neck.

Where in the nervous system does it occur?

The place where the pyramidal decussation occurs, that is, the point from which the nerve bundles pyramidals on the left side of the body will cross and enter the right hemisphere and those on the right in the left, is located in the brain stem.

In the medulla oblongata you can find the pyramids, the bundles of nerve fibers that carry motor information from the brain to the rest of the body. And it is in this structure also where the pyramidal decussation point is found. Specifically, it can be found in the lowest part of the medulla oblongata, putting this structure in contact with the spinal cord.

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Why does the decussation of the pyramids exist?

It is legitimate to ask what sense it makes for nerve fibers to cross at the pyramidal decussation and cause the movement of one side of the body to be carried by the contralateral cerebral hemisphere. It is a question that has sought an answer since the moment the accusation was discovered.

This question, in reality, is not something that has a clear answer. A possible explanation of this fact was the one proposed by Ramón y Cajal, who proposed that the pyramidal decussation was related to that of the sensory pathways: in the optic chiasm there is also produces decussation of a large part of the optic nerve fibers, which is adaptive for perception by allowing both hemispheres have complete information of what both eyes perceive and can generate complete and locatable images in the space.

In this sense, the displacement necessary to react to a possible threat would be that of the muscle groups contrary to that of the part of the brain that perceives them. If there were no pyramidal decussation, the information would first have to travel to the other hemisphere to be subsequently processed and reacted, which would slow it down. Decussation allows the correct muscles to be activated at the right time.

However, it must be taken into account that, although it is a plausible theory that would explain the decussation as something evolutionary, we are facing a hypothesis that should not be taken as the truth absolute. It could be interesting to further explore the possible cause and meaning of the decussation of the pyramids.

Bibliographic references

• Kandel, E.R.; Schwartz, J.H. & Jessell, T.M. (2001). Principles of neuroscience. Fourth edition. McGraw-Hill Interamericana. Madrid.
• Ramon y Cajal, S. (1898). Structure of the optic chiasm and general theory of the crossing of the neural pathways. Rev. Trim. Micrograph 3: 15–65.