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The decussation of the pyramids: their parts and characteristics

The decussation of the pyramids: their parts and characteristics

May 9, 2024

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

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

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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 motoneurons of the anterior horn of the spinal cord, where they connect with the motor neurons that will end up causing movement.

This system calls itself by the type of neurons that configure them, and they generally send information regarding voluntary motor control. One of the main nervous 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 in one part of the brain cross the opposite side of the body .

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Crossing the nerve pathways: pyramidal decussation

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

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

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

In what part of the nervous system is it produced?

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

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

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Why is there a decussation of the pyramids?

It is legitimate to ask what sense it is that the nerve fibers cross in 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 been answered since the decussation 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 argued that the pyramidal decussation was related to that of the sensory pathways: in the optic chiasm there is also decussation of a large part of the optical nerve fibers, which is adaptive in the face of perception by allowing both hemispheres to have an information complete of what both eyes perceive and can generate complete and locatable images in space.

In this sense, the displacement necessary to react to a possible threat would be that of muscle groups contrary to the part of the brain that perceives them. If there is no pyramidal decussation, the information would first travel to the other hemisphere and then process and react, which would be slower. Decusing allows you to activate the right muscles at the right time .

However, we must bear in mind 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 absolute truth. It might be interesting to explore to a greater extent 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.
  • Ramón y Cajal, S. (1898). Structure of the optic chiasm and general theory of the intersections of nerve pathways. Rev. Trim. Micrograph 3: 15-65.

Neuroanatomy - The Brainstem (May 2024).

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