The decussation of the pyramids: its parts and characteristics.

Strange part of the body in which nerve fibers pass from left to right and vice versa.

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. So is our ability to move. There are multiple beams that govern the latter, being especially relevant for voluntary movement those that are part of the pyramidal system.

But if we observe from where they originate to where they arrive, we will see a detail that may seem peculiar: at a particular point 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 pyramidswhich we will discuss in this article.

From one hemibody to the other

It is denominated pyramidal system to the system or set of nervous routes of motor type that go from the cerebral cortex to the motoneurons of the anterior horn of the spinal cord, where they will connect with the motoneurons that will end up causing the movement.

This system is named after the type of neurons that make them up, and they 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 at which most of the motor fibers from one part of the brain cross over to the opposite hemisphere..

Crossing the nerve pathways: pyramidal decussation

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

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

From this decussation arise two large bundles of neuronsThe anterior corticospinal (which is ipsilateral) and lateral corticospinal (made up of the majority of decussating nerve fibers). The lateral corticospinal is associated with the 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 decussate in the pyramidal decussation of the medulla oblongata, it ends up doing so to a large extent within the spinal cord itself, reducing to about 2% the percentage of fibers that remain ipsilateral. It is responsible for the proximal areas of the limbs, trunk and neck.

Where in the nervous system does it occur?

The place where pyramidal decussation occurs, i.e. the point from which the pyramidal nerve bundles from the left side of the body cross into the right hemisphere and those from the right into 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 also in this structure where the pyramidal decussation point is located. Specifically, it can be found in the lowest part of the medulla oblongata, putting this structure in contact with the spinal cord.

Why does the decussation of the pyramids exist?

It is legitimate to ask what sense it makes that 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. This is a question that has sought an answer from the moment decussation was discovered.

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

In this sense, the necessary displacement to react to a possible threat would be that of the muscle groups opposite to that of the perceiving part of the brain. In the absence of pyramidal decussation, the information would first have to travel to the other hemisphere to be processed and reacted to, which would be slower. Decussation makes it possible to activate the right muscles at the right time..

However, it should be noted that, although this is a plausible theory that would explain the decussation as evolutionary, it is a hypothesis that should not be taken as the absolute truth. It could be interesting to explore further the possible cause and meaning of the decussation of the pyramids.

Bibliographical 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 cross-linking of nerve pathways. Micrographic Trim. Rev. 3: 15-65.