Our nervous system is made up of a large number of fibers and beams 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 look from where they originate to where they go, we will see a detail that may seem peculiar: at a certain 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 decusation of the pyramids , which we will comment on in this article.
From one body to another
The pyramidal system is the system or set of motor-type nerve pathways from the cerebral cortex to the motor neurons in the anterior spinal cord shaft, where they connect with the motor neurons that will eventually cause the movement.
This system calls itself by the type of neurons that make it up, and generally sends out information regarding voluntary motor control. One of the main nerve bundles in this system is the corticospinal, which is linked to 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 of the motor fibres from one part of the brain cross over to the opposite hemisphere .
Crossing the nerve pathways: pyramidal decusation
We call the pyramidal decusation a crossing made by the pyramidal fibres , passing the nerve fibres from the left side of the brain to the right hemicorpe and those from the right side to the left. This means that the part of the brain that controls our right side is the left hemisphere, and the injury to the left hemisphere could cause paralysis and other conditions on the right side of the body.
However, despite the fact that most of the nerve fibers cross over to the contralateral hemicorpus, between 15 and 20% of the nerve fibers do not pass through the decustation , continuing to function in an ipsilateral manner (i.e. the nerve pathway continues from the brain to its destination in the same hemicorpus).
From this decusation, two large bundles of neurons arise, the anterior corticospinal (which is ipsilateral) and the lateral corticospinal (formed by most of the decaying nerve fibers). The lateral corticospinal is associated with the fine movement of more 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 decay in the pyramidal decusation of the spinal bulb, to a large extent ends up doing so within the spinal cord itself, reducing to about 2% the percentage of fibers that remain ipsilateral. It takes care of the proximal areas of the extremities, trunk and neck.
Where in the nervous system does it occur?
The place where the pyramidal decusation takes place, that is to say 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 will enter the left hemisphere, is located in the brain stem .
In the spinal cord, 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’s in that structure too that the pyramid decusation point is found. Specifically, it can be found in the lowest part of the spinal bulb, bringing this structure into contact with the spinal cord.
Why does the decusation of the pyramids exist?
It is legitimate to ask what is the point of nerve fibers crossing in pyramidal decusation and causing movement of one side of the body to be carried by the contralateral brain hemisphere. This is a question that has been searching for an answer since the moment the decusation was discovered.
This question is not really something that has a clear answer. A possible explanation for this fact was proposed by Ramón y Cajal , who stated that pyramidal decusation was related to that of the sensory pathways: in the optic chiasm there is also decusation of a large part of the optic nerve fibres, which is adaptive in terms of perception by allowing both hemispheres to have complete information about what both eyes perceive and to generate complete and localizable images in space.
In this sense, the movement necessary to react to a possible threat would be that of the muscle groups opposite to that of the part of the brain that perceives them. If there were no pyramidal decusation, the information would have to travel first to the other hemisphere to be processed and reacted later, so it would be slower. Decusation allows the right muscles to be activated at the right time .
However, we must take into account that, although it is a plausible theory that would explain the decusation as something evolutionary, we are before a hypothesis that should not be taken as the absolute truth. It might be interesting to explore further the possible cause and meaning of the decusation of the pyramids.
- Kandel, E.R.; Schwartz, J.H. & Jessell, T.M. (2001). Principles of neuroscience. Fourth edition. McGraw-Hill Interamerican. Madrid.
- Ramón y Cajal, S. (1898). Structure of the optic chiasm and general theory of nerve tract crossovers. Micrographic Trim. 3: 15-65.