Does the left side of the brain control the right side of the body?

In this post we are going to answer the question ‘’Does the left side of the brain control the right side of the body?’’ We will explain to you how the left hemisphere controls the right side of the body, all the brain mechanisms and the reasons why this happens.

Does the left side of the brain control the right side of the body?

Yes, the left side of the brain control the right side of the body.

Although this statement is not true for the total body of the human being, it is true that we can affirm that a good part of the motor functions is controlled by the opposite side of our brain. That is, the functions that involve, for example, moving the right leg, are controlled by the motor cortex of our left hemisphere. What is this phenomenon due to?

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 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 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 to the opposite side of the body.

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 motor neurons that will eventually cause movement.

This system names itself by the type of neurons that configure them, and generally send information regarding voluntary motor control. One of the main nerve bundles in this system is 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 fibers from one part of the brain cross into the opposite half body.

Crossing the nerve pathways: pyramidal decussation

To begin the explanation, we have to resort to the concept of decussation or chiasma (from the Greek “x”).

We call pyramidal decussation the crossing made by the pyramidal fibers, passing the nerve fibers from the left side of the brain to the right half body 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 the lesion of the left hemisphere 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 pass through decussation, continuing to function ipsilaterally (that is, the nerve pathway continues from the brain to their fate in the same hemibody).

From this decussation, two large bundles of neurons emerge, the anterior corticospinal (which is ipsilateral) and the lateral corticospinal (configured by most of the nerve fibers that decuss). 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 decline in the pyramidal decussation of the medulla oblongata, largely ends up doing so within the spinal cord itself, reducing the percentage of fibers that remain ipsilateral to around 2%. It takes care of the proximal areas of the extremities, trunk and neck.

In what part of the nervous system is it produced?

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

In the medulla oblongata they can find the pyramids, the bundles of nerve fibers that are going to 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 found. Specifically, it can be found in the lowest part of the medulla oblongata, putting this structure in contact with the spinal cord.

Why is there the decussation of the pyramids?

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

This question is not really something that has a clear answer. A possible explanation for this fact was the one proposed by Ramón y Cajal, who suggested that the pyramidal decussation was related to that of the sensory pathways:

Decussation of a large part of the optic nerve fibers also occurs in the optic chiasm, which is adaptive with regard to perception by allowing both hemispheres to have complete information on what both eyes perceive and complete and localizable images can be generated in space.

In this sense, the movement 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 was no pyramidal decussation, the information would have to travel first to the other hemisphere to later be processed and reacted, which would be slower. Decussation allows you to activate the correct muscles at the right time.

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

But why would the body “indulge” itself by crossing certain nerve pathways?

As always, nature does not indulge itself, and the explanation must be found in terms of evolution and Natural Selection, that is, survival. And since it is about survival, we must talk about the flight responses and the speed to execute them, in addition to the sense through which we essentially detect dangers: sight.

It is well known that our eyes, due to optical issues, invert the image and thus “project” it onto the retina.

Part of the nerves that collect this visual information, since ours is binocular (where the field of vision of one eye overlaps in the centric regions to the other) must decuse so that the visual cortex (occipital lobe, in the back of the head) of both hemispheres collect a real image of the physical environment.

Thus, the right fibers of our eyes collect information from the left region of the total visual field, and the left fibers collect information from the right region of the visual field. The right visual cortex ends up processing the left information from the visual field thanks to these decisions, and vice versa.

And what does that have to do with motor response? As we have said, the design of our body has been “shaped” by a vital and constant need to be as effective as possible. That opposite side motor control exerted by each hemisphere is an effective design.

Let us think of a case of flight, where we are sitting calmly at the foot of a tree and suddenly we perceive that on our right a snake appears ready to attack our right arm. This perception of the snake is “processed” by the cerebral cortex of the left hemisphere. To avoid the peck in the most effective way we need to withdraw the right arm as quickly as possible.

If this decussation of the motor pathways does not exist, of the “cables” that carry the information of “move on!”, The left visual cortex should send the information to the motor cortex of the right hemisphere to initiate the motor response of withdrawing the right arm.

However, what it does is that it sends the information to the motor cortex of the same hemisphere and this initiates the response of withdrawal from the opposite side thanks to this crossing of roads, this path being significantly faster, especially if it is to avoid being poisoned.

Another example of the engineering work that is the human being and the rest of the living beings with which it shares structures.

FAQS: Does the left side of the brain control the right side of the body?

Why does right side of brain control left side of body?

Generally, the left hemisphere is dominant for language and mathematical ability, while the right hemisphere is dominant for spatial perception and musical ability.

What does the left side of the brain control?

It is often said that the left hemisphere is the one behind logical and analytical areas, such as mathematics, while the artistic is related to the right hemisphere. It is also assumed that the right hemisphere controls the left side of the body, while the left does the same but with the right side.

What side of brain controls right side of body?

The left side of the brain controls the movements of the right side of the body, and the right side of the brain controls the movements of the left side of the body.

What is the difference between the left side of the brain and the right side?

The right hemisphere is in charge of giving a global vision of things. The left one focuses on the details. The left hemisphere specializes in what is said and the right in how it is said.

Which side of the brain controls vision?

The occipital lobe, located at the back of the brain, processes light and other visual information that it receives from the eyes.

In this post we answered the question ‘’Does the left side of the brain control the right side of the body?’’ We explained to you how the left hemisphere controls the right side of the body, all the brain mechanisms and the reasons why this happens.

If you have any questions or comments please let us know!

References

Kandel, E. R., Schwartz, J. H., Jessell, T. M., Siegelbaum, S., Hudspeth, A. J., & Mack, S. (Eds.). (2000). Principles of neural science (Vol. 4, pp. 1227-1246). New York: McGraw-hill.

Hutsler, J.; Galuske, R.A.W. (2003). Hemispheric asymmetries in cerebral cortical networks.Trends in Neurosciences. 26 (8): 429–435.

Llinás, R. R. (2003). The contribution of Santiago Ramon y Cajal to functional neuroscience. Nature Reviews Neuroscience, 4(1), 77-80.