Let’s think for a moment about a human brain. It is a highly complex structure in which we can perceive the existence of two clearly differentiated parts, the two cerebral hemispheres.
We also know that each of these hemispheres has some more specialized functions in different aspects , for example, speech being in the left hemisphere (generally) or having seen that while the right hemisphere is more holistic or global, the left hemisphere is more logical and analytical. However, these two hemispheres are not loose and separated from each other , but somewhere in the brain’s anatomy it is possible to find a point of union. This junction point is the so-called corpus callosum .
What is corpus callosum?
The corpus callosum is the main bundle of nerve fibers that connects both hemispheres of the brain. This structure is mainly made up of neural axons coated with myelin, thus forming part of the white substance of the brain. Within the white matter the corpus callosum is considered an interhemispheric commissure, as it connects and exchanges information between structures in the different hemispheres. There are other interhemispheric commissures in the human brain, but they are much smaller than the corpus callosum.
This structure is located in the midline of the brain, at the bottom of the interhemispheric fissure, and is mostly hidden from external observation as it is partially covered by the cortex. It is leaf or comma shaped, possessing different parts that connect different parts of the brain to each other .
The areas connected by this brain structure are mostly cortical areas, although there are some exceptions. Generally, subcortical structures are connected to other structures and commissures.
Parts of the corpus callosum
Although the corpus callosum is considered a single structure, it has traditionally been divided into several parts. Specifically, the corpus callosum could be divided into the following four sections .
1. Pico or rostrum
Located in the lower front part of the corpus callosum, it is the most anterior part of this structure. It is born from the terminal lamina and is connected to the optic chiasm.
2. Genuine or knee
This is the part of the corpus callosum that curves into the brain , first moving towards the frontal lobes to form into smaller forceps. The fibres of this part of the corpus callosum connect the prefrontal cortexes of the two hemispheres, allowing their information to be integrated .
After the genuine or knee, is the body, which ends up thickening in the back. It connects to the septum and trigone , which is in turn an important connecting structure between regions of the brain, such as the thalamus, hippocampus and other areas of the limbic system.
The most posterior and final part of the corpus callosum is formed by the fibers that end up being associated with other projection and associative fibers. It connects with the occipital lobe to form the major forceps, and also links to the lateral ventricle to the point of forming one of its lower walls . It also connects with the pineal gland and the havenular commissure (which connects the havenular nuclei of both hemispheres).
Functions of this part of the brain
The main function of the corpus callosum is to transmit information from one hemisphere to another , allowing interhemispheric communication. In this way, the fact that the functions of each of the hemispheres are partly different does not prevent them from acting as an integrated whole, allowing the precise execution of the different processes and actions carried out by the human being.
In this sense also is linked to learning and information processing , by joining and linking the different brain nuclei. On the other hand, if, for example, a part of one brain hemisphere is injured, thanks to the corpus callosum the opposite hemisphere can take care of those functions that are left unattended.
Furthermore, some studies show that apart from this function, the corpus callosum also influences vision, specifically eye movement , by transmitting information about the eye muscles through it. This is natural, since in eye movements coordination between the two bodies, in this case the eyes, is crucial.
What happens when it’s sectioned?
The corpus callosum is an important structure when integrating the information received and processed by both brain hemispheres. Although the absence of connection between hemispheres at the corpus callosum level does not mean a complete loss of functionality (since although it is the main interhemispheric commissure, it is not the only one ), the total or partial disconnection of the cerebral hemispheres can be an important handicap for carrying out various activities.
Among other things, this kind of disconnection between parts of the brain can lead to what is known as callus disconnection syndrome .
In this syndrome we have seen how patients with a divided brain (that is, who present a disconnection between both hemispheres) have shown difficulties such as lack of coordination, repetition or perseverance when carrying out sequenced activities such as combing, feeding or dressing, sometimes performing the same action twice due to the lack of motor integration.
It also makes it very difficult to learn and retain new information by not being able to coordinate the information correctly (although not impossible, it requires much more effort than usual), as well as it can cause alexia (inability to read) and agraphia (inability to write).
In addition, significant alterations can occur at the sensory level. For example, it has been shown that subsequent lesions of the corpus callosum can cause severe difficulties in discriminating between somatic stimuli , leading to somatic agnosias or lack of recognition from tactile stimuli. Memory and language problems are also common.
Callosotomy: when sectioning the corpus callosum can be good
Despite the disadvantages that this type of surgical intervention may entail, in the presence of some very serious disorders, the division of the corpus callosum or callosotomy has been evaluated and successfully applied for medical purposes, as a lesser evil.
The most typical example is that of resistant epilepsy , in which the sectioning of parts of the corpus callosum is used as a method of reducing severe epileptic seizures, preventing the epileptoid impulses from travelling from one hemisphere to another. Despite the problems it can cause on its own, callosotomy increases the quality of life of these patients, because the difficulties it can cause are less than those produced by continuous epileptic seizures , thus reducing the risk of death and improving the quality of life.
On the other hand, over time it is possible that the brain may reorganize to allow for mental processes that during the first weeks after the operation seemed eliminated or seriously damaged, although recovery is usually not complete.
Conditions affecting the corpus callosum
It has been previously indicated that the division of the corpus callosum can have limiting effects, although sometimes its section can be considered in order to improve the symptoms of some disorder. However, the corpus callosum can be cut or damaged in an accidental or natural way , and there are many diseases that can affect this area of the brain. Some of these alterations may occur from the following.
1. Head injuries
In the event of a blow or trauma, the corpus callosum can easily be damaged, mainly due to its great consistency and density. Usually there is a tear in the substance , or diffuse axonal damage as a result of the blow-counterblow to the skull bones. If we speak of point-specific effects, the greatest effect is usually on the splenius.
Although it is not frequent due to the bilateral irrigation of the corpus callosum, it is possible to find cases in which hemorrhages or ischemia produce an affectation of the white substance of the corpus callosum . In this way, the alterations in blood flow are able to leave the communication between the two hemispheres that takes place in the corpus callosum practically cut off, without the need for a solid element to come into contact with this part of the brain and break it.
3. Demyelinating disorders
Being a structure formed by white substance, coated with myelin, disorders such as multiple sclerosis greatly affect the corpus callosum . This type of disorder means that the messages sent by the brain are not sent as efficiently or even that many neurons die, so that in the corpus callosum the perceptions and functionalities of both hemispheres cannot be easily integrated. In this way, mental processes involving regions on both sides of the brain are greatly affected, or directly cannot be carried out.
4. Brain Tumors
Although their compaction means that there are generally not many tumours that affect the corpus callosum , some are very aggressive such as lymphoma or glioblastoma multiforme , which is usually located in the white substance, if they can infiltrate this specific structure and cause serious damage or “strangle” it by the pressure exerted by the growth of the cancerous parts.
In the case of glioblastoma it usually produces a typical butterfly-shaped pattern with greater involvement of the central area.
Although not very frequent, it is possible to find malformations in some subjects that cause them, from birth, to have fewer connections than usual. Other types of congenital malformations can cause easy rupture (and consequent bleeding) of blood vessels in the brain, which can also affect the corpus callosum.
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