Interhemispheric commissures: what are they and what are their functions?

Although the corpus callosum is the largest, there are other commissures that connect the cerebral hemispheres

The human brain is composed of two large cerebral hemispheres which, although they have their own functions, must communicate constantly so that information is transmitted and there is good structural and functional connectivity.

The interhemispheric commissures, which include the corpus callosum, are responsible for this interconnection to occur correctly.

In this article we explain what are the main interhemispheric commissures and what are they?where they are located, what relevant functions they carry out and what problems are caused by damage to these structures.

Interhemispheric commissures: definition and neuroanatomical location

The interhermispheric commissures are a set of structures made up of nerve fibers that connect different regions of the two cerebral hemispheres. These commissures are part of the white matter, which is composed of association fibers, commissural fibers and projection fibers.

Although the best known commissure is the corpus callosum, there are others that also play an important role in brain interconnectivity, such as the anterior commissure, the posterior commissure and the fornix. The anterior and posterior commissures are made up of small bundles of interhemispheric fibers, located in front of and behind the corpus callosum.

The anterior commissure connects the olfactory bulbs, the amygdaloid nuclei, and the medial and inferior temporal lobes. The fibers of the posterior commissure connect areas in the occipital lobes, mainly the areas related to pupillary response and eye movement control.

The largest structure of all commissures is the corpus callosum, located at the top of the diencephalon, forming a large part of the roof of the lateral ventricles located at the top of the diencephalon, forming a large part of the roof of the lateral ventricles. Smaller commissures include: the anterior commissure, located caudally to the tribune of the corpus callosum and rostral to the main part of the fornix; and the hippocampal commissure, formed by fibers originating in the hippocampal formations and crossing the midline as a thin layer inferior to the splenium of the corpus callosum.

The following is a description of the major interhemispheric commissures of the brain.

1. The corpus callosum: characteristics and functions

The corpus callosum is the main interhemispheric commissure. It is formed by bundles of white matter and is located deep in the longitudinal fissure of the brain. The main function of this structure is to transmit information from one cerebral hemisphere to another, thus promoting interhemispheric communication. It also plays a role in learning processes and problem solving.

The fact that there is a structure that acts as a bridge between hemispheres implies that the brain, despite having two distinct hemispheres, acts as an integrated whole that allows the management and execution of different cognitive tasks and functions. On the other hand, it has been suggested that structural abnormalities of the corpus callosum are related to diseases such as multiple sclerosis and other cognitive dysfunctions.

This structure is made up of the following parts:

Peak or rostrum

Located in the inferior frontal area of the corpus callosum, It arises from the lamina terminalis and connects to the optic chiasm.

Knee

It is the part of the corpus callosum that curves inward, its anterior end. Its fibers connect areas of the prefrontal cortex of both hemispheres of the two hemispheres.

Body

Posterior to the knee, it constitutes the superior face of the corpus callosum and connects to the septum and trigone. connects with the septum and trigone.

Splenium or buckle

Constitutes the posterior end of the corpus callosum, and is formed by fibers is formed by fibers that connect with the occipital lobe to form the greater forceps. It is also connected to the lateral ventricle, forming its inferior walls.

2. The anterior commissure: characteristics and functions

The anterior commissure is another interhemispheric commissure formed by a bundle of nerve fibers connecting the temporal lobes. It is located inferior to the peak of the corpus callosum and above the optic chiasm. It is a smaller structure than the corpus callosum and the only mixed commissure (with two types of nerve fibers) (with two types of nerve fibers).

It has been suggested that this commissure may play an important role in Pain sensations (specifically, in acute pain). In addition, its connections with the temporal lobes and the amygdala indicate that it is also involved in memory, memory processes, and the amygdala. involved in memory processes, emotion-associated processes, hearing and language or speech.

The anterior commissure has also been related to basic processes such as olfaction, instinctive behavior and sexual behavior. However, research on subjects with lesions in this brain structure and adjacent areas postulates that it may be involved in numerous functions, ranging from attentional processes to color perception.

3. The posterior commissure: definition, location and characteristics

The fibers of the posterior commissure cross the midline of the brain, above the orifice of the aqueduct of Sylvius (located in the third ventricle). This structure is composed of a rounded band of white fibers. It appears to play an important role in the bilateral pupillary light reflex.

Most of the fibers of the posterior commissure originate in the posterior commissural nucleus (Darkschewitsch nucleus), located in the nuclei of the periaqueductal gray substance at the rostral end of the cerebral aqueduct, in front of the oculomotor nucleus.

4. The fornix: characteristics, structure and functions

The fornix is a white matter structure, also called trigone, which originates in the hippocampus. originates in the hippocampus and passes through the mammillary bodies of the hypothalamus. It is composed of fibers organized in several subregions such as the alveus, the fimbria or the columns of the fornix.

These columns are connected to each other by the commissure of the fornix, a bundle of nerve fibers that serve as a connection between them before forming the body to integrate the formations coming from the hippocampus. When the columns meet at the midline they form the body of the fornix.

It has been suggested that the function of the fornix is to connect different structures to allow information to be transmitted from one side to the other. Also It has also been linked to memory consolidation processes. In fact, patients with lesions in this structure often have problems in this cognitive function.

Research also suggests that lesions in the fornix can lead to deficits in declarative memory (which allows us to recall events voluntarily) and episodic memory (which stores autobiographical memories or specific events that are part of our personal history).

Disorders related to lesions in these structures

Injuries to the main interhemispheric commissure of the brain, the corpus callosum, can cause severe sensory and motor deficits. The main disorder caused by sectioning this structure is callosal disconnection syndrome, which causes difficulties in motor coordination involving incoordination, repetition or perseveration in actions involving sequential movements (such as combing one's hair or tying shoelaces).

Another of the clinical consequences that may occur in callosal disconnection syndrome are difficulty in consolidating learning and retention of new information.. Agraphia (total or partial inability to write) and alexia (partial or total loss of the ability to read) may also occur.

Other types of conditions such as tumors, traumatic brain injury, stroke, malformations or degenerative diseases may be responsible for the clinical signs and symptoms characteristic of damage to the nerve fibers of the interhemispheric commissures connecting the two cerebral hemispheres.

Bibliographic references:

• Peña-Casanova, J. (2007). Neurology of behavior and Neuropsychology. Editorial médica Panamericana.
• Sperry, R. W., Gazzaniga, M. S., & Bogen, J. E. (1969). Interhemispheric relationships: the neocortical commissures; syndromes of hemisphere disconnection.
• Ventura, R. L. (2003). The cerebral interhemispheric disconnection syndrome. Journal of psychiatry and mental health Hermilio Valdizan, 4, 29 - 42.