Interhemispheric commissures: what are they and what functions do they have?

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

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

In this article we explain What do they consist of and what are the main interhemispheric commissures?, where they are located, what relevant functions they carry out and what problems cause damage to these structures.

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Interhemispheric commissures: definition and neuroanatomical location

The interhermispheric commissures are a set of structures formed by 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 have a role important in brain interconnectivity, such as the anterior commissure, the posterior commissure, 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, tonsillar nuclei, and medial and inferior temporal lobes. Fibers from the posterior commissure connect areas in the occipital lobes, primarily areas related to pupillary response and control of eye movement.

The largest structure of all the commissures is the corpus callosum., located in the upper part of the diencephalon, forming much of the roof of the lateral ventricles. The smaller commissures include: the anterior commissure, located caudally to the rostrum of the corpus callosum and rostrally to the main part of the fornix; and the hippocampal commissure, made up of fibers that originate from the hippocampal formations and cross the midline as a thin layer inferior to the splenium corpus callosum.

The main interhemispheric commissures of the brain are described below.

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 different hemispheres, acts as an integrated whole that allows the management and execution of different tasks and cognitive 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:

beak or rostrum

Located in the lower frontal zone of the corpus callosum, 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 the two hemispheres.

Body

Posterior to the knee, it forms the upper face of the corpus callosum and connects with the septum and trine.

Splenium or impeller

It forms the posterior end of the corpus callosum, and consists of fibers that connect with the occipital lobe to form the forceps major. It is also linked to the lateral ventricle, forming its lower walls.

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2. The anterior commissure: characteristics and functions

The anterior commissure is another of the interhemispheric commissures formed by a bundle of nerve fibers that connect the temporal lobes. It is located in the lower part of the beak 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).

It has been suggested that this commissure could have an important role in pain sensations (specifically, in acute pain). In addition, its connections with the temporal lobes and the amygdala indicate that it also intervenes in memory processes, processes associated with emotions, in hearing and language or speech.

The anterior commissure has also been linked to basic processes such as olfaction, instinctive behavior, and sexual drive. However, the investigations carried out in subjects with lesions in this brain structure and in adjacent areas postulate that could be involved in numerous functions, ranging from attentional processes to the perception of colors.

3. The posterior commissure: definition, location and characteristics

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

Most of the fibers of the posterior commissure originate from the nucleus of the posterior commissure (Darkschewitsch's nucleus). located in the nuclei of the periaqueductal gray matter at the rostral end of the cerebral aqueduct, opposite the nucleus oculomotor.

4. The fornix: characteristics, structure and functions

The fornix is ​​a structure of white matter, also called trigone, which originates in the hippocampus and passes through the mammillary bodies of the hypothalamus. It is composed of fibers organized into various 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 that come from the hippocampus. When the columns join in 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 passed back and forth. Also has been related to memory consolidation processes; in fact, patients with lesions in this structure often have problems in this cognitive function.

The investigations carried out also suggest that injuries to the fornix can cause deficits in declarative memory (which allows us to remember events voluntarily) and episodic memory (responsible for storing autobiographical memories or specific events that are part of our history staff).

Disorders related to injuries to these structures

Lesions in 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 the callous disconnection syndrome, which causes difficulties in motor coordination that involve lack of coordination, repetition or perseveration in actions that involve movements in sequence (such as combing one's hair or tying one's shoelaces).

Another of the clinical consequences that can occur in callous disconnection syndrome is Difficulty consolidating learning and retaining new information. Agraphia (partial or total inability to write) and alexia (partial or total loss of the ability to read) may also occur.

Other types of conditions such as tumors, head injuries, cerebrovascular accidents, malformations or degenerative diseases may be factors responsible for producing the characteristic clinical signs and symptoms of damage to the nerve fibers of the interhemispheric commissures that connect both hemispheres cerebral.

Bibliographic references:

• Pena-Casanova, J. (2007). Behavioral neurology and neuropsychology. Panamerican medical editorial.
• Sperry, R. W., Gazzaniga, M. S., & Bogen, J. AND. (1969). Interhemispheric relationships: the neocortical commissures; syndromes of hemisphere disconnection.
• Venture, R. L. (2003). Cerebral interhemispheric disconnection syndrome. Journal of Psychiatry and Mental Health Hermilio Valdizan, 4, 29 - 42.