Interhemispheric commissures: what they are and what functions they have

The human brain is made up of two large cerebral hemispheres which, although they have their own functions, must be constantly communicated for information to be transmitted and for there to be good structural and functional connectivity.

The interhemispheric commissures, including the corpus callosum, are responsible for the smooth running of this interconnection.

In this article we tell you what they consist of and what are the main interhemispheric commissures, Where they are located, what relevant functions they perform and what problems lead to damage to these structures.

    Interhemispheric commissures: definition and neuroanatomical localization

    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 white matter, which is made up of association fibers, commissural fibers, and projection fibers.

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

    The previous commissioner connects the olfactory bulbs, the nuclei of the tonsils, and the medial and inferior temporal lobes. The fibers of the posterior commissure connect the areas of the occipital lobes, mainly the areas related to pupillary response and control of eye movements.

    The largest structure of all the commissures is the corpus callosum, Located at the top of the diencephalon, forming a large part of the roof of the lateral ventricles. The smaller commissures include: the anterior commissure, located caudally in the tribune of the corpus callosum and rostrally to the main part of the fornix; and the commissure of the hippocampus, formed by fibers that originate from the hippocampal formations and cross the midline as a thin layer under the splenium of the 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 made up of bundles of white matter and lies deep in the longitudinal crack 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 the hemispheres implies that the brain, although it has two different hemispheres, it 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 linked to diseases such as multiple sclerosis and other cognitive dysfunctions.

    This structure includes the following parts:

    Small stand

    Located in the lower frontal area of ​​the corpus callosum, it arises from the terminal leaf and connects to the optic chiasm.

    Knee

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

    Cos

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

    Splenium or roller

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

      2. The anterior commissure: characteristics and functions

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

      It has been suggested that this commissure may play an important role in pain sensations (in particular, in acute pain). In addition, their connections to the temporal lobes and the amygdala indicate that intervenes in memory processes, processes associated with emotions, hearing and language or speech.

      The previous commissure has also been linked to basic processes such as smell, instinctual behavior, and sexual behavior. However, research in subjects with lesions of this brain structure and adjacent areas postulates that it may be involved in many functions, ranging from attentional processes to color perception.

      3. The posterior commissure: definition, location and characteristics

      The fibers of the posterior commissure run through the midline of the brain, above the orifice of the Silvio aqueduct (located in the third ventricle). This structure consists of a rounded band of white fibers. It seems to play 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 nucleus), located in the nuclei of periaqueductal gray matter at the rostral end of the cerebral aqueduct, in front of the oculomotor nucleus.

      4. The oven: characteristics, structure and functions

      The fornix is ​​a structure of white matter, also called a trine, which it originates in the hippocampus and crosses the mammary bodies of the hypothalamus. It is composed of fibers organized into several sub-regions 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 originate from the hippocampus. When the columns meet in the middle line, 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 side to side. too much has 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 fornix injuries can lead to deficits in declarative memory (which allows us to remember events on purpose) and episodic memory (responsible for storing autobiographical memories or specific events that are part of our history. personal).

      Injury-related disorders in these structures

      Injury to the main interhemispheric corner of the brain, the corpus callosum, can lead to severe sensory and motor deficits. The main disorder caused by sectioning this structure is callus disconnection syndrome, which results in motor coordination difficulties that involve lack of coordination, repetition, or persistence in actions involving sequential movements (such as combing or knotting ropes).

      Another of the clinical consequences that can occur in callus disconnection syndrome is the difficulty of consolidating learning and retaining new information. It can also produce spelling (total or partial inability to write) and Alexia (partial or total loss of reading ability).

      Other conditions such as tumors, head trauma, stroke, deformities or degenerative diseases may be factors responsible for the clinical signs and symptoms characteristic of lesions of the nerve fibers of the interhemispheric commissures that connect the two cerebral hemispheres.

      Bibliographical references:

      • Peña-Casanova, J. (2007). Behavioral neurology and neuropsychology. Pan American Medical Editorial.
      • Sperry, RW, Gazzaniga, MS and Bogen, JE (1969). Interhemispheric relations: neocortical commissures; hemisphere disconnection syndromes.
      • Ventura, RL (2003). Cerebral hemispherical disconnection syndrome. Journal of Psychiatry and Mental Health Hermilio Valdizan, 4, 29-42.

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