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Commissural pathways

Commissural pathways are white matter fiber bundles that bridge the midline and connect identical cortical structures in the opposite hemispheres of the brain.

In contrast to commisural fibers, there are association fibers, that connect different cortical regions within the same hemisphere, and projection fibers, that connect a cortecal region to lower parts of the brain or the spinal cord. 

The most recognised and well understood commissural pathways are the corpus callosumanterior commissure, posterior commissure, habenular commissure, and the hippocampal commissure (commissure of the fornix).

The main function of commissural pathways is to ensure integration of functions between left and right structures of the brain. Thus, they are involved in many functions including cognitive functions like memory and  several motor and perceptual functions. 

This article will discuss the anatomy and function of commissural pathways.

Key facts about the commissural pathways
Definition White matter fiber bundles that bridge the midline and connect identical cortical structures in the opposite hemispheres of the brain.
Main commissures Corpus callosum, anterior commissure, posterior commissure, habenular commissure, and the hippocampal commissure (commissure of fornix).
Corpus callosum Parts: Body, genu and splenium
Function: Connects corresponding regions of almost all parts of the cerebral cortex of the two hemispheres
Anterior commissure Parts: Anterior and posterior bundle
Function: Connects structures of the olfactory pathway, the frontal cortex, temporal pole and parahippocampal gyri
Posterior commissure Function: Connects the language processing centres of both cerebral hemispheres
Habenular commissure Function: Connects the habenular nuclei of the two cerebral hemispheres
Hippocampal commissure  Function: Connects the hippocampi of the two cerebral hemispheres
General function Integration of functions between left and right structures of the brain.
Involved in memory and several motor and perceptual functions.
Contents
  1. Corpus callosum
  2. Anterior commissure
  3. Posterior commissure
  4. Habenular commissure
  5. Commissure of fornix
  6. Sources
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Corpus callosum

The corpus callosum is the main and largest of the commisural pathways that connects the right and left cerebral hemispheres. During embryonic development, this commissural pathway may fail to develop, leading to a congenital condition called split brain effect as the two cerebral hemispheres are not connected. An after-birth or 'postnatal' implication of this condition is that if one hand, for example, is trained to perform an act, the other hand may not be able to do so.

The absence of a fully developed corpus callosum have a significant relationship with impaired verbal processing speed and problem-solving.

The corpus callosum is subdivided into 3 parts. The central part is called the body or trunk. The anterior end is bent on itself to form the genu, while the elongated posterior end is known as the splenium. These are the major parts of the corpus callosum, but they are are divisible into sub-parts for localization of structures.

A thin lamina of nerve fibres, which form the rostrum of the corpus callosum, connects the genu to the upper end of the lamina terminalis. The corpus callosum is intimately related to the lateral ventricle, and its lower surface gives attachment to the septum pellucidum. As a true commissural pathway, its fibres interconnect the corresponding regions of almost all parts of the cerebral cortex of the two hemispheres.

From the genu, fibres run forwards into the frontal lobes, the fibres of the two sides forming a fork-like structure called the forceps minor. Many fibres of the splenium run backwards into the occipital lobe to form a similar structure called the forceps major (each half of this structure bulges into the posterior horn of the corresponding lateral ventricle, forming the bulb of the posterior horn).

Fibres of the trunk of the corpus callosum (and some from the splenium) run laterally, and intersect the fibres of the coronal radiata as they do so. As fibres of the trunk and of the splenium pass laterally, they form a flattened band referred to as the tapetum which is closely related to the posterior and inferior horn of the lateral ventricle.

Anterior commissure

The anterior commissure is located in the anterior wall of the third ventricle at the upper end of the lamina terminalis. It runs across the midline in front of the anterior columns of the fornix, above the basal forebrain and beneath the medial and ventral aspect of the anterior limb of the internal capsule.

The anterior commissure is divided into an anterior and a posterior bundle when traced laterally, and the posterior half passes underneath the lentiform nucleus of the basal ganglia.

This commissural pathway mainly interconnects the regions of the two cerebral hemispheres concerned with the olfactory pathway such as the olfactory bulb, the anterior olfactory nucleus, the piriform cortex, the entorhinal area, the anterior perforated substance, and the amygdaloid complex.

The anterior commissures also links other parts of the two hemispheres such as caudal part of the orbital frontal cortex, the temporal pole, the rostral superior temporal region, the major part of the inferotemporal area, the parahippocampal gyri and other regions of the two temporal lobes, as well as the frontal lobes of the two hemispheres.

Posterior commissure

The posterior commissural pathway is major part of the epithalamus and forms one of the stalks that attach the pineal body (pineal gland) to the posterior wall of the third ventricle.

The posterior commissure is the inferior lamina or stalk of the pineal gland and is important in language processing and connects the language processing centres of both cerebral hemispheres. It is closely related in function to the splenium of the corpus callosum, and injury to it may lead to disorders such as alexia.

Habenular commissure

The superior lamina or stalk of the pineal body is called the habenular commissure and connects the habenular nuclei of the two cerebral hemispheres. Thus it is also a vital part of the epithalamus.

Commissure of fornix

The hippocampal commissure, or commissure of fornix, is a bundle of fibres interconnecting the hippocampi of the two cerebral hemispheres. More specifically, this commissure connects a part of the body of the fornix called the crus on both sides of the midline.

The fornix, which is composed predominantly of fibres arising from the hippocampus, has a body that when traced posteriorly divides at the middle line into two parts called crura (pleural of crus).

The commissure of the fornix is one of the four bundles of fibres contained in the fornix of the brain. The remaining three being the postcommissural fornix lying behind the anterior commissure, the precommissural fornix, which descends anteriorly to the anterior commissure, and a bundle of fibres running above the splenium of the corpus callosum. This bundle of running fibres is referred to as the dorsal fornix.

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