The two hemispheres of the central nervous system are connected by numerous fibres that bridge the midline to ensure normal balanced functions of several structures. For example, the decussating optic fibres at the optic chiasma, which ensure binocular vision. In a broader view, such crossing fibres are involved in numerous functions including cognitive functions like memory, integration of several motor and perceptual functions, and play a significant role in the development of interhemispheric specialization. In most cases, these interconnecting fibres form recognisable bundles referred to as commissures. Thus, commissural pathways are fibre bundles interconnecting similar regions or structures in the left and right sides of the midbrain, spinal cord, cerebral hemispheres or cerebellum. However, some fibre bundles do not fulfil this criterion as they connect different regions of the two sides or other parts of the brain to the spinal cord, and such fibres are referred to as projection fibres; as well as another group of fibre bundles called association fibres, which connect regions within the same cerebral hemisphere of the brain.
An example of commissures within the spinal cord is the ventral white commissure of the spinal cord. Some other commissures within the central nervous system include the ventral supraoptic commissure of Gudden, the dorsal supraoptic commissure of Meynert, and the anterior hypothalamic commissure of Ganser. However, the most recognised and well understood of the commissural pathways are the habenular commissure, anterior commissure, posterior commissure, corpus callosum, and the hippocampal commissure or commissure of the fornix, all of which are located in the cerebral hemispheres.
The Corpus Callosum
This is the largest of the commissural pathways connecting 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, which suggests that the corpus callosum specifically plays a vital role in problem solving strategies, verbal processing speed, and in executive performance.
The corpus callosum is subdivided into a central part called the trunk and an anterior end that is bent on itself to form the genu, and an elongated posterior end known as the splenium. These are the major parts of the corpus callosum, but 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.
The Anterior, Posterior And Habenular Commissures
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 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. It also links the 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. 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. 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 The Fornix
The hippocampal commissure 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.