Coronal cross-section of brain showing the corpus callosum at top and the anterior commissure below
|Latin||fibra commissuralis, fibrae commissurales telencephali|
|TA|| A14.1.00.017 |
|Anatomical terms of neuroanatomy|
The commissural fibers or transverse fibers are axons that connect the two hemispheres of the brain. In contrast to commissural fibers, association fibers connect regions within the same hemisphere of the brain, and projection fibers connect each region to other parts of the brain or to the spinal cord.
The commissural fibers make up tracts that include the corpus callosum, the anterior commissure, and the posterior commissure.
The corpus callosum is the largest commissural tract in the human brain. It consists of about 200-300 million axons that connect the two cerebral hemispheres. The corpus callosum is essential to the communication between the two hemispheres.
A recent study of individuals with agenesis of the corpus callosum suggests that the corpus callosum plays a vital role in problem solving strategies, verbal processing speed, and executive performance. Specifically, the absence of a fully developed corpus callosum is shown to have a significant relationship with impaired verbal processing speed and problem solving.
Another study of individuals with multiple sclerosis provides evidence that structural and microstructural abnormalities of the corpus callosum are related to cognitive dysfunction. Particularly, verbal and visual memory, information processing speed, and executive tasks were shown to be impaired when compared to healthy individuals. Physical disabilities in multiple sclerosis patients also seem to be related to abnormalities of the corpus callosum, but not to the same extent of other cognitive functions.
Using diffusion tensor imaging, researchers have been able to produce a visualization of this network of fibers, which shows the corpus callosum has an anteroposterior topographical organization that is uniform with the cerebral cortex.
The anterior commissure (also known as the precommissure) is a tract that connects the two temporal lobes of the cerebral hemispheres across the midline, and placed in front of the columns of the fornix. The great majority of fibers connecting the two hemispheres travel through the corpus callosum, which is over 10 times larger than the anterior commissure, and other routes of communication pass through the hippocampal commissure or, indirectly, via subcortical connections. Nevertheless, the anterior commissure is a significant pathway that can be clearly distinguished in the brains of all mammals.
Using diffusion tensor imaging, researchers were able to approximate the location of the anterior commissure where it crosses the midline of the brain. This tract can be observed to be in the shape of a bicycle as it branches through various areas of the brain. Through diffusion tensor imaging results, the anterior commissure was categorized into two fiber systems: 1) the olfactory fibers and 2) the non-olfactory fibers.
The posterior commissure (also known as the epithalamic commissure) is a rounded nerve tract crossing the middle line on the dorsal aspect of the upper end of the cerebral aqueduct. It is important in the bilateral pupillary light reflex.
Evidence suggests the posterior commissure is a tract that plays a role in language processing between the right and left hemispheres of the brain. It connects the pretectal nuclei. A case study described recently in The Irish Medical Journal discussed the role the posterior commissure plays in the connection between the right occipital cortex and the language centers in the left hemisphere. This study explains how visual information from the left side of the visual field is received by the right visual cortex and then transferred to the word form system in the left hemisphere though the posterior commissure and the splenium. Disruption of the posterior commissure can cause alexia without agraphia. It is evident from this case study of alexia without agraphia that the posterior commissure plays a vital role in transferring information from the right occipital cortex to the language centers of the left hemisphere.
The lyra or hippocampal commissure.
Age-related decline in the commissural fiber tracts that make up the corpus callosum indicate the corpus callosum is involved in memory and executive function. Specifically, the posterior fibers of the corpus callosum are associated with episodic memory. Perceptual processing decline is also related to diminished integrity of occipital fibers of the corpus callosum. Evidence suggests that the genu of the corpus callosum does not contribute significantly to any one cognitive domain in the elderly. As fiber tract connectivity in the corpus callosum declines due to aging, compensatory mechanisms are found in other areas of the corpus callosum and frontal lobe. These compensatory mechanisms, increasing connectivity in other parts of the brain, may explain why elderly individuals still display executive function as a decline of connectivity is seen in regions of the corpus callosum.
Older adults compared to younger adults show poorer performance in balance exercises and tests. A decline in white matter integrity of the corpus callosum in older individuals may explain declines in the ability to balance. Changes in the white matter integrity of the corpus callosum may also be related to cognitive and motor function decline as well. Decreased white matter integrity effects proper transmission and processing of sensorimotor information. White matter degeneration of the genu of the corpus callosum is also associated with gait, balance impairment, and the quality of postural control.
The corpus callosum allows for communication between the two hemispheres and is found only in placental mammals (the eutherians), while it is absent in monotremes and marsupials, as well as other vertebrates such as birds, reptiles, amphibians and fish. The anterior commissure serves as the primary mode of interhemispheric communication in marsupials,and which carries all the commissural fibers arising from the neocortex (also known as the neopallium), whereas in placental mammals the anterior commissure carries only some of these fibers).
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The corpus callosum, also callosal commissure, is a wide, thick nerve tract, consisting of a flat bundle of commissural fibers, beneath the cerebral cortex in the brain. The corpus callosum is only found in placental mammals. It spans part of the longitudinal fissure, connecting the left and right cerebral hemispheres, enabling communication between them. It is the largest white matter structure in the human brain, about ten centimetres in length and consisting of 200–300 million axonal projections.
The vertebrate cerebrum (brain) is formed by two cerebral hemispheres that are separated by a groove, the longitudinal fissure. The brain can thus be described as being divided into left and right cerebral hemispheres. Each of these hemispheres has an outer layer of grey matter, the cerebral cortex, that is supported by an inner layer of white matter. In eutherian (placental) mammals, the hemispheres are linked by the corpus callosum, a very large bundle of nerve fibers. Smaller commissures, including the anterior commissure, the posterior commissure and the fornix, also join the hemispheres and these are also present in other vertebrates. These commissures transfer information between the two hemispheres to coordinate localized functions.
The fornix is a C-shaped bundle of nerve fibers in the brain that acts as the major output tract of the hippocampus. The fornix also carries some afferent fibres to the hippocampus from structures in the diencephalon and basal forebrain. The fornix is part of the limbic system. While its exact function and importance in the physiology of the brain are still not entirely clear, it has been demonstrated in humans that surgical transection – the cutting of the fornix along its body – can cause memory loss. There is some debate over what type of memory is affected by this damage, but it has been found to most closely correlate with recall memory rather than recognition memory. This means that damage to the fornix can cause difficulty in recalling long-term information such as details of past events, but it has little effect on the ability to recognize objects or familiar situations.
The internal capsule is a white matter structure situated in the inferomedial part of each cerebral hemisphere of the brain. It carries information past the basal ganglia, separating the caudate nucleus and the thalamus from the putamen and the globus pallidus. The internal capsule contains both ascending and descending axons, going to and coming from the cerebral cortex. It also separates the caudate nucleus and the putamen in the dorsal striatum, a brain region involved in motor and reward pathways.
A neural pathway is the connection formed by axons that project from neurons to make synapses onto neurons in another location, to enable a signal to be sent from one region of the nervous system to another. Neurons are connected by a single axon, or by a bundle of axons known as a nerve tract, or fasciculus. Shorter neural pathways are found within grey matter in the brain, whereas longer projections, made up of myelinated axons, constitute white matter.
The longitudinal fissure is the deep groove that separates the two cerebral hemispheres of the vertebrate brain. Lying within it is a continuation of the dura mater called the falx cerebri. The inner surfaces of the two hemispheres are convoluted by gyri and sulci just as is the outer surface of the brain.
The cingulum in neuroanatomy, is a nerve tract – a collection of fibers, projecting from the cingulate gyrus to the entorhinal cortex in the brain, allowing for communication between components of the limbic system. It forms the white matter core of the cingulate gyrus, following it from the subcallosal gyrus of the frontal lobe beneath the rostrum of corpus callosum to the parahippocampal gyrus and uncus of the temporal lobe.
The posterior cerebral artery (PCA) is one of a pair of arteries that supply oxygenated blood to the occipital lobe, part of the back of the human brain. It begins near where the posterior communicating artery and the basilar artery join, and connects with the middle cerebral artery of the same side and internal carotid artery via the posterior communicating artery.
The anterior commissure is a white matter tract connecting the two temporal lobes of the cerebral hemispheres across the midline, and placed in front of the columns of the fornix. The great majority of fibers connecting the two hemispheres travel through the corpus callosum, which is over 10 times larger than the anterior commissure, and other routes of communication pass through the hippocampal commissure or, indirectly, via subcortical connections. Nevertheless, the anterior commissure is a significant pathway that can be clearly distinguished in the brains of all mammals.
The projection fibers consist of efferent and afferent fibers uniting the cortex with the lower parts of the brain and with the spinal cord. In human neuroanatomy, bundles of axons called tracts, within the brain, can be categorized by their function into association fibers, projection fibers, and commissural fibers.
A commissure is the location at which two objects abut or are joined. The term is used especially in the fields of anatomy and biology.
Association fibers are axons that connect cortical areas within the same cerebral hemisphere.
The inferior longitudinal fasciculus is traditionally considered one of the major occipitotemporal association tracts. It connects the anterior temporal lobe and the extrastriate cortex of the occipital lobe, running along the lateral and inferior wall of the lateral ventricle.
Pure alexia, also known as agnosic alexia or alexia without agraphia or pure word blindness, is one form of alexia which makes up "the peripheral dyslexia" group. Individuals who have pure alexia have severe reading problems while other language-related skills such as naming, oral repetition, auditory comprehension or writing are typically intact.
In human neuroanatomy, brain asymmetry can refer to at least two quite distinct findings:
A nerve tract is a bundle of nerve fibers (axons) connecting nuclei of the central nervous system. In the peripheral nervous system this is known as a nerve, and has associated connective tissue. The main nerve tracts in the central nervous system are of three types: association fibers, commissural fibers, and projection fibers. A tract may also be referred to as a commissure, fasciculus or decussation. A commissure connects the two cerebral hemispheres at the same levels. Examples are the posterior commissure and the corpus callosum. A decussation is a connection made by fibres that cross at different levels (obliquely), such as the sensory decussation. Examples of a fascicle are the subthalamic fasciculus and the lenticular fasciculus.
This article describes anatomical terminology that is used to describe the central and peripheral nervous systems - including the brain, brainstem, spinal cord, and nerves.
Disconnection syndrome is a general term for a collection of neurological symptoms caused -- via lesions to associational or commissural nerve fibres -- by damage to the white matter axons of communication pathways in the cerebrum(not to be confused with the cerebellum), independent of any lesions to the cortex. The behavioral effects of such disconnections are relatively predictable in adults. Disconnection syndromes usually reflect circumstances where regions A and B still have their functional specializations except in domains that depend on the interconnections between the two regions.
This article incorporates text in the public domain from page 843 of the 20th edition of Gray's Anatomy (1918)
The nerve fibres which make up the white matter of the cerebral hemispheres are categorized on the basis of their course and connections. They are association fibres, which link different cortical areas in the same hemisphere; commissural fibres, which link corresponding cortical areas in the two hemispheres; or projection fibres, which connect the cerebral cortex with the corpus striatum, diencephalon, brain stem and the spinal cord.
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