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Commissure
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A commissure (/ˈkɒməʃər/) is the location at which two objects abut or are joined. The term is used especially in the fields of anatomy and biology.
- The most common usage of the term refers to the brain's commissures, of which there are at least nine. Such a commissure is a bundle of commissural fibers as a tract that crosses the midline at its level of origin or entry (as opposed to a decussation of fibers that cross obliquely). The nine are the anterior commissure, posterior commissure, corpus callosum, commissure of fornix (hippocampal commissure), habenular commissure, ventral supraoptic decussation, Meynert's commissure, anterior hypothalamic commissure of Gasner, and the interthalamic adhesion. They consist of fibre tracts that connect the two cerebral hemispheres and span the longitudinal fissure. In the spinal cord, there are the anterior white commissure, and the gray commissure. Commissural neurons refer to neuronal cells that grow their axons across the midline of the nervous system within the brain and the spinal cord.
- Commissure also often refers to cardiac anatomy of heart valves. In the heart, a commissure is the area where the valve leaflets abut. When such an abutment is abnormally stiffened or even fused, valvular stenosis results, sometimes requiring commissurotomy.
- The term may also refer to the junction of the upper and lower lips (see labial commissure of mouth).
- It may refer to the junction of the upper and lower mandibles of a bird's beak,[1] or alternately, to the full-length apposition of the closed mandibles, from the corners of the mouth to the tip of the beak.[2]
- It may refer to the nasal and temporal meeting points of the upper and lower eyelids (the medial and lateral canthi).
- In the vulva, the joining points of the two folds of the labia majora create two commissures—the anterior commissure just anterior to the prepuce of the clitoris, and the posterior commissure of the labia majora, directly posterior to the frenulum of the labia minora and anterior to the perineal raphe.
In biology, the meeting of the two valves of a brachiopod or clam is a commissure; in botany, the term is used to denote the place where a fern's laterally expanded vein endings come together in a continuous marginal sorus.
See also
[edit]References
[edit]- ^ Coues, Elliott (1890). Handbook of Field and General Ornithology. London: Macmillan and Co. p. 155. OCLC 263166207.
- ^ Campbell, Bruce; Lack, Elizabeth, eds. (1985). A Dictionary of Birds. Carlton, England: T and A D Poyser. p. 105. ISBN 0-85661-039-9.
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Commissure
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Etymology and General Definition
Etymology
The term "commissure" originates from the Latin noun commissura, denoting "a joining together," "joint," "juncture," or "seam," derived from the past participle commissus of the verb committere, meaning "to join," "connect," or "entrust."[1][7] This Latin root entered English in the Middle English period around the early 15th century (before 1425), borrowed via Middle French, where it initially referred broadly to a suture, seam, or any point of connection in physical or metaphorical senses.[8][1] The term has been used in anatomical literature since at least the 17th century; for example, English physician Thomas Willis employed it in his 1664 work Cerebri Anatome to describe the anterior commissure of the brain.[9]General Definition
A commissure is the location where two structures abut, meet, or are joined, often forming a seam or band.[1] The term derives from the Latin commissura, meaning "a joining together."[1] In general biology and anatomy, a commissure refers to a connecting band, particularly of fibers or tissues, that unites similar or corresponding parts.[10] This structural union facilitates continuity between components without implying active physiological exchange. Commissure differs from a suture, which specifically denotes an immovable fibrous joint between bones, such as those in the skull.[11] It also contrasts with a synapse, which is the functional junction for signal transmission between neurons rather than a mere structural link.[12]In Neuroanatomy
Commissural Fibers in the Brain
Commissural fibers are bundles of white matter tracts in the brain that cross the midline to connect homologous regions of the cerebral hemispheres, facilitating interhemispheric communication.[2][4] These fibers consist primarily of myelinated axons, which enable rapid transmission of neural signals across the hemispheres.[13] In the classification of cerebral white matter tracts, there are three main categories: commissural fibers, which interconnect corresponding cortical regions between the hemispheres; association fibers, which link different cortical areas within the same hemisphere; and projection fibers, which extend between the cerebral cortex and subcortical structures or the spinal cord.[14] Commissural fibers specifically bridge the two hemispheres.[15] They differ from ipsilateral association fibers, which connect regions within the same hemisphere without crossing the midline.[16] Anatomically, commissural fibers are located predominantly in the forebrain, traversing the midsagittal plane to link corresponding cortical areas.[2] The largest such tract is the corpus callosum, which exemplifies their role in interhemispheric connectivity.[13] Commissural fibers form during embryogenesis, originating from the lamina terminalis in the telencephalon as early as the 7th gestational week, with progressive thickening into a commissural plate that gives rise to major bundles.[17] Myelination of these fibers begins in late gestation but accelerates postnatally, with significant progression in the corpus callosum observed between 3 and 6 months of age, reaching a peak during childhood and continuing into adolescence to support maturing neural efficiency.[18][19]Specific Brain Commissures
The corpus callosum is the largest commissural fiber tract in the human brain, consisting of over 200 million myelinated axons that interconnect homologous regions of the neocortex across the two cerebral hemispheres.[5] It is situated superior to the lateral ventricles and extends from the frontal to the occipital lobes, facilitating interhemispheric communication for sensory, motor, and cognitive integration. Anatomically, it is divided into four main segments: the rostrum, which tapers anteriorly and connects prefrontal areas; the genu, curving forward to link frontal lobes; the body, the central portion bridging parietal and temporal regions; and the splenium, the thick posterior end associating occipital and posterior temporal cortices.[20] These subdivisions reflect topographic organization, with fibers maintaining somatotopic arrangements based on cortical origin.[21] The anterior commissure is a smaller, more compact bundle of approximately 3 million fibers located in the lamina terminalis, anterior to the fornix and third ventricle.[22] It primarily connects contralateral temporal lobes, including the olfactory bulbs, anterior temporal cortices, and amygdaloid complexes, as well as portions of the orbitofrontal cortex and olfactory tubercles.[23] This tract plays a key role in bilateral transfer of olfactory and limbic information, with its fibers decussating compactly to form a handlebar-shaped structure visible on axial MRI sections.[24] The posterior commissure, a slender fiber tract in the dorsal midbrain just caudal to the third ventricle and inferior to the pineal gland, interconnects the pretectal nuclei and adjacent midbrain structures across the midline.[25] Composed of thin, crossing axons, it forms part of the periaqueductal gray matter's dorsal boundary and contributes to pathways for vertical gaze control and the consensual pupillary light reflex by linking bilateral olivary pretectal nuclei to the Edinger-Westphal nuclei.[26] The hippocampal commissure, also known as the psalterium or forniceal commissure, is a minor midline structure within the callosal sulcus, posterior to the corpus callosum's genu and superior to the third ventricle.[27] It connects the contralateral hippocampal formations through the dorsal aspect of the fornix, comprising a small number of crossing fibers that link the fimbriae and alveus to support limited interhemispheric hippocampal signaling.[28] Less prominent than other commissures, it appears as a thin, arched band on midsagittal sections, emphasizing its subtle role in bilateral memory-related circuitry.[29] The habenular commissure lies in the epithalamus, superior to the posterior commissure and within the pineal stalk's superior lamina, connecting the medial and lateral habenular nuclei of each hemisphere.[30] This diminutive tract, visible on midsagittal MRI as a short midline bridge, facilitates interhemispheric coordination of habenular outputs, which integrate limbic inputs from the basal ganglia and septum to influence reward, aversion, and stress responses via the stria medullaris thalami.[31]Functions and Clinical Significance
Physiological Roles
Commissures in the brain, particularly the corpus callosum, play a crucial role in interhemispheric integration by enabling the transfer and coordination of information between the cerebral hemispheres, which supports unified sensory perception and motor activities such as bimanual tasks.[5] This integration allows for the processing of sensory inputs and motor outputs across hemispheres, facilitating coordinated actions like independent finger movements in both hands.[13] For instance, callosal connections contribute to visuomotor coordination and perceptual unity, ensuring that stimuli processed in one hemisphere can influence responses in the other.[32] In cognitive functions, commissures support language processing and emotional regulation through interhemispheric communication. The corpus callosum facilitates the transfer of linguistic information from the dominant (typically left) hemisphere to the non-dominant one, aiding in language lateralization and comprehension across modalities.[33] Similarly, the anterior commissure contributes to emotional processing by connecting regions involved in affective responses, such as the temporal lobes and amygdala, thereby integrating emotional signals bilaterally.[34] Specific commissures mediate distinct physiological processes. The posterior commissure interconnects the superior colliculi, playing a key role in coordinating vertical gaze movements and the pupillary light reflex for adaptive visual responses.[35] The habenular commissure links the habenular nuclei, supporting bilateral signaling in reward and aversion pathways within the limbic system, which helps regulate motivational and emotional behaviors.[36] Commissures also promote bilateral synchronization essential for higher-order functions like attention, learning, and sleep. Through the corpus callosum, interhemispheric functional connectivity is maintained, enabling synchronized neural activity during attentional tasks and cognitive learning processes that require hemispheric cooperation.[37] During sleep, these structures facilitate the bilateral propagation of slow waves, contributing to the consolidation of memory and overall hemispheric balance.[38]Disorders and Research
Agenesis of the corpus callosum (AgCC) is a congenital malformation characterized by the partial or complete absence of the corpus callosum, leading to disrupted interhemispheric communication and associated cognitive and motor deficits, such as reduced processing speed, impaired novel problem-solving, and challenges in social cognition.[39] These deficits arise from the lack of axonal connections between brain hemispheres, often resulting in developmental delays that can be diagnosed prenatally or postnatally via magnetic resonance imaging (MRI), which reveals the absence of callosal tissue and secondary signs like enlarged ventricles.[40] Split-brain syndrome, also known as callosal disconnection syndrome, occurs following corpus callosotomy—a surgical severing of the corpus callosum performed to alleviate intractable epilepsy by preventing seizure spread between hemispheres.[41] This procedure results in functional independence of the cerebral hemispheres, manifesting in symptoms like alien hand syndrome, where one hand performs involuntary actions seemingly outside conscious control, due to the loss of interhemispheric integration.[42] Patients may also exhibit difficulties in tasks requiring bilateral coordination, highlighting the commissure's role in unifying perceptual and motor functions across hemispheres.[43] In multiple sclerosis (MS), demyelination of commissural tracts, particularly in the corpus callosum, contributes to coordination deficits by impairing signal transmission along white matter pathways, leading to symptoms such as ataxia and bimanual incoordination.[44] Lesions in the callosum disrupt axonal integrity, exacerbating motor and cognitive impairments, with quantitative MRI showing reduced callosal volume correlated with upper-extremity dysfunction and overall disability progression.[45] Key research highlights include the use of diffusion tensor imaging (DTI), an MRI-based technique that quantifies white matter fiber integrity through metrics like fractional anisotropy, enabling non-invasive mapping of commissural damage in conditions like AgCC and MS.[46] Pioneering 1960s experiments by Roger Sperry on split-brain patients demonstrated hemispheric specialization and disconnection effects, while subsequent studies revealed neuroplastic adaptations, such as compensatory intrahemispheric pathways that mitigate some deficits over time.[47][48] As of 2025, recent advances link reduced corpus callosum size in autism spectrum disorders (ASD) to altered early development, with studies showing decreased central callosal volume associated with repetitive behaviors and social challenges, potentially informing targeted interventions.[49] Additionally, preclinical research demonstrates the potential of stem cell therapies, such as induced neural stem cell transplantation into demyelinated lesions including the corpus callosum, to promote functional remyelination in animal models of demyelination, paving the way for translational human applications.[50]In Other Biological Contexts
Botanical Commissures
In botany, a commissure refers to the seam or face by which two carpels cohere in a multicarpellary ovary, forming a plane of union that contributes to the structure of the gynoecium.[51] This junction is particularly evident in syncarpous ovaries where carpels fuse postgenitally or congenitally, ensuring coordinated development of the fruit.[52] The term also applies more broadly to points of attachment or coherence in other plant structures. A prominent example occurs in the Apiaceae family (umbellifers, such as the carrot family), where the commissure serves as the ridge or plane along which the two mericarps of a schizocarp fruit are joined.[53] In these dry, indehiscent fruits, the commissure facilitates splitting at maturity, allowing the mericarps to separate and disperse seeds via wind or attachment mechanisms, as seen in species like Daucus carota.[54] This structure often features vittae (oil canals) on the commissural face, which aid in chemical defense and identification in taxonomic studies. Structurally, the commissure provides cohesion during fruit development, preventing premature separation of carpels and supporting ovule attachment along its plane.[55] In schizocarps, it enables precise dehiscence, enhancing seed dispersal efficiency while maintaining fruit integrity until ripe.[56] Evolutionarily, commissures emerged in angiosperms as part of syncarpy, promoting synchronized carpel fusion that improved enclosure of ovules and fruit protection, a key innovation repeated across lineages for enhanced reproductive success.[52] This fusion likely evolved from separate carpels in early angiosperms, adapting to diverse pollination and dispersal strategies.[57]Zoological and Peripheral Commissures
In zoology, a commissure refers to the line or edge where the two valves of brachiopod or bivalve shells meet, particularly along the hinge-free margin opposite the hinge.[58][59] In brachiopods, this structure forms a snugly fitting join that ensures precise alignment of the dorsal and ventral valves during closure.[60] For bivalves, such as clams, the commissure represents the shell margin where the valves converge, often parallel to the pallial line where the mantle attaches.[59] These commissures facilitate shell operation by allowing controlled opening for water flow and food capture, while enabling tight closure to protect internal organs and exclude predators.[60][59] In peripheral human anatomy, commissures denote junction points of soft tissues outside the central nervous system. The labial commissure, or oral commissure, is the corner of the mouth where the upper and lower lips meet at their vermilion borders, serving as an attachment site for muscles involved in facial expression, speech, and chewing.[61] Similarly, the palpebral commissures are the medial and lateral canthi, where the upper and lower eyelids converge to form the boundaries of the palpebral fissure and aperture, maintaining eye protection and lubrication.[62]References
- https://en.wiktionary.org/wiki/commissure