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. * 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 fiber The commissural fibers or transverse fibers are axons that connect the two hemispheres of the brain. Huge numbers of commissural fibers make up the commissural tracts in the brain, the largest of which is the corpus callosum. In contrast to ...

s as a

tract Tract may refer to: Geography and real estate * Housing tract, an area of land that is subdivided into smaller individual lots * Land lot or tract, a section of land * Census tract, a geographic region defined for the purpose of taking a census ...

that crosses the midline at its level of origin or entry (as opposed to a

decussation Decussation is used in biological contexts to describe a crossing (due to the shape of the Roman numeral for ten, an uppercase 'X' (), ). In Latin anatomical terms, the form is used, e.g. . Similarly, the anatomical term Chiasm (anatomy), chi ...

of fibers that cross obliquely). The nine are the

anterior commissure The anterior commissure (also known as the precommissure) is a white matter nerve tract, tract (a bundle of axons) connecting the two temporal lobes of the cerebral hemispheres across the midline, and placed in front of the columns of the Fornix o ...

posterior commissure The posterior commissure (also known as the epithalamic commissure) is a rounded band of white fibers crossing the middle line on the dorsal aspect of the rostral end of the cerebral aqueduct. It is important in the bilateral pupillary light re ...

corpus callosum The corpus callosum (Latin for "tough body"), 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 ...

commissure of fornix The fornix (from ; : fornices) 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 fibers to the hippocampus from structures in the diencephalon and basal ...

(hippocampal commissure),

habenular commissure The habenular commissure is a nerve tract of commissural fibers that connects the habenular nuclei on both sides of the habenular trigone in the epithalamus. The habenular commissure is part of the habenular trigone (a small depressed triangula ...

, ventral supraoptic decussation, Meynert's commissure, anterior hypothalamic commissure of Gasner, and the

interthalamic adhesion The interthalamic adhesion (also known as the massa intermedia, intermediate mass or middle commissure) is a flattened band of tissue that connects both parts of the thalamus at their medial surfaces. The medial surfaces form the upper part of th ...

. They consist of fibre

s that connect the two

cerebral hemisphere 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 ...

s and span the

longitudinal fissure The longitudinal fissure (or cerebral fissure, great longitudinal fissure, median longitudinal fissure, interhemispheric fissure) is the deep groove that separates the two cerebral hemispheres of the vertebrate brain. Lying within it is a continu ...

. In the spinal cord, there are the

anterior white commissure The anterior white commissure (ventral white commissure) is a bundle of nerve fibers which cross the midline of the spinal cord just anterior (in front of) to the gray commissure ( Rexed lamina X). A delta fibers (Aδ fibers) and C fibers carr ...

, and the gray commissure. ''Commissural neurons'' refer to neuronal cells that grow their

axons An axon (from Greek ἄξων ''áxōn'', axis) or nerve fiber (or nerve fibre: see spelling differences) is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action pot ...

across the midline of the nervous system within the brain and the spinal cord. * ''Commissure'' also often refers to cardiac anatomy of

heart valve A heart valve is a biological one-way valve that allows blood to flow in one direction through the chambers of the heart. A mammalian heart usually has four valves. Together, the valves determine the direction of blood flow through the heart. Hea ...

s. 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 Stenosis () is the abnormal narrowing of a blood vessel or other tubular organ or structure such as foramina and canals. It is also sometimes called a stricture (as in urethral stricture). ''Stricture'' as a term is usually used when narrowing ...

results, sometimes requiring

commissurotomy A commissurotomy () is a surgical incision of a commissure in the body, as one made in the heart at the edges of the commissure formed by cardiac valves, or one made in the brain to treat certain psychiatric disorders. Patients with scleroderma, ...

. * The term may also refer to the junction of the upper and lower

lip The lips are a horizontal pair of soft appendages attached to the jaws and are the most visible part of the mouth of many animals, including humans. Mammal lips are soft, movable and serve to facilitate the ingestion of food (e.g. sucklin ...

s (see labial commissure of mouth). * It may refer to the junction of the upper and lower mandibles of a bird's beak, or alternately, to the full-length apposition of the closed mandibles, from the corners of the mouth to the tip of the beak. * It may refer to the nasal and temporal meeting points of the upper and lower

eyelid An eyelid ( ) is a thin fold of skin that covers and protects an eye. The levator palpebrae superioris muscle retracts the eyelid, exposing the cornea to the outside, giving vision. This can be either voluntarily or involuntarily. "Palpebral ...

s (the medial and lateral

canthi The canthus (: canthi, palpebral commissures) is either corner of the eye where the upper and lower eyelids meet. More specifically, the inner and outer canthi are, respectively, the medial and lateral ends/angles of the palpebral fissure. The ...

). * In the

vulva In mammals, the vulva (: vulvas or vulvae) comprises mostly external, visible structures of the female sex organ, genitalia leading into the interior of the female reproductive tract. For humans, it includes the mons pubis, labia majora, lab ...

, the joining points of the two folds of the

labia majora In primates, and specifically in humans, the labia majora (: labium majus), also known as the outer lips or outer labia, are two prominent Anatomical terms of location, longitudinal skin folds that extend downward and backward from the mons pubis ...

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 Biology is the scientific study of life and living organisms. It is a broad natural science that encompasses a wide range of fields and unifying principles that explain the structure, function, growth, History of life, origin, evolution, and ...

, the meeting of the two valves of a

brachiopod Brachiopods (), phylum (biology), phylum Brachiopoda, are a phylum of animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear e ...

clam Clam is a common name for several kinds of bivalve mollusc. The word is often applied only to those that are deemed edible and live as infauna, spending most of their lives halfway buried in the sand of the sea floor or riverbeds. Clams h ...

is a commissure; in

botany Botany, also called plant science, is the branch of natural science and biology studying plants, especially Plant anatomy, their anatomy, Plant taxonomy, taxonomy, and Plant ecology, ecology. A botanist or plant scientist is a scientist who s ...

, the term is used to denote the place where a

fern The ferns (Polypodiopsida or Polypodiophyta) are a group of vascular plants (plants with xylem and phloem) that reproduce via spores and have neither seeds nor flowers. They differ from mosses by being vascular, i.e., having specialized tissue ...

's laterally expanded vein endings come together in a continuous marginal

sorus A sorus (: sori) is a cluster of sporangia (structures producing and containing spores) in ferns and fungi. A coenosorus (: coenosori) is a compound sorus composed of multiple, fused sori. Etymology This Neo-Latin word is from Ancient Greek � ...

References

Anatomy Anatomical terminology Parts of a bird beak {{biology-stub

See also

References