The sensory decussation or decussation of the lemnisci is a decussation or crossover of axons from the gracile nucleus and cuneate nucleus, which are responsible for fine touch, proprioception and two-point discrimination of the body. The fibres of this decussation are called the internal arcuate fibres and are found at the superior aspect of the closed medulla superior to the motor decussation. It is part of the second neuron in the posterior column–medial lemniscus pathway.
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The ventral supraoptic decussation is the crossover (decussation) point for signals from the left and right eye, en route respectively to the right and left sides of the visual cortex. Occupying the posterior part of the commissure of the optic chiasma is a strand of fibers, the Ventral supraoptic decussation (commissure of Gudden, Gudden's inferior commissure), which is not derived from the optic nerves; it forms a connecting link between the medial geniculate bodies.
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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.
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In 1840 he coined the term "horseshoe-shaped commissure of Wernekinck" as a name for the decussation of the brachium conjunctivum.
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The fibres that make up the sensory decussation are responsible for fine touch, proprioception and two-point discrimination of the whole body excluding the head.
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Since siamese cats, like albino tigers, also tend to cross their eyes (strabismus), it has been proposed that this behavior might compensate the abnormal amount of decussation.
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The other 10% of the fibers stay uncrossed in the anterior corticospinal tract. The pyramidal decussation marks the border between the spinal cord and the medulla oblongata.
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The figure shows the cross section of the closed medulla at the level of the sensory decussation. Number 9 illustrates the sensory decussation at the posterior column. At the level of the closed medulla in the posterior white column, two large nuclei namely the gracile nucleus and the cuneate nucleus can be found. The two nuclei receive the impulse from the two ascending tracts: fasciculus gracilis and fasciculus cuneatus.
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The shell has a minute, gemmuliferous decussation. The smallish aperture is oblong. The outer lip is slightly inflated and smooth on the inside. The sinus is not deep.
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After the two tracts terminate upon these nuclei, the heavily myelinated fibres arise and ascend anteromedially around the periaqueductal gray as internal arcuate fibres. These fibres decussate (cross) to the contralateral (opposite) side, so called the sensory decussation. The ascending bundle after the decussation is called the medial lemniscus. Unlike other ascending tracts of the brain, fibres of the medial lemniscus do not give off collateral branches as they travel along the brainstem.
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Complete crossing (decussation) of the nerves at the optic chiasm in birds has also stimulated research. Complete decussation of the optic tract has been seen as a method of ensuring the open eye strictly activates the contralateral hemisphere. Some evidence indicates that this alone is not enough as blindness would theoretically prevent USWS if retinal nerve stimuli were the sole player. However, USWS is still exhibited in blinded birds despite the absence of visual input.
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Between the two pyramids can be seen a decussation of fibers which marks the transition from the medulla to the spinal cord. The medulla is above the decussation and the spinal cord below. ;From behind The appearance of a cadaveric brainstem from behind, with major parts labelled The most medial part of the medulla is the posterior median sulcus. Moving laterally on each side is the gracile fasciculus, and lateral to that is the cuneate fasciculus.
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Damage to the body above the pyramidal decussation will cause contralateral motor deficits. For example, if there is a lesion at the pre-central gyrus in the right cerebral cortex, then the left side of the body will be affected. Whereas damage below the pyramidal decussation will result in ipsilateral motor deficits. For example, spinal cord damage on the left side of the lateral corticospinal tract at the thoracic level can cause motor deficits to the left side of the body.
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Neurons of the dorsal column nuclei send axons that form the internal arcuate fibers, crossing over at the sensory decussation to form the medial lemniscus, ultimately synapsing with third-order neurons of the thalamus.
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The DL neurons are involved in distal limb control. Therefore, these DL neurons are found specifically only in the cervical and lumbosacral enlargements within the spinal cord. There is no decussation in the lateral corticospinal tract after the decussation at the medullary pyramids. The anterior corticospinal tract descends ipsilaterally in the anterior column, where the axons emerge and either synapse on lower ventromedial (VM) motor neurons in the ventral horn ipsilaterally or descussate at the anterior white commissure where they synapse on VM lower motor neurons contralaterally .
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The two pyramids contain the motor fibers that pass from the brain to the medulla oblongata and spinal cord. These are the corticobulbar and corticospinal fibers that make up the pyramidal tracts. About 90% of these fibers leave the pyramids in successive bundles and decussate (cross over) in the anterior median fissure of the medulla oblongata as the pyramidal decussation or motor decussation. Having crossed over at the middle line, they pass down in the posterior part of the lateral funiculus as the lateral corticospinal tract.
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Axons in the lateral corticospinal tract leave out of the tract and into the anterior horns of the spinal cord. It controls fine movement of ipsilateral limbs (albeit contralateral to the corresponding motor cortex) as it lies distal to the pyramidal decussation. Control of more central axial and girdle muscles comes from the anterior corticospinal tract.Blumenfeld, Neuroanatomy Through Clinical Cases, 2002 Damage to different parts of the body will cause deficits, depending on whether the damage is above (rostral) or below (caudal) the pyramidal decussation.
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At the caudal part of the medulla these tracts cross over in the decussation of the pyramids obscuring the fissure at this point. Some other fibers that originate from the anterior median fissure above the decussation of the pyramids and run laterally across the surface of the pons are known as the anterior external arcuate fibers. The region between the anterolateral and posterolateral sulcus in the upper part of the medulla is marked by a pair of swellings known as olivary bodies (also called olives). They are caused by the largest nuclei of the olivary bodies, the inferior olivary nuclei.
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The anterior median fissure (ventral or ventromedian fissure) contains a fold of pia mater, and extends along the entire length of the medulla oblongata: It ends at the lower border of the pons in a small triangular expansion, termed the foramen cecum. Its lower part is interrupted by bundles of fibers that cross obliquely from one side to the other, and constitute the pyramidal decussation. Some fibers, termed the anterior external arcuate fibers, emerge from the fissure above this decussation and curve lateralward and upward over the surface of the medulla oblongata to join the inferior peduncle.
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The internal arcuate fibers or internal arcuate tract are the axons of second- order sensory neurons that compose the gracile and cuneate nuclei of the medulla oblongata. These second-order neurons begin in the gracile and cuneate nuclei in the medulla. They receive input from first-order sensory neurons, which provide sensation to many areas of the body and have cell bodies in the dorsal root ganglia of the dorsal root of the spinal nerves. Upon decussation (crossing over) from one side of the medulla to the other, also known as the sensory decussation, they are then called the medial lemniscus.
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The acute apex is very minute. The surface is cut into a finely densely granulated pattern by the decussation of numerous spiral striae with close, regular, impressed lines of increment. The base of the shell is slightly convex, encircled by numerous unequal lirae. The oblique aperture is subrhomboidal.
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They involve protrusion of intra-abdominal contents through a weakness at the site of passage of the umbilical cord through the abdominal wall. Umbilical hernias in adults are largely acquired, and are more frequent in obese or pregnant women. Abnormal decussation of fibers at the linea alba may contribute.
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Some afferent decussations. Pyramidal decussations. Anatomically, the contralateral organization is manifested by major decussations (latin: the Latin notation for ten, 'deca', is an uppercase 'X') and chiasmas (after the Greek uppercase letter 'Χ', chi). A decussation denotes a crossing of bundles of axonal fibres inside the central nervous system.
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Deep dissection of brain-stem showing decussation The decussation of superior cerebellar peduncle is the crossing of fibers of the superior cerebellar peduncle across the midline, and is located at the level of the inferior colliculi. It comprises the cerebellothalamic tract, which arises from the dentate nucleus (therefore also known as dentatothalamic tract), as well as the cerebellorubral tract, which arises from the globose and emboliform nuclei and project to the contralateral red nucleus to eventually become the rubrospinal tract. It is also known as horseshoe-shaped commissure of Wernekinck. It is important as an anatomical landmark, as lesions above it cause contralateral cerebellar signs, while lesions below it cause ipsilateral cerebellar signs.
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Anterolateral corticospinal tract is a subdivision of the corticospinal tract in the spinal cord. It is formed by approximately 2% of the corticospinal fibers that do not cross to the opposite side of the brainstem in the pyramidal decussation. This tract descends in the lateral white column anterior to the lateral corticospinal tract.
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Drawing of the shell of Ariophanta interrupta. The shell of this species is left-handed (sinistral). The shell is flatly convex above, rather coarsely, obliquely, plicately striated and decussated with fine impressed lines, the decussation is sometimes obsolete, more tumid and smoother beneath. The shell color is brownish horny, darker below the periphery, and gradually becoming paler again beneath.
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The geniculate fibers are the fibers in the region of the genu of the internal capsule; they originate in the motor part of the cerebral cortex, and, after passing downward through the base of the cerebral peduncle with the cerebrospinal fibers, undergo decussation and end in the motor nuclei of the cranial nerves of the opposite side.
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The length of the shell varies between 15 mm and 24 mm. The shell is dark chocolate, covered by rows of lighter colored granulations, caused by the decussation of small flexuous rather numerous longitudinal ribs and elevated revolving lines. The aperture is light chocolate.G.W. Tryon (1884) Manual of Conchology, structural and systematic, with illustrations of the species, vol.
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The N13 is best measured over the fifth cervical spine. Further conduction in the posterior columns passes through the synapse at the cervicomedullary junction and enters the lemniscal decussation. A scalp P14 peak is generated at this level. As conduction continues up the medial lemniscus to upper midbrain and into the thalamus, a scalp negative peak is detected, the N18.
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The length of the shell varies between 4 mm and 5 mm. The decussation not so deep as in Mitromorpha aspera, so that the surface is smoother, the tuberculation smaller. Sometimes the clathration of the body whorl is only seen on the upper portion, the longitudinal costulae becoming obsolete below. G.W. Tryon (1884) Manual of Conchology, structural and systematic, with illustrations of the species, vol.
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Figure 3. The decussation of the optic radiation in the cortex is an example of a type IV crossing This type is usually not called chiasm. Such a looping occurs, for example, in the optic tract between the optic chiasm and the optic tectum. Another example is the optic radiation which rotates the retinal map on the visual cortex by 180° (see Figure 3).
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In jawless vertebrates (hagfish and lamprey), the optic tracts do cross in the midline, but only after entering the ventral side of the central nervous system. After crossing the tracts insert on the dorsal optic tectum as in all other vertebrates. Therefore, given the obvious and undisputed homology, the optic chiasm is called chiasm also in these clades, even though the crossing is technically a decussation.
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The optic nerve leaves the orbit (eye socket) via the optic canal, running postero- medially towards the optic chiasm, where there is a partial decussation (crossing) of fibers from the temporal visual fields (the nasal hemi-retina) of both eyes. The proportion of decussating fibers varies between species, and is correlated with the degree of binocular vision enjoyed by a species.Textbook of Veterinary Anatomy, 4th Edition.
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The paramedian reticular nucleus (in Terminologia Anatomica, or paramedian medullary reticular group in NeuroNames) sends its connections to the spinal cord in a mostly ipsilateral manner, although there is some decussation. It projects to the vermis in the anterior lobe, the pyramis and the uvula. The paramedian nucleus also projects to the contralateral PRN, the gigantocellular nucleus, and the nucleus ambiguous.Jouvet, M. Handbook of clinical neurology vol 3.
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This induces the roof plate to begin to secrete BMP, which will induce the alar plate to develop sensory neurons. The alar plate and the basal plate are separated by the sulcus limitans. Additionally, the floor plate also secretes netrins. The netrins act as chemoattractants to decussation of pain and temperature sensory neurons in the alar plate across the anterior white commissure, where they then ascend towards the thalamus.
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It is characterized by the presence of an oculomotor nerve (CN III) palsy and cerebellar ataxia including tremor and involuntary choreoathetotic movements. Neuroanatomical structures affected include the oculomotor nucleus, red nucleus, corticospinal tracts and superior cerebellar peduncle decussation. It has a similar cause, morphology, signs and symptoms to Weber's syndrome; the main difference between the two being that Weber's is more associated with hemiplegia (i.e. paralysis), and Benedikt's with hemiataxia (i.e.
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Damages to these structures produce the ipsilateral presentation of paralysis or palsy due to the lack of cranial nerve decussation (aside from the trochlear nerve) before innervating their target muscles. The paralysis may be brief or it may last for several days, many times the episodes will resolve after sleep. Some common symptoms of alternating hemiplegia are mental impairment, gait and balance difficulties, excessive sweating and changes in body temperature.
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There are multiple theories that explain the outcome of allochiria. The current and most widely accepted explanation of allochiria is Hammond's Theory. This theory assumes that there is an almost complete decussation of sensory fibers within the grey matter. He concludes that with a lesion on one posterior side, this would reach center in the corresponding hemisphere, and thus, the sensation is then referred by this hemisphere to the opposite side of the body.
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These are the neural tracts which descend in the ventral horn of the spinal cord, carrying signals for voluntary movement of skeletal muscle. From their origin in the primary motor cortex, these nerves pass via the corona radiata to gather in the internal capsule before crossing over to the opposite side (decussation) in the medullary pyramids and proceeding down the spinal cord to meet lower motor neurons in the anterior grey column.
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This fracture resistance is why tooth enamel is three times stronger than its constituent hydroxyapatite crystallites that make up its enamel rods. Enamel tufts do not normally lead to enamel failure, due to these defects stabilizing potential fractures. The processes involved include them creating ‘‘stress shielding’’ by increasing the compliance of enamel next to the dentin. Decussation is another factor by which cracks form wavy stepwise extensions that arrest their further development.
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The neurons in these two nuclei (the dorsal column nuclei) are second-order neurons. Their axons cross over to the other side of the medulla and are now named as the internal arcuate fibers, that form the medial lemniscus on each side. This crossing over is known as the sensory decussation. At the medulla, the medial lemniscus is orientated perpendicular to the way the fibres travelled in their tracts in the posterior column.
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The plantaris muscle is innervated by the tibial nerve, a branch of the sciatic nerve in the sacral plexus. Signaling for contraction begins in the frontal lobe of the brain with the pre-central gyrus (primary motor cortex). Upper motor neurons are stimulated and send a signal through the internal capsule and down the corticospinal tract. Decussation of the lateral corticospinal tract occurs in the medullary pyramids, then the fibers continue down the contralateral side of the spinal cord.
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The rest of the whorls (7 to 8 in all) are traversed spirally by three strong cords, the central one narrowest, all closely beaded by the decussation of close, regular, elevated lamellae of increment, which sharply sculpture the interstices. Two lamellae arise from each bead of the superior spiral cord. The sutures are very deeply, narrowly channelled. The body whorl is angled at the periphery, and bears 7 concentric lirae on the base, the inner ones smaller.
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White matter tracts within a human brain, as visualized by MRI tractography 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.
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In 1857, Broca contributed to Charles-Édouard Brown-Séquard's work on the nervous system, conducting vivisection experiments, where specific spinal nerves were cut to demonstrate the spinal pathways for sensory and motor systems. As a result of this work. Brown-Séquard became known for demonstrating the principle of decussation, where a vertebrate's neural fibers cross from one lateral side to another, resulting in phenomenon of the right side of that animals brain controlling the left side of the other.Schiller, 1979, pp.
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The alar plate and the basal plate are separated by the sulcus limitans. Additionally, the floor plate also secretes netrins. The netrins act as chemoattractants to decussation of pain and temperature sensory neurons in the alar plate across the anterior white commissure, where they then ascend towards the thalamus. Following the closure of the caudal neuropore and formation of the brain's ventricles that contain the choroid plexus tissue, the central canal of the caudal spinal cord is filled with cerebrospinal fluid.
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The cerebellothalamic tract or the tractus cerebellothalamicus, is part of the superior cerebellar peduncle. It originates in the cerebellar nuclei, crosses completely in the decussation of the superior cerebellar peduncle, bypasses the red nucleus, and terminates in posterior division of ventral lateral nucleus of thalamus. The ventrolateral nucleus has different divisions and distinct connections, mostly with frontal and parietal lobes. The primary motor cortex and premotor cortex get information from the ventrolateral nucleus projections originating in the interposed nucleus and dentate nuclei.
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These studies have measured the output of the auditory nerve (AN), with and without OCB stimulation. In 1956, Galambos activated the efferent fibres of the cat by delivering shock stimuli to the floor of the fourth ventricle (at the decussation of the COCB). Galambos observed a suppression of the compound action potentials of the AN (referred to as the N1 potential) evoked by low-intensity click stimuli. This basic finding was repeatedly confirmed (Desmedt and Monaco, 1961; Fex, 1962; Desmedt, 1962; Wiederhold, 1970).
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Decussated fibers later reach and connect these segments with the higher centers. The optic chiasm is the primary cause of decussation; nasal fibers of the optic nerve cross (so each cerebral hemisphere receives contralateral—opposite—vision) to keep the interneuronal connections responsible for processing information short. All sensory and motor pathways converge and diverge to the contralateral hemisphere.Excerpt from Cunningham's Textbook of Anatomy Although sensory pathways are often depicted as chains of individual neurons connected in series, this is an oversimplification.
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The protoconch is composed of four whorls, the first minutely punctate, the second and third being decussated by arcuate riblets, while the fourth whorl has this decussation on its lower half, but one series of riblets has become obsolete on the upper half. The residue of the shell is, in some specimens, marked by three or four incised lines, the only other sculpture being the lines of growth, which are sinuous and more noticeable just below the suture. The body whorl is large and inflated. The aperture is ovate.
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The corpora quadrigemina are four mounds, called colliculi, in two pairs – a superior and an inferior pair, on the surface of the tectum. The superior colliculi process some visual information, aid the decussation of several fibres of the optic nerve (some fibres remain ipsilateral), and are involved with saccadic eye movements. The tectospinal tract connects the superior colliculi to the cervical nerves of the neck, and co-ordinates head and eye movements. Each superior colliculi also sends information to the corresponding lateral geniculate nucleus, with which it is directly connected.
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As the primary motor axons travel down through the cerebral white matter, they move closer together and form part of the posterior limb of the internal capsule. They continue down into the brainstem, where some of them, after crossing over to the contralateral side, distribute to the cranial nerve motor nuclei. (Note: a few motor fibers synapse with lower motor neurons on the same side of the brainstem). After crossing over to the contralateral side in the medulla oblongata (pyramidal decussation), the axons travel down the spinal cord as the lateral corticospinal tract.
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Here they form two prominences called the medulla oblongatary pyramids. Below the prominences, the majority of axons cross over to the opposite side from which they originated, known as decussation. The axons that cross over move to the outer part of the medulla oblongata and form the lateral corticospinal tract, whereas the fibres that remain form the anterior corticospinal tract. About 80% of axons cross over and form the lateral corticospinal tract; 10% do not cross over and join the tract, and 10% of fibres travel in the anterior corticospinal tract.
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There, partial decussation occurs, and about 53% of the fibers cross to form the optic tracts. Most of these fibers terminate in the lateral geniculate body. Based on this anatomy, the optic nerve may be divided in the four parts as indicated in the image at the top of this section (this view is from above as if you were looking into the orbit after the top of the skull had been removed): 1. the optic head (which is where it begins in the eyeball (globe) with fibers from the retina; 2.
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The genu is the flexure of the internal capsule. It is formed by fibers from the corticonuclear tracts. The fibers in this region are named the geniculate fibers; they originate in the motor part of the cerebral cortex and after passing downward through the base of the cerebral peduncle with the cerebrospinal fibers, undergo decussation and end in the motor nuclei of the cranial nerves of the opposite side. It contains the corticobulbar tract, which carries upper motor neurons from the motor cortex to cranial nerve nuclei that mainly govern motion of striated muscle in the head and face.
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The accessory cuneate nucleus is located lateral to the cuneate nucleus in the medulla oblongata at the level of the sensory decussation (the crossing fibers of the posterior column/medial lemniscus tract). It receives sensory input about position and movement (proprioception) from the upper limb by way of cervical spinal nerves and transmits that information to the cerebellum. These fibers are called cuneocerebellar (cuneate nucleus → cerebellum) fibers. In this function, the accessory cuneate nucleus is the upper extremity equivalent of Clarke's column, also called the nucleus thoracicus, which is the source of spinocerebellar connections for proprioception from the lower limb.
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The corticospinal tract serves as the motor pathway for upper motor neuronal signals coming from the cerebral cortex and from primitive brainstem motor nuclei. Cortical upper motor neurons originate from Brodmann areas 1, 2, 3, 4, and 6 and then descend in the posterior limb of the internal capsule, through the crus cerebri, down through the pons, and to the medullary pyramids, where about 90% of the axons cross to the contralateral side at the decussation of the pyramids. They then descend as the lateral corticospinal tract. These axons synapse with lower motor neurons in the ventral horns of all levels of the spinal cord.
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The muscles that would receive signals from these damaged upper motor neurons result in spastic paralysis. With a lesion in the brainstem, this affects the majority of limb and trunk muscles on the contralateral side due to the upper motor neurons decussation after the brainstem. The cranial nerves and cranial nerve nuclei are also located in the brainstem making them susceptible to damage from a brainstem lesion. Cranial nerves III (Oculomotor), VI (Abducens), and XII (Hypoglossal) are most often associated with this syndrome given their close proximity with the pyramidal tract, the location which upper motor neurons are in on their way to the spinal cord.
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A promising method of identifying the neuroanatomical structures responsible for USWS is continuing comparisons of brains that exhibit USWS with those that do not. Some studies have shown induced asynchronous SWS in non-USWS-exhibiting animals as a result of sagittal transactions of subcortical regions, including the lower brainstem, while leaving the corpus callosum intact. Other comparisons found that mammals exhibiting USWS have a larger posterior commissure and increased decussation of ascending fibres from the locus coeruleus in the brainstem. This is consistent with the fact that one form for neuromodulation, the noradrenergic diffuse modulatory system present in the locus coeruleus, is involved in regulating arousal, attention, and sleep-wake cycles.
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From 1722 to 1741 he was a member of the Académie Royale des Sciences. Petit is remembered for his anatomical studies of the eye, as well as physiological research of the sympathetic nervous system. As a military physician, Petit noticed that there was a striking correlation between soldiers' head wounds and contralateral motor effects, which he documented in a 1710 treatise called Lettres d’un medecin des hopitaux du roi a un autre medecin de ses amis. The early 18th century contributions of Pourfour du Petit He performed pioneer investigations on the internal structure of the spinal cord, and gave an early, detailed description of the decussation of the pyramids.
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Brown-Séquard was a keen observer and experimentalist. He contributed largely to our knowledge of the blood and animal heat, as well as many facts of the highest importance on the nervous system. He was the first scientist to work out the physiology of the spinal cord, demonstrating that the decussation of the fibres carrying pain and temperature sensation occurs in the cord itself. His name was immortalised in the history of medicine with the description of a syndrome which bears his name (Brown-Séquard syndrome) due to the hemisection of the spinal cord, which he described after observing accidental injury of the spinal cord in farmers cutting sugar cane in Mauritius.
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The nucleus of the trochlear nerve is located in the midbrain, at an intercollicular level between the superior colliculus and inferior colliculus. It is a motor nucleus, and so is located near the midline, embedded within the medial longitudinal fasciculus (see diagram at right). The oculomotor nerve and trochlear nerve are the only two cranial nerves with nuclei in the midbrain, other than the trigeminal nerve, which has a midbrain nucleus called the mesencephalic nucleus of trigeminal nerve, which functions in preserving dentition. Oddly, fibers from the trochlear nucleus cross over in the trochlear decussation of the midbrain, located in the superior medullary velum to exit dorsally, the only cranial nerve to do so.
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It arises from the outer surfaces of the alveolar processes of the maxilla and mandible, corresponding to the three pairs of molar teeth and in the mandible, it is attached upon the buccinator crest posterior to the third molar;Google Books Woelfel's Dental Anatomy: Its Relevance to dentistry, Rickne C. Scheid, Julian B. Woelfel. and behind, from the anterior border of the pterygomandibular raphe which separates it from the constrictor pharyngis superior. The fibers converge toward the angle of the mouth, where the central fibers intersect each other, those from below being continuous with the upper segment of the orbicularis oris, and those from above with the lower segment; the upper and lower fibers are continued forward into the corresponding lip without decussation.
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The uppermost and lowermost fibers of the buccinator pass across the lips from side to side without decussation. Superficial to this stratum is a second, formed on either side by the caninus and triangularis, which cross each other at the angle of the mouth; those from the caninus passing to the lower lip, and those from the triangularis to the upper lip, along which they run, to be inserted into the skin near the median line. In addition to these, fibers from the quadratus labii superioris, the zygomaticus, and the quadratus labii inferioris intermingle with the transverse fibers above described, and have principally an oblique direction. The proper fibers of the lips are oblique, and pass from the under surface of the skin to the mucous membrane, through the thickness of the lip.
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Sensory information from the upper half of the body is received at the cervical level of the spinal cord and carried in the cuneate tract, and information from the lower body is received at the lumbar level and carried in the gracile tract. The gracile tract is medial to the more lateral cuneate tract. The axons of second-order neurons of the gracile and cuneate nuclei are known as the internal arcuate fibers and when they cross over the midline, at the sensory decussation in the medulla, they form the medial lemniscus which connects with thalamus; the axons synapse on neurons in the ventral nuclear group which then send axons to the postcentral gyrus in the parietal lobe. All of the axons in the DCML pathway are rapidly conducting, large, myelinated fibers.
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Spinal cord tracts - tracts of the DCML pathway shown upper right. The DCML pathway is made up of the axons of first, second, and third- order sensory neurons, beginning in the dorsal root ganglia. The axons from the first-order neurons form the ascending tracts of the gracile fasciculus, and the cuneate fasciculus which synapse on the second-order neurons in the gracile nucleus and the cuneate nucleus known together as the dorsal column nuclei; axons from these neurons ascend as the internal arcuate fibers; the fibers cross over at the sensory decussation and form the medial lemniscus which connects with thalamus; the axons synapse on neurons in the ventral nuclear group which then send axons to the postcentral gyrus in the parietal lobe. The gracile fasciculus carries sensory information from the lower half of the body entering the spinal cord at the lumbar level.
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A lesion on either the left or right side would affect both the anterior and posterior routes on that side because of their close physical proximity to one another. So, a lesion on the left side would inhibit muscle innervation from both the left posterior and anterior routes, thus paralyzing the whole left side of the face (Bell’s palsy). With this type of lesion, the bilateral and contralateral inputs of the posterior and anterior routes, respectively, become irrelevant because the lesion is below the level of the medulla and the facial motor nucleus. Whereas at a level above the medulla a lesion occurring in one hemisphere would mean that the other hemisphere could still sufficiently innervate the posterior facial motor nucleus, a lesion affecting a lower motor neuron would eliminate innervation altogether because the nerves no longer have a means to receive compensatory contralateral input at a downstream decussation.
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