Editing Pyramidal tracts (section)

==Structure==
[[File:Bentralki (1).tif|thumb|upright=1.4|Pyramidal tracts]]
The term ''pyramidal tracts'' refers to [[upper motor neuron]]s that originate in the [[cerebral cortex]] and terminate in the [[spinal cord]] (''corticospinal'') or [[brainstem]] (''corticobulbar''). Nerves emerge in the [[cerebral cortex]], pass down and may [[decussation|cross sides]] in the [[medulla oblongata]], and travel as part of the [[spinal cord]] until they [[synapse]] with [[interneuron]]s in the [[grey column]] of the spinal cord.<ref name=GUYTONHALL2005>{{cite book|last=Hall|first=Arthur C. Guyton, John E.|title=Textbook of medical physiology|date=2005|publisher=W.B. Saunders|location=Philadelphia|isbn=978-0-7216-0240-0|pages=687–690|edition=11th}}</ref>

There is some variation in terminology. The ''pyramidal tracts'' definitively encompass the [[corticospinal tract]]s, and many authors also include the [[corticobulbar tract]]s.<ref>{{cite book|last1=Anthoney|first1=Terence R.|title=Neuroanatomy and the neurologic exam : a thesaurus of synonyms, similar-sounding non-synonyms, and terms of variable meaning|date=1994|publisher=CRC Press|location=Boca Raton|isbn=9780849386312|pages=458–460|url=https://books.google.com/books?id=whrU2XEvGFIC&q=corticobulbar+is+a+pyramidal+tract&pg=PA458|url-status=live|archive-url=https://web.archive.org/web/20180503163021/https://books.google.com/books?id=whrU2XEvGFIC&pg=PA458&dq=corticobulbar+is+a+pyramidal+tract#v=onepage&q=corticobulbar%20is%20a%20pyramidal%20tract&f=false|archive-date=2018-05-03}}</ref>

===Corticospinal tract===
{{Further|Corticospinal tract}}
Nerve fibres in the corticospinal tract originate from [[pyramidal cells]] in layer V of the [[cerebral cortex]]. Fibres arise from the [[primary motor cortex]] (about 30%), [[supplementary motor area]] and the [[premotor cortex]] (together also about 30%), and the [[somatosensory cortex]], [[parietal lobe]], and [[cingulate gyrus]] supplies the rest.<ref name=GUYTONHALL2005 /> The cells have their [[soma (biology)|bodies]] in the cerebral cortex, and the [[axon]]s form the bulk of the pyramidal tracts.<ref name=HARRISONS2008 /> The nerve axons travel from the cortex through the [[posterior limb of internal capsule]], through the [[cerebral peduncle]] and into the [[brainstem]] and anterior [[medulla oblongata]]. Here they form two prominences called the [[medullary pyramids (brainstem)|medulla oblongatary pyramids]]. Below the prominences, the majority of axons cross over to the opposite side from which they originated, known as [[Medullary pyramids (brainstem)#Decussation|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]].<ref name=GUYTONHALL2005 /> 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. {{Citation needed|date=February 2013}}

The nerve axons traveling down the tract are the [[efferent nerve fiber]]s of the [[upper motor neuron]]s. These axons travel down the tracts in the [[white matter]] of the spinal cord until they reach the [[vertebra]]l level of the muscle that they will innervate.<ref name=Arslan2001/> At this point, the axons [[synapse]] with [[lower motor neuron]]s. The majority of axons do not directly synapse with lower motor neurons, but instead synapse with an [[interneuron]] that then synapses with a lower motor neuron. This generally occurs in the [[anterior grey column]].<ref name=GUYTONHALL2005 /> Nerve axons of the lateral corticospinal tract that did not cross over in the medulla oblongata do so at the level of the spinal cord they terminate in.<ref name=YOUNG2007 />

These tracts contain more than 1 million axons and the majority of the axons are myelinated. The corticospinal tracts myelinate largely during the first and second years after birth. The majority of nerve axons are small (<4μm) in diameter. About 3% of nerve axons have a much larger diameter (16μm) and arise from [[Betz cell]]s, mostly in the leg area of the primary motor cortex. These cells are notable because of their rapid conduction rate, over 70m/sec, the fastest conduction of any signals from the brain to the spinal cord.<ref name=GUYTONHALL2005 />

[[File:Lower pons horizontal KB.svg|thumb|right|Horizontal section through the lower part of the pons, showing the fibers of the corticospinal tract (#19) passing through the pontine nuclei]]

===Corticobulbar tract===
{{Further|Corticobulbar tract}}
Fibres from the [[:wikt:ventral|ventral]] [[motor cortex]] travel with the corticospinal tract through the internal capsule, but terminate in a number of locations in the [[midbrain]] ([[cortico-mesencephalic tract]]), [[pons]] ([[Corticopontine tract]]), and [[medulla oblongata]] ([[cortico-bulbar tract]]).<ref name=YOUNG2007>{{cite book|last1=Young|first1=Paul A.|title=Basic clinical neuroscience|date=2007|publisher=Lippincott Williams & Wilkins|location=Philadelphia, Pa.|isbn=9780781753197|pages=69–70|edition=2nd|url=https://books.google.com/books?id=dCh_gIIUnEoC&q=corticobulbar+is+a+pyramidal+tract&pg=PA69|url-status=live|archive-url=https://web.archive.org/web/20170313215307/https://books.google.com/books?id=dCh_gIIUnEoC&pg=PA69&dq=corticobulbar+is+a+pyramidal+tract#v=onepage&q=corticobulbar%20is%20a%20pyramidal%20tract&f=false|archive-date=2017-03-13}}</ref> The [[upper motor neuron]]s of the corticobulbar tract synapse with interneurons or directly with the lower motor neurons located in the motor [[cranial nerve nuclei]], namely [[oculomotor]], [[trochlear motor neuron|trochlear]], motor nucleus of the [[trigeminal nerve]], [[abducens]], [[facial nerve]] and [[Accessory nerve|accessory]] and in the [[nucleus ambiguus]] to the [[hypoglossal]], [[vagus nerve|vagus]] and [[accessory nerve]]s.<ref name=YOUNG2007 /> These nuclei are supplied by nerves from both sides of the brain, with the exception of the parts of the facial nerve that control muscles of the lower face. These muscles are only innervated by nerves from the contralateral (opposite) side of the cortex.<ref name=YOUNG2007 />