View source for Pyramidal tracts

{{Short description|Central nervous system pathways}}
{{Infobox tract
| Name            = Pyramidal tracts
| Latin           =tractus pyramidalis<br />tractus corticospinalis 
| Image           = Gray684.png
| Caption         = Deep dissection of brain-stem. Lateral view. ("pyramidal tract" visible in red, and "pyramidal decussation" labeled at lower right.)
| Image2          = Spinal cord tracts - English.svg
| Caption2        = [[Spinal cord tracts]], with ''pyramidal tracts'' labeled at upper left
| IsPartOf        =
| Components      =
| Artery          =
| Vein            =
| From = [[Cerebral cortex]] 
| To = [[Spinal cord]] (''corticospinal'') or [[brainstem]] (''corticobulbar'')
| Decussation = Many fibres in the [[medullary pyramids]] 
|System=|Function=}}

The '''pyramidal tracts''' include both the [[corticobulbar tract]] and the [[corticospinal tract]]. These are aggregations of [[efferent nerve fiber]]s from the [[upper motor neuron]]s that travel from the [[cerebral cortex]] and terminate either in the [[brainstem]] (''corticobulbar'') or [[spinal cord]] (''corticospinal'') and are involved in the [[Motor control|control of motor functions]] of the body.

The corticobulbar tract conducts [[action potential|impulses]] from the [[brain]] to the [[cranial nerve]]s.<ref>Chapter 9 of "Principles of Physiology" (3rd edition) by Robert M. Berne and Mathew N. Levy. Published by Mosby, Inc. (2000) {{ISBN|0-323-00813-5}}.</ref> These nerves control the muscles of the face and neck and are involved in facial expression, mastication, swallowing, and other motor functions.

The [[corticospinal tract]] conducts impulses from the brain to the spinal cord. It is made up of a [[lateral corticospinal tract|lateral]] and [[anterior corticospinal tract|anterior tract]]. The corticospinal tract is involved in voluntary movement. The majority of fibres of the corticospinal tract [[medullary pyramids#Decussation|cross over]] in the [[medulla oblongata]], resulting in muscles being controlled by the opposite side of the brain. The corticospinal tract contains the [[axon]]s of the pyramidal cells, the largest of which are the [[Betz cell]]s, located in the [[primary motor cortex]].

The pyramidal tracts are named because they pass through the [[Medullary pyramids|pyramids]] of the [[medulla oblongata]]. The corticospinal fibers converge to a point when descending from the [[internal capsule]] to the brain stem from multiple directions, giving the impression of an inverted pyramid. Involvement of the pyramidal tract at any level leads to [[pyramidal signs]].

The [[myelin]]ation of the pyramidal fibres is incomplete at birth and gradually progresses in cranio-caudal direction and thereby progressively [[Child development stages#Motor, speech, vision and hearing|gaining functionality]]. Most of the myelination is complete by two years of age and thereafter it progresses very slowly in cranio-caudal direction up to twelve years of age.

==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=978-0-8493-8631-2|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=978-0-7817-5319-7|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 />

==Function==
The nerves within the corticospinal tract are involved in [[Motor system|movement]] of [[muscle]]s of the body. Because of the crossing-over of fibres, muscles are supplied by the side of the brain opposite to that of the muscle.<ref name=GUYTONHALL2005 /> The nerves within the corticobulbar tract are involved in movement in muscles of the head. They are involved in swallowing, [[phonation]], and movements of the tongue.<ref name=YOUNG2007 /> By virtue of involvement with the [[facial nerve]], the corticobulbar tract is also responsible for transmitting [[facial expression]].<ref name=Arslan2001>{{cite book|last1=Arslan|first1=Orhan|title=Neuroanatomical Basis of Clinical Neurology|date=2001|publisher=CRC Press|isbn=1-4398-0613-6|page=368|url=https://books.google.com/books?id=x3Gmj9_m1D0C&q=corticobulbar&pg=PA368|url-status=live|archive-url=https://web.archive.org/web/20180503163021/https://books.google.com/books?id=x3Gmj9_m1D0C&pg=PA368&dq=corticobulbar#v=onepage&q=corticobulbar&f=false|archive-date=2018-05-03}}</ref> With the exception of lower muscles of facial expression, all functions of the corticobulbar tract involve inputs from both sides of the brain.<ref name=Arslan2001 />

The [[extrapyramidal system]] refers to tracts within the spinal cord involved in involuntary movement but not part of the pyramidal tracts.<ref name=GUYTONHALL2005 /> Their functions include the control of posture and [[muscle tone]].{{citation needed|date=June 2014}}

==Clinical significance==
{{See|Pyramidal signs}}
# Damage to the fibres of the corticospinal tracts, anywhere along their course from the cerebral cortex to the lower end of the spinal cord, can cause an [[upper motor neuron syndrome]]. 
# A few days after the injury to the upper motor neurons, a pattern of motor signs and symptoms appears, including [[spasticity]], hyperactive reflexes, a loss of the ability to perform [[dexterity|fine movements]], and an [[extensor]] [[plantar]] response known as the [[Babinski sign]].<ref>{{cite book|title = Neuroscience|date = 2001|publisher = Sinauer Assoc.|location = Sunderland, Mass|isbn = 0-87893-742-0|pages = Damage to Descending Motor Pathways: The Upper Motor Neuron Syndrome|edition = 2.|url = https://www.ncbi.nlm.nih.gov/books/NBK10898/|url-status = live|archive-url = https://web.archive.org/web/20180503163021/https://www.ncbi.nlm.nih.gov/books/NBK10898/|archive-date = 2018-05-03}}</ref> 
# Symptoms generally occur alongside other sensory problems. 
# Causes may include disorders such as [[stroke]]s,<ref>{{cite journal |last1=Topcuoglu |first1=MA |last2=Saka |first2=E |last3=Silverman |first3=SB |last4=Schwamm |first4=LH |last5=Singhal |first5=AB |title=Recrudescence of Deficits After Stroke: Clinical and Imaging Phenotype, Triggers, and Risk Factors. |journal=JAMA Neurology |date=1 September 2017 |volume=74 |issue=9 |pages=1048–1055 |doi=10.1001/jamaneurol.2017.1668 |pmid=28783808|pmc=5710180 }}</ref> [[cerebral palsy]],<ref>{{cite journal |last1=Vitrikas |first1=K |last2=Dalton |first2=H |last3=Breish |first3=D |title=Cerebral Palsy: An Overview. |journal=American Family Physician |date=15 February 2020 |volume=101 |issue=4 |pages=213–220 |pmid=32053326}}</ref><ref>{{cite journal |last1=Farag |first1=Sara M. |last2=Mohammed |first2=Manal O. |last3=EL-Sobky |first3=Tamer A. |last4=ElKadery |first4=Nadia A. |last5=ElZohiery |first5=Abeer K. |title=Botulinum Toxin A Injection in Treatment of Upper Limb Spasticity in Children with Cerebral Palsy |journal=JBJS Reviews |date=March 2020 |volume=8 |issue=3 |article-number=e0119 |doi=10.2106/JBJS.RVW.19.00119 |pmid=32224633|pmc=7161716 |doi-access=free }}</ref> [[subdural hemorrhage]], [[abscess]]es and [[tumour]]s, neurodegenerative diseases such as [[multiple system atrophy]], inflammation such as [[meningitis]] and [[multiple sclerosis]], and trauma to the spinal cord, including from [[spinal disc herniation|slipped discs]].<ref name="HARRISONS2008">{{cite book|editor-first1=Anthony S. |editor-last1=Fauci |editor-first2=T. R. |editor-last2=Harrison|title=Harrison's principles of internal medicine|date=2008|publisher=McGraw-Hill Medical|location=New York|isbn=978-0-07-147692-8|pages=147–149|edition=17th}}</ref>
# If the [[corticobulbar tract]] is damaged on only one side, then only the lower face will be affected, however if there is involvement of both the left and right tracts, then the result is [[pseudobulbar palsy]]. This causes problems with swallowing, speaking, and [[emotional lability]].<ref name="HARRISONS2008" />
# Severe disabling involuntary movements such as [[hemiballismus]] or severe [[chorea]] might exhaust the patient and become a life-threatening situation. 
# In the past, this condition was treated by partial section of the pyramidal tract either at the [[primary motor cortex]] or at the [[cerebral crus]] (pedunculotomy).<ref>{{Cite book|url=https://books.google.com/books?id=Y6CMiJ3XincC&q=neuroanatomy+william+demyer|title=Neuroanatomy|last=DeMyer|first=William|date=1998|publisher=Williams & Wilkins|isbn=978-0-683-30075-8|language=en|url-status=live|archive-url=https://web.archive.org/web/20180303110819/https://books.google.co.th/books?id=Y6CMiJ3XincC&dq=neuroanatomy+william+demyer&hl=en&sa=X&ved=0ahUKEwjj9pWt9M7ZAhUBppQKHatODk4Q6AEIJzAA|archive-date=2018-03-03}}</ref>

==Additional images==
<gallery>
 File:Gray683.png|Dissection of brain-stem. Lateral view.
 File:Gray689.png|Superficial dissection of brain-stem. Ventral view.
 File:Gray764.png|The motor tract.
 File:Medulla spinalis - tracts - English.svg
 File:Gray672.png|Diagram of the principal fasciculi of the spinal cord, from [[Gray's anatomy]]
</gallery>

== In popular culture ==
In ''[[National Lampoon's European Vacation]]'', the Griswold family wins a vacation on a game show called ''Pig in a Poke'' when their opponents fail to correctly answer a question about the pyramidal tracts, despite Clark Griswold (played by Chevy Chase) mistakenly answering that they are a housing development outside Cairo.<ref>{{cite web|url=https://www.imdb.com/title/tt0089670/quotes|title=Quotes from "National Lampoon's European Vacation"|access-date=3 May 2018|via=www.imdb.com|url-status=live|archive-url=https://web.archive.org/web/20170325180929/http://www.imdb.com/title/tt0089670/quotes|archive-date=25 March 2017}}</ref>

==References==
{{Reflist}}

==External links==
* {{BrainstemWisconsin|01Pyramid}}
* [http://www.thebrain.mcgill.ca/flash/i/i_06/i_06_cl/i_06_cl_mou/i_06_cl_mou.html McGill]
* {{BrainMaps|Corticospinal%20tract|Corticospinal tract}}

{{Mesencephalon}}
{{Pons}}
{{Spinal cord}}
{{Neural tracts}}
{{Authority control}}

[[Category:Central nervous system pathways]]
[[Category:Motor system]]