Corticospinal tract
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2008-06-17T16:41:32Z
春秋君
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The '''corticospinal''' or '''pyramidal tract''' is a massive collection of [[axon]]s that travel between the [[cerebral cortex]] of the [[brain]] and the [[spinal cord]].
The '''corticospinal tract''' mostly contains motor [[axon]]s. It actually consists of two separate tracts in the spinal cord: the [[lateral corticospinal tract]] and the [[anterior corticospinal tract]]. An understanding of these tracts leads to an understanding of why for the most part, one side of the body is controlled by the opposite side of the brain.
Also the [[corticobulbar tract]] is considered to be a pyramidal tract. The corticobulbar tract carries signals that control motor neurons located in cranial nerve brain nuclei rather than motor neurons located in the spinal cord.<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>
The [[neurons]] of the pyramidal tracts are [[pyramidal neurons]], but that is not how the pyramidal tract got its name, as most of the pyramidal neurons send their axons elsewhere.<ref>[http://williamcalvin.com/LEM/LEMend.htm William H. Calvin and Derek Bickerton, Lingua ex Machina, glossary (MIT Press)<!-- Bot generated title -->]</ref> Instead, it got its name from the shape of the corticospinal axon tracts: when the pyramidal tract passes the [[medulla]], it forms a dense bundle of nerve fibres that is shaped somewhat like a pyramid.<ref>[http://thebrain.mcgill.ca/flash/a/a_06/a_06_cl/a_06_cl_mou/a_06_cl_mou.html The Brain From Top To Bottom<!-- Bot generated title -->]</ref>
==The motor pathway==
The corticospinal tract originates from [[pyramidal cells]] in [[neocortex|layer V]] of the [[motor cortex]].
===Upper motor neurons===
The motor neuron cell bodies in the [[motor cortex]], together with their axons that travel down through the [[brain stem]] and [[spinal cord]], are referred to as [[upper motor neuron]]s.
====Decussation and synapses====
The [[neuron]]al cell bodies in the motor cortex send long axons to the motor [[cranial nerve]] nuclei mainly of the contralateral side of the [[midbrain]] ([[cortico-mesencephalic tract]]), [[pons]] ([[cortico-pontine tract]]), [[medulla oblongata]] ([[cortico-bulbar tract]]); the bulk of these fibers, however, extend all the way down to the [[spinal cord]] (corticospinal tract).
* Most of the cortico-spinal fibers (about 80%) cross over to the contralateral side in the [[medulla oblongata]] ([[pyramidal decussation]]). Those that cross in the medulla oblongata travel in the [[lateral corticospinal tract]].
* 10% enter the lateral corticospinal tract on the same side
* The remainder of them (10%) cross over at the level that they exit the [[spinal cord]], and these travel in the [[anterior corticospinal tract]].
Whichever of these two tracts it travels in, a cortico-spinal [[axon]] will [[synapse]] with another neuron in the [[Anterior horn (spinal cord)|ventral horn]]. This ventral horn neuron is considered a second-order neuron in this pathway, but is not part of the corticospinal tract itself.
====From cerebral to motor neurons====
The motor axons move closer together as they travel down through the cerebral [[white matter]], and form part of the ''posterior limb'' of the [[internal capsule]].
The motor fibers continue down into the [[brainstem]]. The bundle of corticospinal axons is visible as two column-like structures ("[[Pyramid of medulla oblongata|pyramids]]") on the ventral surface of [[medulla oblongata]] - this is where the name '''pyramidal tract''' comes from.
After the decussation, the axons travel down the [[spinal cord]] as the [[lateral corticospinal tract]]. Fibers that do not cross over in the [[medulla oblongata]] travel down the separate [[anterior corticospinal tract]], and most of them cross over to the contralateral side in the [[spinal cord]], shortly before reaching the [[lower motor neuron]]s.
===Lower motor neurons===
In the spinal cord, the axons of the upper motor neuron connect (most of them via [[interneurons]], but to a lesser extent also via direct [[synapse]]s) with the [[lower motor neuron]]s (LMNs), located in the [[ventral horn]] of the spinal cord.
In the [[brain stem]], the [[lower motor neuron]]s are located in the motor [[cranial nerve]] nuclei ([[occulomotor]], [[trochlear]], motor nucleus of the [[trigeminal nerve]], [[abducens]], [[facial]], [[accessory]], [[hypoglossal]]). The [[lower motor neuron]] [[axons]] leave the [[brain stem]] via motor cranial nerves and the [[spinal cord]] via anterior roots of the [[spinal nerve]]s respectively, end-up at the [[neuromuscular plate]] and provide motor innervation for voluntary muscles.
==Sensory pathways==
* [[Spinothalamic tract]]
* [[Spinocerebellar tract]]
* [[Visual pathway]]
* [[Olfactory system]]
* [[Posterior column pathway]]
==Corticospinal tract damage==
see [[upper motor neuron]].
==Extrapyramidal motor pathways==
These are motor pathways that lie outside the corticospinal tract and are beyond voluntary control. Their main function is to support voluntary movement and help control posture and [[muscle tone]]. See [[extrapyramidal motor system]].
==Additional images==
<gallery>
Image:Gray683.png|Dissection of brain-stem. Lateral view.
Image:Gray689.png|Superficial dissection of brain-stem. Ventral view.
Image:Gray764.png|The motor tract.
Image:Medulla spinalis - tracts - English.svg
</gallery>
==References==
{{Reflist}}
==External links==
* {{GPnotebook|959774759}}
* {{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}}
{{Medulla and pons of rhombencephalon}}
{{Spinal cord}}
{{Neural tracts}}
[[Category:Central nervous system pathways]]
[[de:Pyramidales System]]
[[ja:皮質脊髄路]]
[[pl:Układ piramidowy]]
[[sr:Пирамидални пут]]
[[fi:Kortikospinaalirata]]
[[pt:Sistema piramidal]]