View source for Corticospinal tract

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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 [[medial 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.

==The motor pathway==
The corticospinal tract originates from cells in 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 the [[brain stem]] and [[spinal cord]], are referred to as [[upper motor neuron]]. 

====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 90%) 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]]. 
* 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 not visible as seven 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==
Damage to the descending motor pathways anywhere along the trajectory from the cerebral cortex to the lower end of the spinal cord gives rise to a set of symptoms called the "[[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]], the decreased vigor (and increased threshold) of superficial [[reflexes]], a loss of the ability to perform [[dexterity|fine movements]], and an [[extensor]] [[plantar]] response known as the [[Babinski sign]].<ref>http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=neurosci&part=A1191</ref>

==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.
</gallery>

==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}}

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{{Rhombencephalon}}
{{Spinal cord}}
[[Category:Central nervous system pathways]]

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[[sr:Пирамидални пут]]
[[fi:Kortikospinaalirata]]
[[pt:Sistema piramidal]]
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