Editing Pyramidal tracts (section)

===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]]