Primary motor cortex
2472926
223683264
2008-07-05T06:54:49Z
CopperKettle
1434427
wikified Betz cells
{{Infobox Brain|
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GraySubject = 189 |
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Image = ba4.png |
Caption = [[Brodmann area 4]] of human brain. |
Image2 = Illu cerebrum lobes.jpg |
Caption2 = Primary motor cortex labeled as somatomotor cortex. |
IsPartOf = [[Precentral gyrus]]|
Components = |
Artery = [[Anterior cerebral artery|Anterior cerebral]] <br/> [[Middle cerebral artery|Middle cerebral]]|
Vein = |
BrainInfoType = ancil |
BrainInfoNumber = 421 |
MeshName = Motor+Cortex |
MeshNumber = A08.186.211.730.885.213.270.548 |
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The '''primary motor cortex''' (or '''M1''') works in association with [[Brodmann area 6|pre-motor]] areas to plan and execute movements. M1 contains large neurons known as [[Betz cell]]s which send long [[axon | axons]] down the [[spinal cord]] to synapse onto alpha motor neurons which connect to the muscles. Pre-motor areas are involved in planning actions (in concert with the [[basal ganglia]]) and refining movements based upon sensory input (this requires the [[cerebellum]]).
Scientists have long considered arrangement of the primary motor area to be similar in all mammals.
==Location==
The human primary motor cortex is located in the [[anatomical terms of location | dorsal]] part of the '''precentral gyrus''' and the [[anatomical terms of location | anterior]] bank of the [[central sulcus]]. The precentral gyrus is in front of the [[postcentral gyrus]] from which it is separated by the central sulcus. Its anterior border is the [[precentral sulcus]], while [[anatomical terms of location | inferiorly]] it borders to the [[lateral fissure]] (''Sylvian fissure''). [[anatomical terms of location | Medially]], it is contiguous with the [[paracentral lobule]].
==Layers==
The ''internal pyramidal layer (layer V)'' of the precentral [[cerebral cortex | cortex]] contains giant (70-100 micrometers) ''[[neuron | pyramidal neuron]]s'' (a.k.a. ''Betz cells''), which send long [[axon]]s to the contralateral motor nuclei of the [[cranial nerve]]s and to the [[lower motor neuron]]s in the ventral horn of the [[spinal cord]]. These axons form the [[corticospinal tract]]. The ''[[Betz cells]]''' along with their long axons are referred to as the [[upper motor neuron]] (UMN).
=="Homunculus" or "Little Man"==
[[Image:Sensory and motor homunculi.jpg|thumb|right|300px|Sensory (left) and motor (right) homunculi.]]
There is a precise somatotopic representation of the different body parts in the primary motor cortex, with the leg area located close to the midline, and the head and face area located laterally on the convex side of the cerebral hemisphere (''motor [[homunculus]]''). The arm and hand motor area is the largest, and occupies the part of precentral gyrus, between the leg and face area.
In humans, the lateral area of the primary motor cortex is arranged from top to bottom in areas that correspond to the buttocks, torso, shoulder, elbow, wrist, fingers, thumb, eyelids, lips and jaw. Interior sections of the motor area folding into the [[medial longitudinal fissure]] correspond with the legs.
This arrangement, elucidated by [[Wilder Penfield]] and others, is called a motor [[homunculus]] (Latin: ''little man''). [http://nawrot.psych.ndsu.nodak.edu/Courses/Psych465.S.02/Movement/Fig.%208-4.jpg Here's] a picture of it.
Not all body parts are equally represented by cell density in the motor area in proportion to their size in the body. Lips, parts of the face and hands enjoy especially large areas of cells in the motor area. Evidence suggests motor cells not used can be recruited by other cells to account for deficiencies arising from trauma such as amputation or paralysis.
==Pathway==
As the motor [[axon]]s travel down through the cerebral [[white matter]], they move closer together and form part of the ''posterior limb'' of the [[internal capsule]].
They continue down into the [[brainstem]], where some of them, after crossing over to the contralateral side, distribute to the [[cranial nerve]] motor nuclei. ('''Note''': a few motor fibers [[synapse]] with [[lower motor neuron]]s on the same side of the [[brainstem]]).
After crossing over to the contralateral side in the [[medulla oblongata]] ([[corticospinal tract | pyramidal decussation]]), the axons travel down the [[spinal cord]] as the ''[[corticospinal tract | lateral corticospinal tract]]''.
Fibers that do not cross over in the [[brainstem]] travel down the separate [[corticospinal tract | ventral corticospinal tract]] and most of them cross over to the contralateral side in the [[spinal cord]], shortly before reaching the [[lower motor neuron]]s.
==Blood supply==
Branches of the [[middle cerebral artery]] provide most of the arterial blood supply for the [[motor cortex | primary motor cortex]].
The medial aspect (leg areas) is supplied by branches of the [[anterior cerebral artery]].
==Pathology==
Lesions of the precentral gyrus result in [[paralysis]] of the contralateral side of the body ([[facial palsy]], arm-/leg [[monoparesis]], [[hemiparesis]]) - see [[upper motor neuron]].
==Additional images==
<gallery>
Image:precentral_sulcus.png|[[Precentral sulcus]]
Image:Central sulcus.png|[[Central sulcus]]
Image:Gray726.png|Surface of left cerebral hemisphere, viewed from the side.
Image:Gray764.png|The motor tract.
Image:Sensory_and_motor_homunculi.jpg|Sensory and motor homunculi
</gallery>
==See also==
* [[Corticospinal tract]]
* [[Motor cortex]]
* [[Upper motor neuron]]
* [[Brodmann area]]
* [[List of regions in the human brain]]
==External links==
* [http://www.thebrain.mcgill.ca/flash/a/a_06/a_06_cr/a_06_cr_mou/a_06_cr_mou.html Overview at mcgill.ca]
{{Prosencephalon}}
[[Category:Cerebrum]]
[[Category:Somatic motor system]]
[[Category:Neuroanatomy]]
[[de:Gyrus praecentralis]]
[[ja:一次運動野]]
[[no:Primary motor cortex]]
[[ro:Girusul precentral]]
[[fi:Motorinen aivokuori]]