There is a precise somatotopic representation of the different body parts in the primary motor cortex, with the leg area located medially (close to the midline), and the head and face area located laterally on the convex side of the cerebral hemisphere (cortical homunculus). The arm and hand motor area is the largest and occupies the part of precentral gyrus, located inbetween the leg and face area.
Function
The precentral gyrus is specialised for sending signals down to the spinal cord for movement.[1] As they travel down through the cerebral white matter, the motor axons 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 neurons on the same side of the brainstem). After crossing over to the contralateral side in the medulla oblongata (pyramidal decussation), the axons travel down the spinal cord as the lateral corticospinal tract. Fibers that do not cross over in the brainstem travel down the separate ventral corticospinal tract and most of them cross over to the contralateral side in the spinal cord, shortly before reaching the lower motor neurons.
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. New research has identified this as the part of the brain that makes sure our words are being properly articulated. This knowledge could help treat speech disorders and neural disorders in the future, the researchers say.[2]
Additional images
Precentral gyrus highlighted in green on coronal T1 MRI images
Precentral gyrus highlighted in green on sagittal T1 MRI images
Precentral gyrus highlighted in green on transversal T1 MRI images
^ This article incorporates text available under the CC BY 4.0 license. Betts, J Gordon; Desaix, Peter; Johnson, Eddie; Johnson, Jody E; Korol, Oksana; Kruse, Dean; Poe, Brandon; Wise, James; Womble, Mark D; Young, Kelly A (July 6, 2023). Anatomy & Physiology. Houston: OpenStax CNX. 12.3 The function of nervous tissue. ISBN978-1-947172-04-3.