The precentral gyrus is a prominent gyrus on the surface of the posterior frontal lobe of the brain.
It is the site of the primary motor cortex.
The precentral gyrus is defined as Brodmann area 4.
The precentral gyrus lies in front of the postcentral gyrus – mostly on the lateral side of each cerebral hemisphere.
The pre and post central gyrus are separated by the central sulcus.
Its anterior border is represented by the precentral sulcus, while inferiorly it borders to the lateral sulcus, known as the Sylvian fissure.
Its internal pyramidal layer (layer V) of the precentral cortex contains giant pyramidal neurons called Betz cells.
Betz cells are 70-100 micrometers in size, and send long axons to the contralateral motor nuclei of the cranial nerves and to the lower motor neurons in the ventral horn of the spinal cord.
These axons form the corticospinal tract, and with the Betz cells along with their long axons are referred to as upper motor neurons (UMN).
Different body parts are geographically represented in the precenteral gyrus primary cortex: the leg area is located medially close to the midline, and the head and face area located laterally on the convex side of the cerebral hemisphere, the cortical homunculus, the arm and hand area is located inbetween the leg and face area.
The arm and hand motor area is the largest area of the precentral gyrus.
As precentral axons of the corticospinal tract travel down through the cerebral white matter, the motor axons move closer together and form part of the posterior limb of the internal capsule.
The axons continue into the brainstem, where some of them, after crossing over to the contralateral side, distribute to the cranial nerve motor nuclei.
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, referred to as 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.
Branches of the middle cerebral artery provide most of the arterial blood supply for the primary motor cortex.
The medial aspect of the primary motor cortex, leg area, is supplied by branches of the anterior cerebral artery.
Lesions of the precentral gyrus result in paralysis of the contralateral side of the body+ facial palsy, arm-/leg monoparesis, hemiparesis.