Primary motor cortex

The primary motor area is a group of networked cells in mammalian brains that controls movements of specific body parts associated with cell groups in that area of the brain. The area is closely linked by neural networks to corresponding areas in the primary somatosensory cortex.

Anatomy
The precentral gyrus lies in front of the postcentral gyrus - mostly on the lateral (convex) side of the cerebral hemispheres - from which it is separated by the central sulcus. Its anterior border is represented by the precentral sulcus, while  inferiorly it borders to the lateral fissure (Sylvian fissure). Medially, it is contiguous with the paracentral lobule.

The internal pyramidal layer (layer V) of the precentral cortex contains giant (70-100 micrometers)  pyramidal neurons (a.k.a. Betz cells), which 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. The Betz cells' along with their long axons are referred to as the upper motor neuron (UMN).

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 (motor homunculus). The arm and hand motor area is the largest and occupies the part of precentral gyrus, located in between the leg and face area.

As the motor axons 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 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.

Branches of the middle cerebral artery provide most of the arterial blood supply for the primary motor cortex. The medial aspect (leg areas) is supplied by branches of the anterior cerebral artery.

Scientists have long considered arrangement of the primary motor area to be about the same in all mammals. In neurological terms, the area is described as M1.

Function
In humans, the lateral area of the posterior prefrontal cortex (the side toward the back) 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).

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.

The primary motor cortex (also known as M1) works in association with pre-motor areas to plan and execute movements. M1 contains large neurons known as Betz cells which send long axons down the spinal cord to synapse directly 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).

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.