Primary motor area

Primary motor cortex
Brodmann area 4 of human brain.
Primary motor cortex shown in green.
Details
Part of	Precentral gyrus
Artery	Anterior cerebral Middle cerebral
Identifiers
Latin	cortex motorius primus
MeSH	A08.186.211.730.885.213.270.548
NeuroNames	ancil-421
NeuroLex ID	Primary motor cortex
FMA	224854
Anatomical terms of neuroanatomy []

The primary motor cortex (Brodmann area 4) is a brain region that in humans is located in the dorsal portion of the frontal lobe. It is the primary region of the motor system and works in association with other motor areas including premotor cortex, the supplementary motor area, posterior parietal cortex, and several subcortical brain regions, to plan and execute movements. Primary motor cortex is defined anatomically as the region of cortex that contains large neurons known as Betz cells. Betz cells, along with other cortical neurons, send long axons down the spinal cord to synapse onto the interneuron circuitry of the spinal cord and also directly onto the alpha motor neurons in the spinal cord which connect to the muscles.

At the primary motor cortex, motor representation is orderly arranged (in an inverted fashion) from the toe (at the top of the cerebral hemisphere) to mouth (at the bottom) along a fold in the cortex called the central sulcus. However, some body parts may be controlled by partially overlapping regions of cortex. Each cerebral hemisphere of the primary motor cortex only contains a motor representation of the opposite (contralateral) side of the body. The amount of primary motor cortex devoted to a body part is not proportional to the absolute size of the body surface, but, instead, to the relative density of cutaneous motor receptors on said body part. The density of cutaneous motor receptors on the body part is generally indicative of the necessary degree of precision of movement required at that body part. For this reason, the human hands and face have a much larger representation than the legs.

For the discovery of the primary motor cortex and its relationship to other motor cortical areas, see the main article on the motor cortex.

The human primary motor cortex is located on the anterior wall of the central sulcus. It also extends anteriorly out of the sulcus partly onto the precentral gyrus. Anteriorly, the primary motor cortex is bordered by a set of areas that lie on the precentral gyrus and that are generally considered to compose the lateral premotor cortex. Posteriorly, the primary motor cortex is bordered by the primary somatosensory cortex, which lies on the posterior wall of the central sulcus. Ventrally the primary motor cortex is bordered by the insular cortex in the lateral sulcus. The primary motor cortex extends dorsally to the top of the hemisphere and then continues onto the medial wall of the hemisphere.