Location
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.
The location of the primary motor cortex is most obvious on histological examination due to the presence of the distinctive Betz cells. Layer V of the primary motor 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 a part of the corticospinal tract. The Betz cells account for only a small percentage of the corticospinal tract. By some measures they account for about 10% of the primary motor cortex neurons projecting to the spinal cord or about 2-3% of the total cortical projection to the spinal cord. Though the Betz cells do not compose the entire motor output of the cortex, they nonetheless provide a clear marker for the primary motor cortex. This region of cortex, characterized by the presence of Betz cells, was termed area 4 by Brodmann.
Read more about this topic: Motor Strip