Anterior corticospinal tract

The anterior corticospinal tract is a descending motor pathway primarily involved in the control of voluntary movements of the axial or truncal musculature, thereby contributing to posture adjustment. It consists of upper motor neurons and functions alongside the lateral corticospinal tract, which is responsible for skilled, fine voluntary movements of the distal muscles.

Anatomy or origin:

The corticospinal tracts—both anterior and lateral—originate from three regions of the cerebral cortex: the primary motor cortex (located in the precentral gyrus, Brodmann area 4), the premotor cortex (Brodmann area 6), and the primary somatosensory cortex (postcentral gyrus, Brodmann areas 3, 1, and 2). At their cortical origins, the anterior corticospinal tract is indistinguishable from the lateral corticospinal tract.

The fibers descend through the corona radiata and converge in the posterior limb of the internal capsule. From there, the corticospinal fibers travel through the crus cerebri (cerebral peduncle) of the midbrain, continue downward through the transverse pontocerebellar fibers of the ventral pons, and then enter the medulla oblongata. In the medulla, the fibers group together on either side of the midline to form swellings called the "pyramids"—hence the corticospinal pathway's alternative name, the pyramidal tract.

Decussation and differentiation:

• At the level of the lower medulla oblongata, approximately 90% of corticospinal fibers cross (decussate) to the contralateral side, and consequently descend in the lateral white column of the spinal cord as the lateral corticospinal tract.

• The remaining uncrossed fibers continue to descend ventrally, forming the anterior corticospinal tract.

Course and function:

The anterior corticospinal tract descends in the anterior while column of the spinal cord. While descending, its fibers eventually cross the midline within their respective spinal cord segments before synapsing with the alpha and gamma lower motor neurons in the anterior horns of the spinal cord. This crossing pattern allows the anterior corticospinal tract to exert bilateral control over axial muscles, assisting in the regulation of posture and trunk movements.