Lateral corticospinal tract

The lateral corticospinal tract, a descending motor pathway, is pivotal for voluntary, skilled (or fine) movements of distal muscles. Within the brain, it runs alongside the anterior corticospinal tract, which controls axial musculature, aiding posture adjustment. Both tracts descend closely through the brain, but their fibers separate at the medullary decussation.

The corticospinal tracts–lateral and anterior–arise from the primary motor cortex (precentral gyrus, area 4), premotor cortex (area 6), and primary sensory cortex (postcentral gyrus, areas 3, 1, 2). Initially indistinguishable, corticospinal fibers descend through the corona radiata and converge in the posterior limb of the internal capsule. From here, they traverse the midbrain’s crus cerebri before continuing through the transverse pontocerebellar fibers of the ventral pons. In the upper medulla, these fibers form the pyramids, prominent swellings on either side of the midline, giving the tract its alternate name, the pyramidal tract. At the lower medulla, approximately 90% of corticospinal fibers decussate, forming the lateral corticospinal tract, which descends in the lateral white column of the spinal cord. There, upper motor neurons of the lateral corticospinal tract synapse with the cell bodies of alpha and gamma lower motor neurons in the anterior horns of the spinal cord.

The remaining uncrossed fibers, descend in the ventral portions of the lower medulla and the spinal cord, forming the anterior corticospinal tract. These fibers decussate at their respective spinal cord segments, synapsing with the lower motor neurons in the anterior horns.

The lateral corticospinal tract’s medullary decussation explains its clinical significance. Lesions above the decussation (e.g., in the cerebral cortex, internal capsule, midbrain, or pons) result in contralateral paralysis or weakness, whereas lesions below the decussation (e.g., in the spinal cord) cause ipsilateral motor deficits.