The axons of the cortical neurons go through the posterior limb of the internal capsule and continue caudally through the midbrain, pons, and upper medulla. At the lower medulla, these fibers cross to the contralateral side and then continue caudally in the lateral and anterior aspect of the spinal cord until they reach the neurons of the lumbosacral somatic motor center (Figure 228.1). The cortical neurons control voluntary movements. The subcortical neurons of the lumbosacral somatic motor system are located in the midbrain, pons, medulla, and cerebellum. The subcortical neurons influence lower extremity movements by their connections with the cortical component of the lumbosacral somatic motor system and by their connections with the lumbosacral somatic motor center. The subcortical neurons control automatic movements.
Figure 228.1.—Schematic representation of the somatic motor system. The green line represents the innervation of the leg. A lesion in this system produces leg weakness. V: ventricles; T: thalamus; UQ: upper quadrant; LQ: lower quadrant; BP: brachial plexus; LSP: lumbosacral plexus. The colored rectangles indicate the location of weakness produced by damage to the different components of the somatic motor system.