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Leg monoparesis is due to lumbosacral somatic motor system injury. The lumbosacral somatic motor system has a central component and a peripheral component. The central component of the lumbosacral somatic motor system includes the cortical and subcortical neurons that influence the peripheral component. The peripheral component of the lumbosacral somatic motor system consists of motor neurons in the lumbosacral enlargement of the spinal cord (lumbosacral motor center), their axons, and the muscles they innervate.
The majority of cortical neurons of the lumbosacral somatic motor system are in the upper third of the postcentral gyrus. The upper third of the postcentral gyrus is located in the mesial surface of the brain (Figure 227.1). The rest of the cortical neurons of the lumbosacral motor system are scattered in the frontal and parietal areas. The axons from these cortical neurons travel in the centrum semiovale close to the lateral ventricles.

Figure 227.1.— 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; FN: facial nerve; 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 various components of the somatic motor system.