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.
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.