Final answer:
The completion of myelination of neural fibers within the corticospinal tract, linking the cerebral cortex and spinal cord, leads to significant motor control improvements in preschool-aged children.
Step-by-step explanation:
Dramatic gains in motor control at the end of preschool years can be attributed to completed myelinization of the fibers linking the cerebral cortex and the spinal cord. The corticospinal tract represents the neurons that send output from the primary motor cortex to the upper motor neurons (UMN) and then to the lower motor neurons (LMN), which finally project to skeletal muscle, causing contraction. Myelination is crucial because it enhances the speed and efficiency of electrical signal transmission along the nervous system, which results in improved voluntary movement control. During preschool years, this process reaches a level of maturity that allows for significant improvements in motor skills.