Final answer:
The white matter is thinner in the upper spinal cord because sensory neurons are taken away at various levels, and motor neurons leave the white matter and enter the gray matter as they travel downward.
Step-by-step explanation:
The white matter of the spinal cord is thinner in the upper spinal cord for two primary reasons. First, sensory neurons are taken away at various levels of the spinal cord, thereby reducing the number of ascending fibers in the white matter as one moves upward. As you ascend the spinal cord, sensory information from the lower parts of the body has already been relayed to the brain, and hence fewer sensory fibers are needed. Secondly, motor neurons leave the white matter going into the gray matter as they go down the spinal cord. As motor commands from the brain descend in the spinal cord, they synapse on motor neurons located in the gray matter, which then relay the signals to the muscles, leading to a reduction in the volume of descending fibers in the white matter toward the upper segments of the spinal cord.