Final answer:
The cell bodies of upper motoneurons are primarily found in the primary motor cortex (precentral gyrus) in the frontal lobe.
Step-by-step explanation:
The cell bodies of upper motoneurons are primarily found in the primary motor cortex (precentral gyrus). The primary motor cortex, located in the frontal lobe, is responsible for generating motor commands and instructing cells in the spinal cord to move skeletal muscles. This region of the cerebral cortex contains the upper motor neurons that stimulate spinal cord neurons, leading to skeletal muscle contraction.