Final answer:
The newest UMN pathway begins in the cerebral cortex. The primary motor cortex in the frontal lobe sends signals down the corticobulbar and corticospinal tracts to reach the lower motor neurons in the brain stem and spinal cord, which then control skeletal muscle movement.
Step-by-step explanation:
The newest upper motor neuron (UMN) pathway originates in the cerebral cortex. Specifically, the neurons located in the primary motor cortex, such as Betz cells, are responsible for sending motor output down to control the musculature through motor neurons. These descending pathways consist of the corticobulbar tract, which targets the brain stem, and the corticospinal tract, which targets the spinal cord.
As for the target of an upper motor neuron, it is the lower motor neuron, which in turn influences skeletal muscle. This connection allows the brain to execute coordinated movement. The part of the frontal lobe that initiates movement by directly connecting to motor neurons is also known as the primary motor cortex.