Final answer:
The second oldest upper motor neuron (UMN) pathway originates in the Cerebral cortex and descends to activate upper motor neurons in the spinal cord's ventral horn.
Step-by-step explanation:
The second oldest upper motor neuron (UMN) pathway originates in the Cerebral cortex. The UMN pathway consists of the corticospinal tract, which controls voluntary movements of skeletal muscles. The axons of Betz cells in the primary motor cortex descend to activate upper motor neurons in the spinal cord, specifically in the ventral horn.