Final answer:
Upper motor neurons target lower motor neurons, and while the specific pathway for audio-visual input movement was not directly identified, the corticobulbar tract involves responses to audio-visual input.
Step-by-step explanation:
The upper motor neurons (UMN) pathways that are involved in movement in response to audio-visual input are not directly specified in the provided options of oldest, second oldest, or newest. However, we know that UMNs are involved in the conscious or voluntary movements of skeletal muscles. The target of an UMN is the lower motor neuron (LMN), which in turn connects to skeletal muscles to elicit movement. The two main descending pathways related to UMNs are the corticospinal tract and the corticobulbar tract. While the corticospinal tract is involved in voluntary movements, especially in response to complex and fine motor activity, the corticobulbar tract is primarily responsible for voluntary movements of the muscles controlled by cranial nerves, including responses to audio-visual input.