Final answer:
The red nucleus in the midbrain is the region whose neurons project to medullary reticular nuclei, activating central pattern generators for locomotion.
Step-by-step explanation:
Neurons projecting from the red nucleus of the midbrain synapse on neurons in the medullary reticular nuclei to result in downstream activation of central pattern generators (CPGs) in locomotion. The red nucleus is known to have connections with motor pathways and is involved in the coordination of movements. The descending signals from the red nucleus are part of the extrapyramidal system, which influences movement control, posture, and muscle tone. The substantia nigra, another midbrain region, has important roles in modulating movement via the basal nuclei but is not the direct source of neurons that synapse on medullary reticular nuclei for CPG activation associated with locomotion.