Final answer:
The first and second order neurons of the dorsal column system synapse in the nuclei of the medulla. The nucleus gracilis is the target of fibers from the lower body and legs, whereas the nucleus cuneatus is the target of fibers from the upper body and arms. The second neurons then cross the midline of the medulla and project to the thalamus.
Step-by-step explanation:
The first and second order neurons of the dorsal column system synapse in the nuclei of the medulla. Specifically, the first order neurons from the lower body and legs synapse in the nucleus gracilis, while the first order neurons from the upper body and arms synapse in the nucleus cuneatus. The second neurons in these pathways then project from the respective nuclei, cross the midline of the medulla, and continue to ascend the brain stem as the medial lemniscus. Finally, they terminate in the thalamus and synapse with the third neuron, which projects its axons to the postcentral gyrus of the cerebral cortex for conscious perception of somatosensory stimuli.