Final answer:
The 1st order neuron of the PCML enters the ipsilateral posterior column and terminates in the nucleus gracilis in the medulla oblongata, where it synapses with the second-order neuron.
Step-by-step explanation:
The 1st order neuron of the posterior column-medial lemniscus pathway (PCML) enters the ipsilateral posterior column and goes into the fasciculus gracilis, which is where the axons from the lower body and legs are carried. These axons of the dorsal root ganglion neuron enter the dorsal root and ascend through the spinal cord. Upon reaching the medulla oblongata, the axons terminate and synapse at the nucleus gracilis, a medullary nucleus. It is here that the first-order neurons of the dorsal column system specifically synapse from the lower body and legs.
Furthermore, the fibers that make up the fasciculus gracilis are part of a somatosensory pathway responsible for carrying information such as touch, vibration, and proprioception from the lower body to the brain. The pathway is highly organized and somatotopically arranged, with the lower body fibers being medial and upper body fibers laterally in the dorsal column.