Final answer:
The first-order neuron of the PCML pathway enters the spinal cord through the dorsal root ganglion. From there, it synapses with the second neuron in the pathway and ascends to the brain to reach the thalamus. Finally, the third-order neuron synapses in the cerebral cortex.
Step-by-step explanation:
The first-order neuron of the posterior column-medial lemniscus (PCML) pathway enters the spinal cord through the dorsal root ganglion. These neurons extend their axons to the dorsal horn of the spinal cord and synapse with the second neuron in their respective pathway.
From there, the second neuron in the PCML pathway has its cell body in the gray matter of the spinal cord and connects to the thalamus. Axons from these second neurons then decussate (cross over) within the spinal cord and ascend to the brain, where they enter the thalamus.
In the thalamus, each second-order neuron synapses with a third-order neuron in its respective pathway. The neurons in the thalamus then project their axons to the spinothalamic tract, which finally synapses in the postcentral gyrus of the cerebral cortex.