Final answer:
The first order neuron in the anterolateral pathway ends in the spinal cord dorsal horn where it synapses with the second-order neuron as part of the spinothalamic tract.
Step-by-step explanation:
The first order neuron of the anterolateral pathway, specifically in the spinothalamic tract, ends in the spinal cord dorsal horn. These neurons begin in the dorsal root ganglion and send their axons to the dorsal horn, where they synapse with the second-order neurons. These second-order neurons then extend their axons, cross within the spinal cord (decussate), and ascend to the thalamus. It is in the thalamus where these second-order neurons synapse with third-order neurons, which then send their axons to synapse in the postcentral gyrus of the cerebral cortex, finalizing the pathway for the processing of somatosensory information.