Final answer:
The second-order neurons in the fast pain pathway terminate in the thalamus, which they reach via the spinothalamic tract after initially synapsing in the spinal cord's dorsal horn.
Step-by-step explanation:
The question pertains to the pathway of fast pain fibers within the human nervous system. Specifically, it asks where the second-order neurons of this pathway terminate. The spinothalamic tract begins with neurons in a dorsal root ganglion, which extend axons to the dorsal horn and synapse with second-order neurons. These neurons, once synapsed, have their cell bodies in the spinal cord gray matter and connect to the thalamus. The axons from the second-order neurons decussate within the spinal cord and ascend to the brain to enter the thalamus. Here, each axon synapses with a third neuron. Therefore, the correct answer to the question is that the second-order neurons in the pathway for fast pain terminate in the thalamus.