Final answer:
The second neurons in the spinothalamic tract cross the midline in the spinal cord, where they are responsible for the transmission of pain and temperature sensations.
Step-by-step explanation:
Spinothalamic Tract Decussation
The 1st-order neuron of the spinothalamic tract synapses with an ipsilateral cell body in the dorsal horn of the spinal cord. After these ascending tracts synapse, they then cross the spinal cord at the level at which they entered. The correct answer to the question is that the axons from the second neurons decussate or cross within the spinal cord, not at the level of the medulla, pons, midbrain, or thalamus. Thus, they cross the midline in the spinal cord much earlier than the medulla, which is where the dorsal column system crosses. The spinothalamic tract is primarily responsible for transmitting pain and temperature sensations to the brain, as compared to the dorsal column system that carries information about touch and proprioception.