Final answer:
The spinothalamic tract is located in the dorsal horn of the gray matter of the spinal cord, where it begins and synapses before ascending to the thalamus.
Step-by-step explanation:
The spinothalamic tract is contained within the gray matter of the spinal cord. It begins with neurons in a dorsal root ganglion, which extend their axons to the dorsal horn, where they synapse with the second neuron in the pathway. The second neuron, which is part of the spinothalamic tract, has its cell body in the dorsal horn and sends axons across the spinal cord (a process known as decussation) and ascends to the thalamus.