Final answer:
First and second order neurons of the anterolateral/spinothalamic tract synapse in the dorsal horn, and the second neuron decussates within the spinal cord before synapsing with the third neuron in the thalamus.
Step-by-step explanation:
The first and second order neurons of the anterolateral/spinothalamic tract synapse in the dorsal horn of the spinal cord. Here, the first neuron, which originates from the dorsal root ganglion, extends its axons to the dorsal horn where it synapses with the second neuron.
The second neuron then decussates, or crosses over within the spinal cord, ascends to the brain and then enters the thalamus, where it synapses with the third neuron. This third neuron projects its axons to the cerebral cortex, terminating in the postcentral gyrus.