Final answer:
Preganglionic sympathetic axons innervate cervical and thoracic viscera by synapsing in the cervical and thoracic spine ganglia, specifically at chain ganglia or paravertebral ganglia within the sympathetic nervous system.
Step-by-step explanation:
The preganglionic sympathetic axons provide innervation to cervical and thoracic viscera within the ganglia at the cervical spine and thoracic spine levels. Specifically, the synapse with the ganglionic neuron occurs at chain ganglia that are adjacent to the spinal segment from which the preganglionic fiber originates.
For instance, the cervical ganglia, referred to as paravertebral ganglia, receive synapses from preganglionic sympathetic neurons. These neurons may ascend through the chain to synapse at a level different from where they emerged from the spinal cord, as in the case of spinal nerve T1, which travels to the superior cervical ganglion.