Final answer:
The adrenal medulla is the organ in the sympathetic nervous system that has only one synapse, with preganglionic neurons directly synapsing onto its cells, leading to the systemic release of epinephrine and norepinephrine.
Step-by-step explanation:
The organ that has only one synapse in the sympathetic nervous system (SNS) when nearly all other target organs have two is the adrenal medulla. In the typical autonomic pathway, a preganglionic neuron originating in the central nervous system (CNS) synapses onto a postganglionic neuron in a ganglion, which in turn synapses onto the target organ. However, the adrenal medulla is unique because the preganglionic neuron directly synapses with the cells in the adrenal medulla, which then release epinephrine and norepinephrine into the bloodstream. This mechanism allows for a rapid systemic response that affects many organ systems at once, contributing to the 'fight or flight' response.