How does the sympathetic innervation of the adrenal medulla differ from typical sympathetic pathways, and why is the adrenal medulla considered a 'misplaced' sympathetic ganglion?

In typical sympathetic pathways, preganglionic fibers synapse in trunk or collateral ganglia on postganglionic neurons that innervate target organs. In the adrenal medulla pathway, some thoracic splanchnic nerve fibers pass through the celiac ganglion without synapsing and instead synapse directly on hormone-producing medullary cells, which release norepinephrine and epinephrine into the blood rather than acting on a local effector. The adrenal medulla is considered a 'misplaced' sympathetic ganglion because embryologically it arises from the same tissue as sympathetic ganglia, so its hormone-releasing cells are viewed as equivalent to postganglionic sympathetic neurons even though they lack nerve processes.

The difference is where the preganglionic fiber synapses and what the postsynaptic cell does. In a standard sympathetic pathway, the preganglionic axon synapses in a sympathetic trunk ganglion or a collateral ganglion such as the celiac ganglion, and a postganglionic neuron then carries the signal to an effector organ. For the adrenal medulla, some preganglionic fibers traveling in the thoracic splanchnic nerves pass through the celiac ganglion without synapsing and terminate by synapsing with the medullary cells of the adrenal gland. When stimulated, those medullary cells secrete norepinephrine and epinephrine into the blood, producing widespread excitatory effects rather than localized organ-specific effects. The reason the adrenal medulla is called a 'misplaced' sympathetic ganglion is that sympathetic ganglia and the adrenal medulla arise from the same embryonic tissue. Therefore, the hormone-releasing cells of the adrenal medulla, although lacking nerve processes, are considered equivalent to postganglionic sympathetic neurons.