Preganglionic fibers are part of the nervous systems of Bajorans and Humans.

In one of the final oral exams while he was at the Starfleet Medical Academy, Julian Bashir incorrectly identified a preganglionic fiber as a postganglionic nerve. The error meant Bashir lost the title of valedictorian to Elizabeth Lense and became salutatorian of the graduating class. (DS9 episodes: "Q-Less", "Explorers")

Bashir mentioned his failure in discerning between a preganglionic fiber and a postganglionic nerve to Jadzia Dax, Benjamin Sisko and Kira Nerys when he first met them on the way to, and on, Deep Space 9 in 2369. (DS9 novelization: Emissary)

In 2371, while Bashir was in a coma after a telepathic attack by the Lethean criminal Altovar, an imaginary version of the Lethean claimed Bashir gave the wrong answer on purpose. (DS9 episode: "Distant Voices")

In 2376, Simon Tarses misidentified a preganglionic fiber as a postganglionic nerve on a medical scan of Ro Laren. Bashir was quick to correct him. (DS9 novel: Warpath)