Cranial nerve zero

The terminal nerve, or cranial nerve zero, was discovered by German scientist Gustav Fritsch in 1878 in the brains of sharks. It was first found in humans in 1913, although its presence in humans remains controversial. However, studies have indicated that the terminal nerve is a common finding in the adult human brain. It projects from the nasal cavity, enters the brain just a little bit ahead of the other cranial nerves as a microscopic plexus of unmyelinated peripheral nerve fascicles.

The nerve is often overlooked in autopsies because it is unusually thin for a cranial nerve, and is often torn out upon exposing the brain. Careful dissection is necessary to visualize the nerve. Its purpose and mechanism of function is still open to debate; consequently, nerve 0 is often not mentioned in anatomy textbooks.

Although very close to (and often confused for a branch of) the olfactory nerve, nerve 0 is not connected to the olfactory bulb, where smells are analyzed. This fact suggests that the nerve is either vestigial or may be related to the sensing of pheromones. This hypothesis is further supported by the fact that nerve zero projects to the medial and lateral septal nuclei, and the preoptic areas, all of which are involved in regulating sexual behavior in mammals.

The zebrafish has been used as a model in recent research.