The terminal nerve, also known as cranial nerve 0 or simply as CN 0, is a nerve that was not included in the seminal classification of the cranial nerves as CN I through CN XII but is now generally classified as a cranial nerve. It was discovered by German scientist Gustav Fritsch in 1878 in the brains of sharks. It was first found in humans in 1913.[1]
A 1990 study has indicated that the terminal nerve is a common finding in the adult human brain.[2][3]
The nerve has been called unofficially by other names, including cranial nerve XIII, zero nerve, nerve N,[4]
and NT.[5]
Structure
The original images (1878) of Fritsch's dogfish shark brain showing the nerve marked by an asterisk
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.[4] Careful dissection is necessary to visualize the nerve. Its purpose and mechanism of function is still open to debate; consequently, nerve zero is often not mentioned in anatomy textbooks.[1]
Development
The zebrafish was used as a developmental model in research from 2004.[6]
The connections between the terminal nerve and the olfactory system have been extensively studied in human embryos. It was found to enter the brain at stages 17 and 18 from olfactory origins.[7]
Function
Although very close to[8] (and often confused for a branch of) the olfactory nerve, the terminal nerve 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 the terminal nerve projects to the medial and lateral septal nuclei and the preoptic areas, all of which are involved in regulating sexual behavior in mammals,[1] as well as a 1987 study finding that mating in hamsters is reduced when the terminal nerve is severed.[9]
Additional images
Three forms of the nerve on the underside of human brains
Brain viewed from below. Gyrus rectus seen at anterior centre.
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Fuller GN, Burger PC (1990). "Nervus terminalis (cranial nerve zero) in the adult human". Clinical Neuropathology. 9 (6): 279–83. PMID2286018. The presence of an additional cranial nerve (the nervus terminalis or cranial nerve zero) is well documented in many non-human vertebrate species. However, its existence in the adult human has been disputed. The present study focused on the structure and incidence of this nerve in the adult human brain. The nerve was examined post-mortem in 10 adult brains using dissection microscopy, light microscopy, transmission electron microscopy, and immunohistochemistry. In all specimens, the nervus terminalis was identified bilaterally as a microscopic plexus of unmyelinated peripheral nerve fascicles in the subarachnoid space covering the gyrus rectus of the orbital surface of the frontal lobes. The plexus appeared in the region of the cribriform plate of the ethmoid and coursed posteriorly to the vicinity of the olfactory trigone, medial olfactory gyrus, and lamina terminalis. We conclude that the terminal nerve is a common finding in the adult human brain, confirming early light microscopic reports.
