Terminal nerve

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The terminal nerve, also called cranial nerve zero (CN 0), is a nerve that isn’t part of the original 12 cranial nerves but is now recognized as a real, separate nerve.

Discovery and presence
- It was first found by German scientist Gustav Fritsch in shark brains in 1878 and later identified in humans in 1913. It is a common feature in adult human brains.

Where it starts and what it looks like
- The terminal nerve enters the brain in front of the other cranial nerves, near the lamina terminalis.
- It begins as a tiny network of unmyelinated nerve fibers in the subarachnoid space close to the cribriform plate and then travels toward the olfactory trigone and the lamina terminalis.
- In embryos it is clearly visible, but many of its nerve cells are lost before birth, so it is easier to miss in adults or during autopsies. Careful dissection is often needed to see it.

Development and research
- The zebrafish has been used as a model to study its development since 2004.
- Researchers have explored how the terminal nerve connects with the smell system in human embryos. Olfactory nerve fibers enter the brain around stage 17, and fibers from the vomeronasal organ and the terminal nerve enter around stages 17 and 18.

Function and significance
- The terminal nerve does not connect to the olfactory bulb, where smells are processed, so its exact role is not clear.
- It may be vestigial (a remnant from evolution) or involved in pheromone detection. It could modulate olfactory input to make pheromones more noticeable.
- The nerve projects to brain areas involved in reproduction (the medial and lateral septal nuclei and the preoptic region).

Evidence and possible conditions
- Some studies show that severing the terminal nerve can reduce mating behavior in animals like hamsters.
- Changes in this nerve might be linked to Kallmann syndrome, a reproductive disorder.