Cranial nerves

What is the structure and function of cranial nerve 0?

Answer: Cranial nerve 0, also called the terminal nerve (nervus terminalis) or cranial nerve XIII, is a thin plexus of unmyelinated fibers that is believed to play a role in sensing pheromones or some vestigial scent function in humans.

nervus terminalis cranial nerve 0

Classically, there are twelve nerves that exit from or enter directly into the brain or brain stem (as opposed to the spinal cord.) Because of their structure, these nerves are called the “cranial nerves.” They are responsible for both motor and sensory functions, and carry out a variety of behaviors including carrying smell (Cranial nerve I), visual (Cranial nerve II), and auditory information into the brain, as well as sending muscle control to the facial muscles (Cranial nerve VII) and autonomic functions like the parasympathetic nervous system (Cranial nerve X).

In the human brain, with careful dissection, it is possible to observe a very thin plexus of unmyelinated fibers that appears at the cribriform plate and runs into the brain posteriorly towards the olfactory gyrus and the septal nuclei. This bundle of nerves is referred to as Cranial nerve 0, or the terminal nerve. In most neuroanatomy textbooks, Cranial nerve 0 is not reported. This nerve runs alongside and very close to the olfactory nerve, or Cranial nerve I, however, it is not believed to be involved with olfactory sensory processing.

Currently, there are theories that Cranial nerve 0 is related to the detection of pheromones during sexual behavior. For one, the fibers terminate in the septal nuclei (medial olfactory area), which are brain structures that are related to reward and pleasure. For example, in the classic Olds and Milner paper with intracranial self stimulation, rats will repeatedly activate these septal regions of the brain. Also, in hamsters which have their terminal nerve severed, there is decreased mating behavior ("Terminal nerve damage impairs the mating behavior of the male hamster"). Furthermore, molecular studies have demonstrated that Cranial nerve 0 produces and releases gonadotropin-releasing hormone (The neglected cranial nerve: nervus terminalis (cranial nerve N)) which can trigger the release of luteinizing hormone from the anterior pituitary, another sign that the terminal nerve is related to sexual behavior.

Cranial nerve 0 was first identified in sharks by the German anatomist and physiologist Gustav Fritsch in 1878. It was first documented in humans in 1913, with rigorous scientific documentation in 1990 (Nervus terminalis (cranial nerve zero) in the adult human.)

The terminal nerve is believed to the be the nerve pathway that sends and receives information from the vomeronasal organ in other nonhuman species.