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Saturday, June 5, 2010

A Primitive Cranial Nerve... Did you know about this?

I can't remember what I was looking for exactly when I stumbled upon a research paper for a cranial nerve that I had absolutely no idea about. For those of you who aren't that medically inclined, here's an intro.

Cranial nerves are nerves that arise from the encephalon. They are responsible for many reflexes and control of many automatic functions, such as gaze coordination, taste etc...
There are 12 cranial nerves (CN), numbered CN I-XII in the order that they enter or arise from the brain. If you're interested, here are the known Cranial Nerves:

CN I - Olfactory Nerve: olfaction, or sense of smell.
CN II - Optic Nerve: vision and visual acuity; afferent limb of the pupillary light reflex.
CN III - Occulomotor Nerve: eye movements, efferent limb of the pupillary light reflex.
CN IV - Trochlear Nerve: eye movements.
CN V - Trigeminal Nerve: sensation in the face, involved in mastication.
CN VI - Abducens Nerve: eye movements.
CN VII - Facial Nerve: muscles of facial expression as well as taste (Chorda Tympani).
CN VIII - Cochleo-Vestibular Nerve: hearing and balance.
CN IX - Glossopharyngeal Nerve: sensation from the pharynx, tonsils, and middle ear; taste.
CN X - Vagus Nerve: sensory fibers from organs in the body to the Central Nervous System. Autonomic organ control.
CN XI - Accessory Nerve: motor to Sternocleidomastoid (head movement) and Trapezius (Shoulder movement) muscles.
CN XII - Hypoglossal Nerve: motor innervation to the tongue.

Apparently there is a thirteenth nerve; CN 0, or the Terminal Nerve.

Projecting from the nasal cavity, it enters the brain slightly ahead of the other cranial nerves (hence the number zero). Being very close to CN I (Olfactory Nerve), CN 0 was often considered a branch of it during autopsies, which is why its existence as a separate cranial nerve was often dismissed. Also, given its very small size, very careful dissection is needed to identify it without severing it and losing it along with unwanted tissue.
Initially discovered in sharks, it was first identified during a human autopsy in 1913, when it was discovered that CN 0 did not connect to the olfactory bulb like CN I did, establishing that CN 0 was NOT a branch of CN I. It was therefore hypothesized to be a vestigial structure with no significant function. One other hypothesis held that CN 0 was responsible for sensing pheromones, and this was supported by the discovery that the nerve projected to areas in the brain responsible for the regulation of sexual behavior in mammals.

Controversy surrounding the existence of this nerve in humans is being settled; one study (published on MedLine) concluded that its presence in human beings is a common occurrence. We need more studies to ascertain whether it is functional!

I am still surprised that I didn't know about this. I hope i'm not the only one, and that this turns out to be an interesting read!

And cheers for my first nerdy post :)