Wednesday, December 17, 2014

Why Do Humans Have Uvula?

I believe many of us have experienced looking at ourselves in the mirror, with our mouth wide open, and wondered what's that dangling-testicle-lookalike-thing doing in there?

That dangling thing, my friend, is called the uvula.
So what exactly is the function of uvula?
The uvula, also known as palatine uvula, is interesting because humans are the only mammal that possess the strange-looking thing. It's small, seemingly unimportant, and often gets cut off (uvulectomy) in western medicine as part of the remedy for snoring. Okay don't freak out just yet; cutting off uvula won't impede speech.

The human uvula is made up of three main compartments: the surface epithelium, the subepithelial area and the area of the glands. Larger and thicker uvulas are related to obstructive sleep apnoea (OSA), and they sometimes collapse into the throat, narrowing the airway leading to breathing difficulty.
Throughout history many different functions and conditions have been attributed to the uvula. The prominent Greek physician Galen thought that the uvula was important for speech and was responsible for the beauty of the voice. The Bedouins of the South Sinai Desert believe that the uvula is the source of thirst, and that removing the uvula could reduce the need for water. In rural Nigeria, it is believed that an elongated uvula is the root cause of all throat problems.

Modern physicians, on the other hand, are more pragmatic in their approach. Some researchers believe that the uvula plays a prominent role in the production of some sounds present in French, Arabic, and some West African languages, and that uvulectomy causes the patients to stop producing these sounds. A group of researchers postulated that the uvula acts as a drain for the mucous secretion from the nasal cavities, directing its flow towards the base of our tongue.

Mukai et al state that the uvula plays an important role in eating and swallowing. He observed that after uvulectomy, patients sometimes burn their throats because even after sensing that the food was not too hot for the mouth it was often too hot for the throat.

The uvula is constantly exposed to airborne antigens. And researchers have found out that the uvula may play some role in local immune protection and be a site for induction of mucosal tolerance to inhaled and ingested antigens.

Traditional uvulectomy is still prevalent in the Middle East and many parts of Africa. The procedure—often done without anaesthetic and uses unsterile knifes and scissors—is performed by traditional healers, Arabic teachers and petty traders. Post-surgery complication may develop and can sometimes lead to death. In one study, 17.2% of 61 children under 15 who were admitted to hospital in Nigeria with complications of uvulectomy died.

Greek physicians in the 4th century BC like Aristotle and Hippocrates referred to the uvula as a source of inflammation. Another Greek physician, Paul of Aegina, recommended cutting the superfluous part of the uvula in cases of enlargement, when it resulted in coughing, inflammation, insomnia or risk of suffocation.

But if it's so unimportant and disease-inducing, why do we evolve it in the first place?

Some scientists believe that the uvula may be a philogenetic remnant from mammals that drink while bending their neck downwards, and may be another marker of human evolution that differentiates man from other animals. Another theory suggests that the uvula is a remnant of the velum, the soft palate that protects animal throats from ingesting insects while running. And the uvula is also said to secret saliva and keeping the pharynx moist and well lubricated. Scientists observed that one of the effects of uvula removal is pharyngeal dryness.

There is no clear answer for the exact function of uvula. It could have multiple functions, playing a role in speech and swallowing, lubrication of the throat and immunity. It seems awkward that humans are the only mammalian species that has it, as if evolution has granted us a special place in the animal kingdom by giving us a seemingly unassuming feature hidden at the back of our throat.

So the uvula will remain a mystery to us. It's ironic that the human race now have the knowledge and technological know-how to explore Mars, and can't solve the mystery of the humble uvula.

reference: G. W. Back et al, Clin. Otolaryngol. 2004, 29, 689-693

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