Biomedical Engineering

Along with the flowers and sunshine of spring comes allergy season. Fortunately, I don't suffer from the itchy, watery eyes, and the spring time sneezing, but several of my friends and colleagues do. As I hear the familiar "AhChoos" and echoes of "God bless you" across the office, I wonder what exactly is a sneeze is and (keeping with the allergy theme), what is an itch? So welcome to the second medical mystery post! (Check out the first post on yawns

A Big Sneeze

A sneeze, also called a sternutation, is a normal, forceful way for your body to remove an irritant from your nose. The nasal passage is lined with sensitive tissue that can be irritated by dust, pollen, physical stimulation, or infection. Sneezing is a flex, so you don't have any control over it.

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Once the inside of your nose has been irritated, a message is sent to the part of your brain called the sneeze center, located on the brain stem. From there the body responds by activating your abdominal muscles, chest muscles, diaphragm, vocal cords, muscles in the back of your throat, and eyelids. (Fun fact: You always close your eyes when you sneeze. It's a subconscious defense mechanism to protect your eyes from debris). That's a lot of muscle activity that works together very precisely, to spew tiny particles and liquid out of your nose and mouth at 100 miles per hour at a range of up to a meter and a half. The liquid comes from the space between the lips and lower gums. A sneeze lasts 2-3 seconds.

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A pollutant in the air isn't the only thing that can make you sneeze. Many people (myself included) experience photic sneezing when exposed to bright light. Scientifically speaking, a photic sneeze is called an Autosomal Dominate Compelling Helio (sun)- Opthalmic (eyes) Outburst Syndrome, also known as ACHOO syndrome! Photic sneezing affects one in three people and is an inherited trait. There is also a rare psychogenic syndrome of intractable "pseudosneezing" related to psychological stress. Sneezing is also common when someone has a cold. The cold causing virus causes swelling and irritation in the nasal passage - a similar situation occurs for allergy sufferers.

Got the Itch

Allergies can also manifest themselves as an itchy spot. However, not all itches are related to allergies. Since the average human body is covered by about 20 square feet of skin, it's reasonable to assume that it will face a vast number of irritants on a daily bases. Itching can be caused by serious conditions such as severe allergies, disease, emotions, and infections.

The common itch, also known as pruritus, is caused by some type of external stimuli, including bugs, dust, clothing, fibers and hairs. An itch is a defense mechanism to an irritant. The irritant, let's say a bug, lands on your skin and begins to rub back and forth across your skin. The bug will eventually scratch the skins surface layer and irritate the receptors in the dermis. After a split second, the receptors send a signal through the fibers in the skin to your spinal cord and then up to the cerebral cortex in your brain and you scratch. Scratching is our way of getting rid of the irritant and relieving the itch. When the irritant is gone, the signal from your skin to your brain is interrupted and your brain doesn't recognize the irritant. If the irritant can't be removed, itching will at least cause enough pain to distract you from the original irritation. In 2008, a study observed participants scratching their lower leg in an MRI. Those performing the test noticed that areas of the brain associated with unpleasant or aversive emotions and memories became significantly less active during scratching. So it may be that scratching suppresses the emotional component of an itch.

Bringing it back to the dreaded allergy season, and summer time in general, itching can be caused by histamines which are chemicals produced in plant and animal tissues. Histamines are also produced in the human body and are released in reaction to certain substances in order to get rid of toxins. Histamines make blood vessels expand and cause the nasal swelling which causes sneezing. The chemicals are also called to action by irritants that come into contact with the skin, causing an itch to get rid of the foreign substance. ^{Image Credit: JustHow2.com}

As annoying as sneezing and itching can be (trust me, reading all the information on itching made my skin crawl), they are perfectly normal body defense mechanisms. It's almost impossible to stop a sneeze or resist the urge to scratch an itch, and you probably shouldn't. Both reactions help rid our body of irritants and keep us healthy (as long as you use a tissue when you sneeze!).

Resources

AHHH . . . CHOO!

What is a Sneeze http://www.weirdfacts.com/facts/3253-sneeze-facts.html#ixzz1rGPUVkba

When you have an itch what is happening under your skin?

Itching Causes & Treatments