What is a sneeze?
A sneeze is a sudden and forceful expiration of air through the nose or sometimes partly through the mouth. Its function is to clear the nasal passages of any irritants. However, sneezing is a common feature of the various nasal disorders even though no irritant is present in the nasal passages. This is seen in upper respiratory tract infections and allergic rhinitis, where the lining of the nasal cavity is inflamed as a result of infectious or immune-mediated factors. Sneezing in these cases does little to clear out the nasal passages but may pass out excessive nasal mucus (rhinorrhea) which is a concomitant feature.
The sneeze reflex can be triggered by even mild stimulation of the the nasal lining. When stimulated, the receptors in the lining send impulses via the fifth cranial nerve (CN V / trigeminal nerve) to sneezing center in the medulla. This triggers the sneeze reflex which is similar to the cough reflex, with the difference being that air is forced out mainly through the nose by depressing the uvula.
Rapid inspiration fills the lung with extra air. The epiglottis and the vocal cords close tightly to build up pressure in the lungs. The expiratory muscles of respiration as well as accessory respiratory muscles contract forcefully. This further increases the pressure in the lungs. Once a sufficiently high pressure is reached, the vocal cords relax, the epiglottis opens and the air rushes out rapidly.
Due to the speed of the moving air, the terminal parts of the airway invaginates thereby creating slitswhich further increases the pressure of the expired air. The depressed uvula in a sneeze reflex pushes the air out though the nose. This entire process happens within seconds. Just as with a cough, the speed of the air rushing out through the nose can be close to 100mph.
Common stimulation and tightly shutting the eyes while sneezing, may cause the lacrimal glands (tear glands) to empty its contents thereby resulting in slightly watery eyes with sneezing. The irritation of the nasal passages triggers the inflammatory response which causes mucus hypersecretion. This excess mucus is also forced out during a sneeze in an aerosol fashion.
Sneeze and Light Reflex
Known as the photic sneeze reflex or ACHOO (autosomal dominant compelling helio-ophthalmic outburst), the sudden exposure to bright light may elicit a sneeze in some people. The exact cause is unknown although there is evidence that there is a familial tendency for this to occur. It may be related to local factors in the eye stimulating the same pathway that sends impulses back to the sneeze center via the trigeminal nerve.
It is believed that optic nerve stimulation due to exposure to sudden bright light triggers impulses in the trigeminal nerve via the opthalmic branch. This elicits the sneeze reflex.
The other theory is that this stems from the medulla as a crossing over of impulses from neighboring centers into the sneezing center. Snatiation reflex, where stretching of the stomach causes sneezing, and sexual excitement or an orgasm triggers sneezing may be due to central causes.