The physiology behind a sneeze: (found in "The Physiological Handbook for Teachers of Yogasana" by Mel Robin. ISBN 1-58736-033-0)
"Should an allergen or a foreign particle of somt sort become lodged in the nose, then a reflexive sneeze may be in order to rid the body of this irritant. Irritants in the nasal tract signal their presence to the medulla which then acts to raise the volume of inspired air to 2-2.5 liters (*). This is the "aaahhh" of the "aaahhhchooo". With the lungs filled with air, the glottis closes, and then as it does so, the diameter of the trachea collapses. In order to "chooo", the medulla instructs the abdominal muscles and the inter costals (muscles between the ribs) to contract so as to compress the air in the lungs, the glottis opens rapidly and the compressed air is blown the the narrowed trachea at almost the speed of sound (Mach 0.85). During the nasal explosion, the eyes close to avoid being sprayed with germs and the muscles of the inner ear contract so as to lessen the impact of the loud noise on the auditory sensors. Strangely, there is a well-known photic-sneeze reflex, in which brief exposure to intense light will trigger a sneeze(*).
- K. J. Rose, The Body in Time, Wiley NY, 1988