It is a reflex mechanism initiated by irritation of the nasal outer membrane (epithelium). The irritants stimulate pain fibers in the trigeminal nerve supplying the nasal epithelium, which transmit afferent nerve fibers (going towards) the medulla oblongata. Efferent (coming from) neve fibers from the medulla oblongata initiate a strong inspiratory effort where in about 2.5 litres of air is inspired. This is followed by strong contraction of the respiratory muscles (including abdominal muscles) building up a high intrapulmonary pressure of about 100 mm Hg. This leads to an explosive outflow of air at velocities reaching upto 965 km/hr.