Sneezing

SNEEZING, expulsive thrust through the nose, caused reflexively when the sensory nerves of the nasal cavity are irritated. Irritants include foreign bodies, certain pungent substances (see Chemical Warfare Agents), accumulated mucus, etc. An act of S. is usually preceded by a tickling sensation in the nose, followed by a strong respiratory movement. When a brief pause occurs after inhalation, the nasopharyngeal part is isolated from the pharynx by the elevation of the soft palate, and the oral cavity is closed from the pharynx side by the contraction of the anterior pillars of the fauces and the apposition of the tongue to the hard palate; during the subsequent forceful exhalation, the soft palate suddenly opens, and air is driven into the nasal cavity under strong pressure. S. can be considered a protective reflex, by means of which the organism protects the respiratory passages from the penetration of any irritating substances. The most sensitive receptors that cause this reflex are located in the anterior part of the septum narium and in the area of the anterior ends of the upper and middle nasal conchae, from where the irritation is transmitted to the endings of the ethmoidal nerve of the trigeminal nerve, then passes through the Gasserian ganglion to the central nucleus of the trigeminal nerve, and finally to the respiratory center. The sneeze reflex is triggered in some individuals by irritation of the retina by bright light. The conducting pathways to the respiratory center through the same stations (Gasserian ganglion, trigeminal nucleus) are in this case the nasociliary nerves of the trigeminal nerve. The motor pathways of the reflex arc go to the expiratory muscles.