Walsh & Hoyt: Types of Tear Secretion

Four different types of lacrimation can be identified in humans: (a) continuous tearing, produced constantly for protection and maintenance of a healthy corneal epithelium and a perfectly smooth and transparent corneal refractive surface; (b) reflex tearing, stimulated by exposure of the free nerve endings in the eye, nose, and face to light, cold, wind, foreign bodies, or irritating gases and liquids; (c) induced tearing, which often develops as an allergically or chemically mediated response to local irritants or by direct nonsynaptic parasympathomimetic action of some drugs on the cAMP-dependent signal transduction pathways in the secretory cells of the lacrimal glands; and (d) psychogenic tearing or tears of emotion, which are unique to humans. Young infants cry without shedding tears during the first days of life, and infants born prematurely may not shed tears for weeks. This delayed capacity for psychogenic weeping suggests that the connections within the CNS that indirectly innervate the lacrimal system are not fully developed in most newborns.