Tears (tear film) are a transparent fluid secreted primarily by thelacrimal glands (tear gland) found in theeyes of allland mammals.[1] According to the mode of production, tears are classified into four types: basal, closed eye, emotional, and reflex.[2] The basal rate of tear secretion is ~0.5–2.2 µL/min,[3] and irritation can increase secretion by up to ~100-fold, reaching ~300 µL/min.[4] Tears are made up of water, electrolytes, proteins, lipids, and mucins that form layers on the surface of eyes.[5] The four types of tears differ significantly in their composition.[5]
Anatomy of lachrymation, showing
a) Lacrimal gland
b) Superior lacrimal punctum
c) Superior lacrimal canal
d) Lacrimal sac
e) Inferior lacrimal punctum
f) Inferior lacrimal canal
g) Nasolacrimal canal
Some of the functions of tears include lubricating the eyes (basal tears), removing irritants (reflex tears), and also aiding theimmune system.[6] Tears also occur as a part of the body's natural pain response.[7] Emotional secretion of tears may serve a biological function by excreting stress-inducing hormones built up through times of emotional distress.[8][9] Tears havesymbolic significance among humans.[7]
Tear film was described in 1946 by Wolff using aslit lamp to have a three-layered structure: lipid, aqueous, and mucous.[10] Tears are composed ofwater,salts,antibodies, andlysozymes (antibacterial enzymes). The composition of each layer determines its function. For example, the diverse lipid classes in the tear filmlipid layer (TFLL) confer uniquephysicochemical properties that support roles such as enabling thin film formation and preventing its collapse onto theocular surface.[11] More recently, Mazyar Yazdani atOslo University Hospital proposed that the TFLL may also contribute to corneal oxygenation, based on its composition-driven properties.[12]
The composition varies among different tear types. The composition of tears caused by an emotional reaction differs from that of tears as a reaction to irritants, such as onion fumes, dust, or allergens. Emotional tears contain higher concentrations of stress hormones such asadrenocorticotropic hormone andleucine enkephalin (a natural pain killer), which suggests that emotional tears play a biological role in balancing stress hormone levels.[13]
The main lipid classes in whole human meibum: 1) Nonpolar lipids including wax esters (WE, 41%), cholesteryl esters (Chl-E, 31%), cholesteryl esters of (O-acyl)-ω-hydroxy fatty acids (Ch-OAHFA, 3%) and triacylglycerols (TAG, 1%); 2) Amphiphilic lipids consisting of (O-acyl)-ω-hydroxy fatty acids (OAHFA, 4%), cholesterol (Chl, 0.5%), free fatty acids (FFA, 0.1%), phospholipids (PL, 0.1%) and ceramides in various proportions (CER, 0.1%). An unknown fraction (19.2%) with nonpolar (e.g., diacylated α,ω-diols, diacylated a,b-diols and other more complex lipids), amphiphilic and non-lipid properties (e.g., denatured proteins, salts, etc.) has also been suggested.[14][12]
The main source isMeibomian glands (or tarsal glands). Other proposed sources with minor roles are the Harderian (especially in rabbits), Moll, and Zeiss glands.[12]
Diverse functions such as producing a smooth optical surface to improve the refraction of light, providing lubrication for blinks and eye movements, thickening the aqueous sub-phase due to Marangoni effect, sealing the lid margins during prolonged eye closure, providing resistance to evaporation, defending against the external environment (e.g., foreign particles and microbes),[15][16] enabling the formation of a thin film, preventing its collapse,[11] and involving in the oxygenation of the cornea.[12]
Thelacrimal glands secrete lacrimal fluid, which flows through the main excretory ducts into the space between the eyeball and the lids.[19] When the eyes blink, the lacrimal fluid is spread across the surface of the eye.[19] Lacrimal fluid gathers in thelacrimal lake which is found in the medial part of the eye. The lacrimal papilla is an elevation in the inner side of the eyelid, at the edge of the lacrimal lake.[19] The lacrimal canaliculi open into the papilla.[19] The opening of each canaliculus is the lacrimal punctum. From the punctum, tears will enter thelacrimal sac,[5] then on to thenasolacrimal duct, and finally into thenasal cavity.[19] An excess of tears, as caused by strongemotion, can cause the nose to run. Quality of vision is affected by the stability of the tear film.[20]
There are three basic types of tears: basal, reflex and emotional.[6]
Category
Description
Basal tears
In healthymammalian eyes, thecornea is continually kept wet and nourished bybasal tears. They lubricate the eye and help keep it clear ofdust. Tear fluid contains water,mucin,lipids,lysozyme,lactoferrin,lipocalin,lacritin,immunoglobulins,glucose,urea,sodium, andpotassium. Some of the substances in lacrimal fluid (such as lysozyme) fight againstbacterialinfection as a part of theimmune system. Lysozyme does this by dissolving a layer in the outer coating, called peptidoglycan, of certain bacteria. It is a typical body fluid with salt content similar to blood plasma. Usually, in a 24-hour period, 0.75 to 1.1 grams (0.03 to 0.04 oz) of tears are secreted; this rate slows with age.[5]
Reflex tears
The second type of tears results from irritation of the eye by foreign particles, or from the presence of irritant substances such asonion vapors,perfumes and other fragrances,tear gas, orpepper spray in the eye's environment, including the cornea, conjunctiva, or nasal mucosa, which triggerTRP channels in theophthalmic nerve.[citation needed] It can also occur with bright light and hot or peppery stimuli to the tongue and mouth. It is also linked with vomiting, coughing, and yawning.[5] Thesereflex tears attempt to wash out irritants that may have come into contact with the eye.
Emotional tears (psychic tears)
The third category, in general, referred to ascrying orweeping, is increased tearing due to strongemotional stress, pleasure (sometimes called 'tears of joy'),anger,suffering,mourning, orphysical pain. This practice is not restricted to negative emotions; many people cry when extremely happy, such as times of intense humor and laughter. In humans,emotional tears can be accompanied by reddening of the face and sobbing—cough-like, convulsive breathing, sometimes involving spasms of the whole upper body. Tears brought about by emotions have a different chemical makeup than those for lubrication; emotional tears contain more of the protein-based hormonesprolactin,adrenocorticotropic hormone, andLeu-enkephalin (a natural painkiller) than basal or reflex tears. Thelimbic system is involved in the production of basic emotional drives, such as anger, fear, etc. The limbic system, or, more specifically the hypothalamus, also has a degree of control over the autonomic system. Theparasympathetic branch of theautonomic nervous system controls the lacrimal glands via the neurotransmitteracetylcholine through both thenicotinic andmuscarinic receptors. When these receptors are activated, the lacrimal gland is stimulated to produce tears.[21]
Some mammals, such ascats,camels,polar bears,seals andaardvarks, have a full translucent third eyelid called anictitating membrane, while others have avestigial nictitating membrane.[22] The membrane works to protect and moisten the eyelid while maintaining visibility. It also contributes to the aqueous portion of the tear film and possibly immunoglobulins.[23] Humans and some primates have a much smaller nictitating membrane; this may be because they do not capture prey or root vegetation with their teeth, so that there is no evolutionary advantage of the third eyelid.[24]
A toddler producing tears due to emotional stress or pain
Thetrigeminal V1 (fifth cranial) nerve bears the sensory pathway of the tear reflexes. When the trigeminal nerve is cut, tears from reflexes will stop, while emotional tears will not. The great (superficial) petrosal nerve from cranial nerve VII provides autonomic innervation to the lacrimal gland.[25] It is responsible for the production of much of the aqueous portion of the tear film.
Crying boyQueenMaria II of Portugal shedding tears and hugging a bust of her late fatherKing Pedro IV (also Emperor of Brazil as Pedro I), 1836
In nearly all human cultures,crying is associated with tears, active tear ducts and abrupt strong respiration, due to strong emotional impetuses. Triggers of crying can vary from sadness andgrief to intense anger, happiness, fear, mirth,frustration, confusion, and any form of overwhelming stimuli. Emotional tears can also be triggered by social and personal experiences, like listening to music,[26] reading social media content, sharing thoughts, and communicating.
Crying is often associated with babies and children. The infants that are unable to vocally communicate have many alternating tones in their crying, attracting the attention of the caregiver and specifically their biological mothers.[27] Blood-related mothers go through physiological changes upon exposure to the crying, with a deceleration in heart rate, followed by a quick acceleration, as well as understanding the vocalizations of the baby's crying.[27] This is a mother-specific case, as the other caregivers, like biological father or adoptive parents, are not able to decode the sound.
Some cultures[which?] consider crying to be undignified and infantile, casting aspersions on those who cry in public settings, excluding circumstances which concerns loss of a relative or a loved one.[citation needed] In most Western cultures, it is more socially acceptable for women and children to cry than men, reflecting masculine sex-role stereotypes.[28] In some[which?]Latin regions, crying among men is more acceptable.[29][30][31] There is evidence for an interpersonal function of crying as tears express a need for help and foster willingness to help in an observer.[28]
Some modernpsychotherapy movements such asRe-evaluation Counseling encourage crying as beneficial to health and mental well-being.[32] An insincere display of grief or dishonest remorse is sometimes calledcrocodile tears in reference to an Ancient Greek anecdote that crocodiles would pretend to weep while luring or devouring their prey.[33] In addition, "crocodile tears syndrome" is a colloquialism forBogorad's syndrome, an uncommon consequence of recovery fromBell's palsy in which faulty regeneration of the facial nerve causes people to shed tears while eating.[34][35]
Bogorad's syndrome, also known as "crocodile tears syndrome", is an uncommon consequence of nerve regeneration subsequent toBell's palsy or other damage to thefacial nerve. Efferent fibers from thesuperior salivary nucleus become improperly connected to nerve axons projecting to thelacrimal glands, causing one to shed tears (lacrimate) on the side of the palsy during salivation while smelling foods or eating. It is presumed[by whom?] that this would cause salivation while crying due to the inverse improper connection of the lacrimal nucleus to the salivary glands, but this would be less noticeable.[34][35] The condition was first described in 1926 by its namesake, Russian neuropathologist F. A. Bogorad, in an article titled "Syndrome of the Crocodile Tears" (alternatively, "The Symptom of the Crocodile Tears") that argued the tears were caused by the act of salivation.[36][37]
Keratoconjunctivitis sicca, commonly known as dry eye, is a prevalent condition of the tear film. Despite the eyes being dry, those affected can still experience watering of the eyes, which is, in fact, a response to irritation caused by the original tear film deficiency. Lack ofMeibomian gland secretion can mean that the tears are not enveloped in a hydrophobic film coat, leading to tears spilling onto the face.
Treatment for dry eyes to compensate for the loss of tear film include eye-drops composed of methyl cellulose or carboxy- methyl cellulose or hemi-cellulose in strengths of either 0.5% or 1% depending upon the severity of drying up of the cornea.[citation needed]
Formeibomian gland dysfunction (MGD), one of the treatments is intense pulsed light (IPL). It is a therapeutic modality that was originally developed for dermatological applications and later adopted in ophthalmology.[38]
Familial dysautonomia is a genetic condition that can be associated with a lack of overflow tears (alacrima) during emotional crying.[39]
Obstruction of thepunctum,nasolacrimal canal, ornasolacrimal duct can cause even normal levels of the basal tear to overflow onto the face (epiphora), giving the appearance of constant psychic tearing. This can have significant social consequences.[citation needed]
Pseudobulbar affect (PBA) is a condition involving episodic uncontrollable laughter or crying. PBA mostly occurs in people with neurological injuries affecting how the brain controls emotions.[40] Scientists believe PBA results from prefrontal cortex damage.[41] PBA often involves crying. Hence, PBA is mistakable for depression. But PBA is neurological; depression is psychological.[42] Patients with PBA do not experience typical depression symptoms like sleep disturbances or appetite loss.
^Willcox, Mark D. P.; Argüeso, Pablo; Georgiev, Georgi A.; Holopainen, Juha M.; Laurie, Gordon W.; Millar, Tom J.; Papas, Eric B.; Rolland, Jannick P.; Schmidt, Tannin A.; Stahl, Ulrike; Suarez, Tatiana; Subbaraman, Lakshman N.; Uçakhan, Omür Ö.; Jones, Lyndon (2017-07-01)."TFOS DEWS II Tear Film Report".The Ocular Surface. TFOS International Dry Eye WorkShop (DEWS II).15 (3):366–403.doi:10.1016/j.jtos.2017.03.006.hdl:10138/297896.ISSN1542-0124.PMC6035753.PMID28736338.
^abFarandos, Nicholas M.; Yetisen, Ali K.; Monteiro, Michael J.; Lowe, Christopher R.; Yun, Seok Hyun (April 2015). "Contact Lens Sensors in Ocular Diagnostics".Advanced Healthcare Materials.4 (6):792–810.doi:10.1002/adhm.201400504.PMID25400274.
^Frey, William H.; Desota-Johnson, Denise; Hoffman, Carrie; McCall, John T. (October 1981). "Effect of Stimulus on the Chemical Composition of Human Tears".American Journal of Ophthalmology.92 (4):559–567.doi:10.1016/0002-9394(81)90651-6.PMID7294117.
^Wolff, E. (1946). "The muco-cutaneous junction of the lidmargin and the distribution of the tear fluid".Trans Ophthalmol Soc UK.66:291–308.
^Frey, William H.; Desota-Johnson, Denise; Hoffman, Carrie; McCall, John T. (October 1981). "Effect of Stimulus on the Chemical Composition of Human Tears".American Journal of Ophthalmology.92 (4):559–567.doi:10.1016/0002-9394(81)90651-6.PMID7294117.
^Barasa, A. (June 2003). "Morphologie et structure du cartilage de la membrane nictitante des Mammifères" [Morphology and structure of the nictitating membrane cartilage in mammals].Morphologie (in French).87 (277):5–12.PMID14717063.INIST15314125.
^Williams, D.L.; Miller, T.R. (2006). "Ophthalmology".The Equine Manual. pp. 1157–1210.doi:10.1016/B978-0-7020-2769-7.50024-1.ISBN978-0-7020-2769-7.The nictitating membrane, or third eyelid, serves to protect the globe, distribute tears, and produce immunoglobulins and part of the precorneal tear film. The caruncle is a small, finely haired prominence within the nasal canthus that may or may not be pigmented.
^Bogorad, F.A. (January 1979). "The symptom of crocodile tears".J Hist Med Allied Sci.34 (1). Translated by Seckersen, A.:74–9.doi:10.1093/jhmas/xxxiv.1.74.PMID381375.
^Russin LA (1939). "Paroxysmal Lacrimation During Eating as a Sequal of Facial Palysyndrome of Crocodile Tears".JAMA.113 (26):2310–1.doi:10.1001/jama.1939.72800510001009.
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