Wikipedia

Tears

(Redirected fromLacrimation)
For other uses, seeTears (disambiguation).

Tearsare a clear liquid secreted by thelacrimal glands(tear gland) found in theeyesof allland mammals.[1]Tears are made up of water, electrolytes, proteins, lipids, and mucins that form layers on the surface of eyes.[2]The different types of tears—basal, reflex, and emotional—vary significantly in composition.[2]

Human tears
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

The functions of tears include lubricating the eyes (basal tears), removing irritants (reflex tears), and also aiding theimmune system.[3]Tears also occur as a part of the body's natural pain response.[4]Emotional secretion of tears may serve a biological function by excreting stress-inducing hormones built up through times of emotional distress.[5][6]Tears havesymbolic significance among humans.[4]

Physiology

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Chemical composition

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Tears are made up of three layers: lipid, aqueous, and mucous.[7]Tears are composed ofwater,salts,antibodies,andlysozymes(antibacterial enzymes); though 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 hormoneandleucine enkephalin(a natural pain killer), which suggests that emotional tears play a biological role in balancing stress hormone levels.[8]

Name Contents Secretors Functions
Lipid layer Oils Meibomian glands(or tarsal glands) Coats the aqueous layer, provides ahydrophobicbarrier that envelops tears and prevents their spilling onto the cheek. These glands are located among thetarsal plates,and thus deposit the tear fluid between the eye proper and the oil barriers of the lids.[2]
Aqueous layer Electrolytes,60 metabolites—Amino Acids (1-Methylhistidine/3-Methylhistidine, Arginine, Asymmetric, Asymmetric dimethylarginine/Symmetric dimethylarginine, Citrulline, Creatine, Glutamine, Homoarginine, Hydroxyproline, Phenylalanine, Proline, Pyroglutamic acid, Serine, Taurine, Theonine, Tryptophan, Tyrosine, Urocanic acid, Valme), Amino Alcohols (Panthenol), Amino Ketones (Allantoin, Creatine), Aromatic Acids (Cinnamic acid, o-Coumaric acid/m-Coumaric acid/p-Coumaric acid), Carbohydrates (N-Acetylneuraminic acid), Carnitines (Acetylcarnitine, Carnitine, hexanoylcarnitine, Palmitoylcarnitine), Cyclic Amines (Niacinamide), Dicarboxylic Acids (Fumaric acid/ Maleic acid), Nucleosides (1-Methyladenosine, Adenosine, Cytidine, Guanosine, Inosine, S-Adenosyl-homocysteine, S-Adenosylmethionine, Uridine, and Xanthosine), Nucleotides (ADP, AMO, CMP, Cytidine diphosphate choline, GMP, IMP, UDP, UMP, UDP-N-acetylgalactosamine/UDP-N-acetylglucosamine), Peptides (Oxidized glutathione), Phospholipids (1-Palmitoyl-lysophosphatidylcholine), Purines and derivatives (Hypoxanthine, Theobromine, Uric acid, Xanthine), Purines and derivatives (4-Pyridoxic acid), Quaternary Amines (Acetylcholine, Glycerophosphocholine, Phosphocholine), and Tricarboxylic Acids (Citric acid), and other substances such as proteins (e.g.,antibodies,[7]lipocalin,lactoferrin,lysozyme,[9]andlacritin) Lacrimal gland Promotes spreading of the tear film, the control of infectious agents, and osmotic regulation.
Mucous layer Mucins Conjunctivalgoblet cells Coats thecornea,provides a hydrophilic layer and allows for even distribution of the tear film.

Drainage of tear film

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Thelacrimal glandssecrete lacrimal fluid, which flows through the main excretory ducts into the space between the eyeball and the lids.[10]When the eyes blink, the lacrimal fluid is spread across the surface of the eye.[10]Lacrimal fluid gathers in thelacrimal lakewhich 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.[10]The lacrimal canaliculi open into the papilla.[10]The opening of each canaliculus is the lacrimal punctum. From the punctum, tears will enter thelacrimal sac,[2]then on to thenasolacrimal duct,and finally into thenasal cavity.[10]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.[11]

Types

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There are three basic types of tears: basal, reflex and emotional.[3]

Category Description
Basal tears In healthymammalianeyes, thecorneais 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 againstbacterialinfectionas 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 (0.03–0.04-ounce avoirdupois) of tears are secreted; this rate slows with age.[2]
Reflex tears The second type of tears results from irritation of the eye by foreign particles, or from the presence of irritant substances such asonionvapors,perfumesand other fragrances,tear gas,orpepper sprayin the eye's environment, including the cornea, conjunctiva, or nasal mucosa, which triggerTRP channelsin 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.[2]Thesereflex tearsattempt to wash out irritants that may have come into contact with the eye.
Emotional tears (psychic tears) The third category, in general, referred to ascryingorweeping,is increased tearing due to strongemotional stress,pleasure,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 tearscan 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 systemis 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. Theparasympatheticbranch of theautonomic nervous systemcontrols the lacrimal glands via the neurotransmitteracetylcholinethrough both thenicotinicandmuscarinicreceptors. When these receptors are activated, the lacrimal gland is stimulated to produce tears.[12]

Nictitating membrane

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Some mammals, such ascats,camels,polar bears,sealsandaardvarks,have a full translucent third eyelid called anictitating membrane,while others have avestigialnictitating membrane.[13]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.[14]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.[15]

A toddler producing tears due to emotional stress or pain

Neurology

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ThetrigeminalV1(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.[16]It is responsible for the production of much of the aqueous portion of the tear film.

Human culture

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Crying boy
QueenMaria II of Portugalshedding tears and hugging a bust of her late fatherKing Pedro IV(also Emperor of Brazil as Pedro I), 1836

In nearly all human cultures,cryingis associated with tears trickling down the cheeks and accompanied by characteristic sobbing sounds. Emotional triggers are most oftensadnessandgrief,but crying can also be triggered byanger,happiness,fear,laughterorhumor,frustration,remorse,or other strong, intense emotions. Emotional tears can also be triggered by listening to music[17]or by reading, watching or listening to various forms of media. Crying is often associated with babies and children. Some cultures[which?]consider crying to be undignified and infantile, casting aspersions on those who cry publicly, except if it is due to the death of a close friend or relative. In most Western cultures, it is more socially acceptable for women and children to cry than men, reflecting masculine sex-role stereotypes.[18]In some[which?]Latin regions, crying among men is more acceptable.[19][20][21]There is evidence for an interpersonal function of crying as tears express a need for help and foster willingness to help in an observer.[18]

Some modernpsychotherapymovements such asRe-evaluation Counselingencourage crying as beneficial to health and mental well-being.[22]An insincere display of grief or dishonest remorse is sometimes calledcrocodile tearsin reference to an Ancient Greek anecdote that crocodiles would pretend to weep while luring or devouring their prey.[23]In addition, "crocodile tears syndrome" is a colloquialism forBogorad's syndrome,an uncommon consequence of recovery fromBell's palsyin which faulty regeneration of the facial nerve causes people to shed tears while eating.[24][25]

Pathology

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Bogorad's syndrome

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Bogorad's syndrome, also known as "Crocodile Tears Syndrome", is an uncommon consequence of nerve regeneration subsequent toBell's palsyor other damage to thefacial nerve.Efferent fibers from thesuperior salivary nucleusbecome 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.[24][25]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.[26][27]

Keratoconjunctivitis sicca (dry eye)

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Keratoconjunctivitis sicca,known in the vernacular as dry eye, is a very common disorder 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 of Meibomian 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]

Familial dysautonomia

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Familial dysautonomiais a genetic condition that can be associated with a lack of overflow tears (Alacrima) during emotional crying.[28]

Obstruction of thepunctum,nasolacrimal canal,ornasolacrimal ductcan 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

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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.[29]Scientists believe PBA results from prefrontal cortex damage.[30]PBA often involves crying. Hence, PBA is mistakable for depression. But PBA is neurological; depression is psychological.[31]Patients with PBA do not experience typical depression symptoms like sleep disturbances or appetite loss.

See also

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References

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  1. ^"Definition of TEAR".merriam-webster.Retrieved2019-08-04.
  2. ^abcdef"eye, human." Encyclopædia Britannica fromEncyclopædia Britannica 2006 Ultimate Reference Suite DVD2009
  3. ^abFarandos, NM; Yetisen, AK; Monteiro, MJ; Lowe, CR; Yun, SH (2014). "Contact Lens Sensors in Ocular Diagnostics".Advanced Healthcare Materials.4(6): 792–810.doi:10.1002/adhm.201400504.PMID25400274.S2CID35508652.
  4. ^ab"Why do we cry? The scientific reasons behind sobbing".The Independent.2014-09-18.Archivedfrom the original on 2022-05-26.Retrieved2019-08-04.
  5. ^"All About Emotional Tears".American Academy of Ophthalmology.2017-02-28.Retrieved2019-08-04.
  6. ^Frey, W. H.; DeSota-Johnson, D.; Hoffman, C.; McCall, J. 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.ISSN0002-9394.PMID7294117.
  7. ^abMoshirfar M, Pierson K, Hanamaikai K, Santiago-Caban L, Muthappan V, Passi SF (July 2014)."Artificial tears potpourri: a literature review".Clin Ophthalmol.8:1419–33.doi:10.2147/OPTH.S65263.PMC4124072.PMID25114502.
  8. ^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.
  9. ^"Ocular Pathology Study Guide: Tear Proteins".medrounds.org.Archived fromthe originalon 2006-06-27.
  10. ^abcde"Tears".Target Health Blog.July 2, 2018.Retrieved2019-07-07.
  11. ^Szczesna DH, Jaroński J, Kasprzak HT, Stenevi U (2006)."Interferometric measurements of dynamic changes of tear film".J Biomed Opt.11(3): 34028.Bibcode:2006JBO....11c4028S.doi:10.1117/1.2209881.PMID16822077.S2CID24410898.
  12. ^Skorucak A."The Science of Tears."ScienceIQ. Accessed September 29, 2006.
  13. ^Barasa, A. (2003-06-01)."Morphology and structure of the nictitating membrane cartilage in mammals".Morphologie.87(277): 5–12.ISSN1286-0115.PMID14717063.
  14. ^"Nictitating Membrane - an overview | ScienceDirect Topics".sciencedirect.Retrieved2022-10-18.
  15. ^"Why do cats have an inner eyelid as well as outer ones?".Scientific American.Retrieved2022-10-18.
  16. ^Dartt, D. A. (2009)."Neural Regulation of Lacrimal Gland Secretory Processes: Relevance in Dry Eye Diseases".Progress in Retinal and Eye Research.28(3): 155–177.doi:10.1016/j.preteyeres.2009.04.003.PMC3652637.PMID19376264.
  17. ^Mori, Kazuma; Iwanaga, Makoto (2017-04-07)."Two types of peak emotional responses to music: The psychophysiology of chills and tears".Scientific Reports.7(1): 46063.doi:10.1038/srep46063.ISSN2045-2322.PMC5384201.
  18. ^abStadel, M; Daniels, JK; Warrens, MJ; Jeronimus, BF (2019)."The gender-specific impact of emotional tears".Motivation and Emotion.1(1): 696–704.doi:10.1007/s11031-019-09771-z.
  19. ^Dianne Hales (October 2005)."Big Boys Don't Cry — and Other Myths About Men and Their Emotions (page 2 of 3)".Reader's Digest.Archived fromthe originalon January 17, 2009.Retrieved2008-07-20.
  20. ^Fran Metcalf (8 May 2008)."These days it's OK for men to cry, say famous guys".The Courier Mail.Retrieved3 June2015.
  21. ^John-Paul Flintoff (August 30, 2003)."Why we cry".The Age.Retrieved2008-07-20.
  22. ^"Re-evaluation Counseling".
  23. ^"No Faking It, Crocodile Tears Are Real".ScienceDaily.Retrieved2019-06-18.
  24. ^abMorais Pérez D, Dalmau Galofre J, Bernat Gili A, Ayerbe Torrero V (1990). "[Crocodile tears syndrome]".Acta Otorrinolaringol Esp(in Spanish).41(3): 175–7.PMID2261223.
  25. ^abMcCoy, FJ; Goodman, RC (Jan 1979). "The crocodile tear syndrome".Plastic and Reconstructive Surgery.63(1): 58–62.doi:10.1097/00006534-197901000-00010.PMID432324.S2CID24362510.
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  27. ^Lester Allen Russin, "Paroxysmal Lacrimation During Eating as a Sequal of Facial Palysyndrome of Crocodile Tears", JAMA. 1939;113(26):2310-2311.
  28. ^Felicia B Axelrod; Gabrielle Gold-von Simson (October 3, 2007)."Hereditary sensory and autonomic neuropathies: types II, III, and IV".Orphanet Journal of Rare Diseases.2(39): 39.doi:10.1186/1750-1172-2-39.PMC2098750.PMID17915006.
  29. ^"Pseudobulbar affect - Symptoms and causes".Mayo Clinic.Retrieved2019-06-18.
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  31. ^"About PBA".PBA Info.2014-03-10.Retrieved2019-06-18.
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