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Anatomy of lacrimation, 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

Tearing (/ˈtɪərɪŋ/), lacrimation or lachrymation (from Latin lacrima, meaning 'tear') is the secretion of tears, a body fluid which cleans and lubricates the eyes in response to irritation.[1] Tears formed through crying are associated with strong internal emotions. This includes sorrow, elation, love, awe, and pleasure.[2] Laughing or yawning can also cause tear production.

Contents

PhysiologyEdit

In humans, the tear film coating the eye, known as the precorneal film, has three distinct layers.[3] Having a thin tear film may hinder one's ability to wear contact lenses, as the volume of oxygen needed is higher than normal while contact lenses prevent oxygen from entering the eye. This will cause eyes with a thin tear film to dry out whilst wearing contact lenses. Special eye drops are available for contact lens wearers with a thin tear film and certain types of contact lenses (Silicone hydrogel lenses) are designed to allow more oxygen through to the eye.[4]

Name Container(s) Secretors Functions
Lipid layer Oils Meibomian glands (or tarsal glands) Coats the aqueous layer, provides a hydrophobic barrier that envelops tears and prevents their spilling onto the cheek. These glands are found among the tarsal plates. Thus, the tear fluid deposits between the eye proper and oil barriers of the lids.[5]
Aqueous layer Electrolytes, 60 metabolites—Amino Acids (1-Methylhistidine/3-Methylhistidine, Arginine, Asymmetric, Asymmetric dimethylarginine/Symmetric dimethylarginine, Citrulline, Creatine, Glutamine, Homoarginine, Hydroxyproline, Phenylalamine, Proline, Pyroglutamic acid, Serine, Taurine, Theonine, Tryptophan, Tyrosine, Urocanic acid, Valme), Amino Alcohols (Panthenoll); 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, Adenoisine, Cytidine, Guanosine, Inosine, S-Adenosyl-homocysteine, S-Adenosyl-methionine, Uridine, and Xamthosine), Nucleotides (ADP, AMO, CMP, Cytidine diphosphate choline, GMP, IMP, UDP, UMP, UDP-N-acetylgalactosamine/UDP-N-acetylglucosamine), Peptides (Oxidized glutathione), Phospholipids (1-Palmitoyl-lysophosphatidylcholime), Purines and derivatives (Hypoxanthine, Theobromine, Uric acid, Xanthine), Purines and derivatives (4-Pyridoxic acid), Quaternary Amines (Acetylcholine, Glycerolphosphocholine, Phosphocholine), and Tricarboxylic Acids (Citric acid)--and other substances such as proteins (e.g., antibodies,[3] lipocalin, lactoferrin, lysozyme,[6] and lacritin) Lacrimal gland Promotes spreading of the tear film, the control of infectious agents and osmotic regulation.
Mucous layer Mucins Conjunctival goblet cells Coats the cornea, provides a hydrophilic layer and allows for even distribution of the tear film.

Drainage of tear filmEdit

The lacrimal glands secrete lacrimal fluid, which flows through the main excretory ducts into the space between the eyeball and lids.[7] When the eyes blink, the lacrimal fluid is spread across the surface of the eye.[7] Lacrimal fluid gathers in the lacrimal 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.[7] The lacrimal canaliculi open into the papilla.[7] The opening of each canaliculus is the lacrimal punctum. From the punctum, tears will enter the lacrimal sac,[5] then on to the nasolacrimal duct, and finally into the nasal cavity.[7] An excess of tears, as caused by strong emotion, can cause the nose to run. [1]

TypesEdit

There are three very basic types of tears:[1]

Category Description
Basal tears In healthy mammalian eyes, the cornea is continually kept wet and nourished by basal tears. They lubricate the eye, and help to keep it clear of dust. Tear fluid contains water, mucin, lipids, lysozyme, lactoferrin, lipocalin, lacritin, immunoglobulins, glucose, urea, sodium, and potassium. Some of the substances in lacrimal fluid (such as lysozyme) fight against bacterial infection as a part of the immune system. Lysozyme does this by dissolving a layer in the outer coating, called peptidoglycan, of certain bacteria. It is a typical body fluid with a 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 is secreted; this rate slows with age.[5] In addition, the basal tears are composed of antioxidants such as Ascorbate, Urate, Cysteine, Glutathione, and Tyrosine. Ascorbate and Urate constitute half of the tears.
Reflex tears The second type of tears results from irritation of the eye by foreign particles, or from the presence of irritant substances such as onion vapors, perfumes and other fragrances, tear gas, or pepper spray in the eye's environment, including the cornea, conjunctiva, or nasal mucosa, which trigger TRP channels in the ophthalmic nerve.[8] 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] These reflex tears attempt to wash out irritants that may have come into contact with the eye.
Crying or weeping (psychic tears) The third category, in general, referred to as crying or weeping, is increased tearing due to strong emotional stress, pleasure, anger, suffering, mourning, or physical pain. This practice is not restricted to negative emotions; many people cry when extremely happy such as, times of intense humour 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 make-up than those for lubrication; emotional tears contain more of the protein-based hormones prolactin, adrenocorticotropic hormone, and Leu-enkephalin (a natural painkiller) than basal or reflex tears. The limbic system is involved in production of basic emotional drives, such as anger, fear, etc. The limbic system, to be specific, the hypothalamus, also has a degree of control over the autonomic system. The parasympathetic branch of the autonomic nervous system controls the lacrimal glands via the neurotransmitter acetylcholine through both the nicotinic and muscarinic receptors. When these receptors are activated, the lacrimal gland is stimulated to produce tears.[9]
 
A toddler producing tears due to emotional stress or pain

Neural aspectsEdit

The trigeminal 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 nott. The lacrimal nerve from cranial nerve VII innervates the lacrimal gland, which is located within the orbit. It is responsible for the production of much of the aqueous portion of the tear film. For this reason, if someone has signs of facial nerve problems, tear production should be checked. Likewise, application of cocaine to the surface of the eye, due to its paralyzing effect on the sensory nerve endings, inhibits the reflex even under exposure to strong tear gases. The motor pathway is autonomic (involuntary), and, in general, uses the pathway of the facial (seventh) nerve in the parasympathetic division. In parasympathetic imitators (such as acetylcholine), more tears are produced, and an anticholinergic drug like atropine inhibits tear production. A newborn infant has insufficient development of nervous control, so s/he "cries without weeping."[citation needed] If the lacrimal gland malfunctions or is damaged (e.g. by surgery), but does not cause any severe drying of the cornea, it is not a serious matter, for the accessory glands are enough for general secretion. In reflex situations, copious tears are produced prodominantly[weasel words] in emergencies.[5][better source needed]

Diseases and disordersEdit

Quality of vision is affected by the stability of the tear film.[10]

"Crocodile tears syndrome", also known as Bogorad's syndrome, is an uncommon consequence of nerve regeneration subsequent to Bell's palsy or other damage to the facial nerve in which efferent fibres from the superior salivary nucleus become improperly connected to nerve axons projecting to the lacrimal glands, causing one to shed tears (lacrimate) 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.[11][12]

Keratoconjunctivitis sicca, known as dry eye, is a very common disorder of the tear film. However, sufferers can 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 the tears are not enveloped in a hydrophobic film coat, leading to tears spilling onto the face.

Familial dysautonomia is a genetic condition that can be associated with a lack of overflow tears (alacrima) during emotional crying.[13]

Obstruction of the punctum, nasolacrimal canal, or nasolacrimal 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.

Pseudobulbar affect (PBA) is a condition that is characterised by episodes of uncontrollable laughter or crying. It mostly occurs in people suffering from certain neurological injuries which affect the way in which their brain controls emotions.[14] Scientists believe PBA may be a result of damage to the prefrontal cortex, which is the area of the brain that controls emotions.[15] Because PBA often involves crying, the condition may be mistaken for depression. However, PBA is a neurological condition, while depression is psychological.[16] People suffering from PBA do not experience some of the typical depression symptoms such as sleep disturbances or loss of appetite.

Social aspectsEdit

 
Crying boy

In nearly all cultures, crying is associated with tears trickling down the cheeks and accompanied by characteristic sobbing sounds. Emotional triggers are most often sadness and grief but crying can also be triggered by anger, happiness, fear, laughter or humor, frustration, remorse, or other strong, intense emotions. In many cultures[which?], crying is 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.[2] In some[which?] Latin regions, crying among men is more acceptable.[17][18][19] There is evidence for an interpersonal function of crying as tears express a need for help and foster willingness to help in an observer.[2]

Some modern therapy movements such as Re-evaluation Counseling teach that crying is beneficial to health and mental well-being, encouraging it positively.[20] An insincere display of grief or dishonest remorse is sometimes called crocodile tears in reference to an Ancient Greek anecdote that crocodiles would pretend to weep while luring or devouring their prey.[21] In addition, in medical terms, someone is said[according to whom?] to have crocodile tears syndrome as an uncommon consequence of recovery from Bell's palsy, in which faulty regeneration of the facial nerve causes sufferers to shed tears while eating.[citation needed]

Chemicals in tearsEdit

On a study conducted by the Weizmann Institute of Science in Rehovot, Israel, emotional tears from women have been found to reduce sexual arousal in men. Also, emotional tears are made up of a different chemical component than those evoked by eye irritants and can relay chemical messages to others.[22] The change in sex drive could be attributed to a drop in testosterone provoked by the tear chemicals, reducing aggression. In animals other than humans, it has been found that some blind mole rats rub tears all over their bodies as a strategy to keep aggressive mole rats away.[23]

Tear composition varies among different tear types. Mainly, tears are composed of water, salts, antibodies and lysozymes (antibacterial enzymes). According to a discovery by Dr William H. Frey II, a biochemist from St. Paul Ramsey Medical Center in Minnesota, the composition of tears caused by emotion differs from that of tears as a reaction to irritations, such as onion fumes, dust or allergy. Emotional tears are composed of more protein-based hormones, such as prolactin, adrenocorticotropic, and leucine enkephalin (a natural pain killer), which is suggested to be the mechanism behind the experience of crying from emotion making an individual feel better.[24]

See alsoEdit

ReferencesEdit

  1. ^ a b Farandos, 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. PMID 25400274.
  2. ^ a b c Stadel, M; Daniels, JK; Warrens, MJ; Jeronimus, BF (2019). "The gender-specific impact of emotional tears". Motivation and Emotion. 1 (1): 1–9. doi:10.1007/s11031-019-09771-z.
  3. ^ a b Moshirfar 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. PMC 4124072. PMID 25114502.CS1 maint: Uses authors parameter (link)
  4. ^ "Silicone Hydrogel Contact Lenses". All About Vision. Retrieved 2019-07-07.
  5. ^ a b c d e "eye, human."Encyclopædia Britannica from Encyclopædia Britannica 2006 Ultimate Reference Suite DVD 2009
  6. ^ "Ocular Pathology Study Guide: Tear Proteins". medrounds.org. Archived from the original on 2006-06-27.
  7. ^ a b c d e "Tears | Jul 02, 2018". www.targethealth.com. Retrieved 2019-07-07.
  8. ^ http://cdn.intechopen.com/pdfs/31117/InTech-Transient_receptor_potential_trp_channels_in_the_eye.pdf
  9. ^ Skorucak A. "The Science of Tears." ScienceIQ.com. Accessed September 29, 2006.
  10. ^ 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. PMID 16822077. Archived from the original on 2012-07-13.CS1 maint: Multiple names: authors list (link)
  11. ^ Morais Pérez D, Dalmau Galofre J, Bernat Gili A, Ayerbe Torrero V (1990). "[Crocodile tears syndrome]". Acta Otorrinolaringol Esp (in Spanish). 41 (3): 175–7. PMID 2261223.
  12. ^ McCoy, FJ; Goodman, RC (Jan 1979). "The crocodile tear syndrome". Plastic and Reconstructive Surgery. 63 (1): 58–62. doi:10.1097/00006534-197901000-00010. PMID 432324.
  13. ^ 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. PMC 2098750. PMID 17915006.
  14. ^ "Pseudobulbar affect - Symptoms and causes". Mayo Clinic. Retrieved 2019-06-18.
  15. ^ "Pseudobulbar Affect: What Is It?". WebMD. Retrieved 2019-06-18.
  16. ^ "About PBA". PBA Info. 2014-03-10. Retrieved 2019-06-18.
  17. ^ Dianne Hales (October 2005). "Big Boys Don't Cry — and Other Myths About Men and Their Emotions (page 2 of 3)". Reader's Digest. Retrieved 2008-07-20.
  18. ^ Fran Metcalf (8 May 2008). "These days it's OK for men to cry, say famous guys". The Courier Mail. Retrieved 3 June 2015.
  19. ^ John-Paul Flintoff (August 30, 2003). "Why we cry". The Age. Retrieved 2008-07-20.
  20. ^ Re-evaluation Counseling site: "The Recovery Process" Archived 2006-12-11 at the Wayback Machine
  21. ^ "No Faking It, Crocodile Tears Are Real". ScienceDaily. Retrieved 2019-06-18.
  22. ^ Weaver, Janelle. Crying Women Turn Men Off". Scientific American Mind, May/June 2011, p. 22, 6.
  23. ^ Dell'Amore, Christine. " Women's Tears Reduce Sex Drive in Men, Study Hints", "National Geographic, January 6, 2011, accessed June 6, 2011.
  24. ^ 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. PMID 7294117.

External linksEdit