Tears: Wikis

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The tear system. A) Tear gland / Lacrimal gland B) Superior lacrimal punctum C) Superior lacrimal canal lacrimation leads to tears D) Tear sac / Lacrimal sac E) Inferior lacrimal punctum F) Inferior lacrimal canal G) Nasolacrimal canal

Tears are the liquid product of a process of crying to clean and lubricate the eyes. The word lacrimation (from L. Lacrima meaning Tear) (also spelled lachrymation) may also be used in a medical or literary sense to refer to crying. Strong emotions, such as sorrow or elation, may lead to crying. The process of yawning may also result in lacrimation. Although most land mammals have a lacrimation system to keep their eyes moist and in response to other stimuli, humans are the only mammal generally accepted to cry emotional tears.[1]

Contents

Physiology

In humans, the tear film coating the eye, known as the precorneal film, has three distinct layers, from the most outer surface:

Name Container(s) Secretors Functions
Lipid layer oils meibomian glands (or tarsal glands) coats the aqueous layer; provides a hydrophobic barrier that retards evaporation and prevents tears 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.[2]
Aqueous layer water and other substances such as proteins (e.g. tear lipocalin, lactoferrin, lysozyme[3] and lacritin) lacrimal gland promotes spreading of the tear film; promotes the control of infectious agents; promotes osmotic regulation
Mucous layer mucin conjunctival goblet cells coats the cornea;provides a hydrophilic layer;allows for even distribution of the tear film; covers the cornea

Having a thin tear film may prevent one's ability to wear contact lenses as the amount of oxygen needed is higher than normal and contact lenses stop oxygen from entering the eye. Eyes with thin tear film will dry out while wearing contact lenses. Special eye drops are available for contact lens wearers. Certain types of contact lenses are designed to let more oxygen through to the eye.

Drainage of tear film

The lacrimal glands secrete lacrimal fluid which flows through the main excretory ducts into the space between the eyeball and lids. When the eyes blink, the lacrimal fluid is spread across the surface of the eye. Lacrimal fluid gathers in the lacrimal lake, and is drawn into the puncta by capillary action, then flows through the lacrimal canaliculi at the inner corner of the eyelids entering the lacrimal sac[2], then on to the nasolacrimal duct, and finally into the nasal cavity. An excess of tears, as with strong emotion, can thus cause the nose to run. [1]

Types

There are three very basic types of tears:

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 the outer coating 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 are secreted; this rate slows with age.
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, tear gas or pepper spray in the eye's environment, including the cornea, conjunctiva, or nasal mucosa. It can also occur with bright light and hot or peppery stimuli to the tongue and mouth. It is also linked with vomiting. 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, generally referred to as crying or weeping, is increased lacrimation due to strong emotional stress, suffering, mourning, or physical pain. This practice is not restricted to negative emotions; many people cry when extremely happy. 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 leucine 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, specifically the hypothalamus, also has a degree of control over the autonomic system. The parasympathetic branch of the autonomic 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.[4]
A toddler producing tears due to emotional stress or pain
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Neural Aspects

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, but not emotional tears. Likewise, application of cocaine to the surface of the eye inhibits the reflex even under exposure to strong tear gases. The motor pathway is autonomic (involuntary), and generally 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." Lest the cornea be damaged in surgery or other failure of lacrimal function occur, it is not a serious matter, for the accessory glands are enough for general secretion. In reflex situations, copious tears are produced mainly in emergencies.

Diseases and disorders

Quality of vision is affected by the stability of the tear film.[5]

"Crocodile tears syndrome" is an uncommon consequence of nerve regeneration subsequent to Bell's palsy or other damage to the facial nerve in which efferent fibers from the inferior salivary nucleus become improperly connected to nerve axons projecting to the lacrimal glands (tear ducts), causing one to shed tears (lacrimate) during salivation while smelling foods or eating. Presumably, one would also salivate while crying due to the inverse improper connection of the lacrimal nucleus to the salivary glands, but this would be less noticeable.[6]

Keratoconjunctivitis sicca, more commonly known as dry eye, is a very common disorder of the tear film. Paradoxically, sufferers can experience watering of the eyes which is in fact a response to irritation caused by the original tear film deficiency.

"Leamy Eye" is a condition whereby there is excessive watering of one eye, seemingly for no apparent reason, in response to environmental stimuli.

Familial dysautonomia is a genetic condition which can be associated with a lack of overflow tears (alacria) during emotional crying.[7]

Societal aspects

Most mammals will produce tears in response to extreme pain[citation needed] or other stimuli, but crying as an emotional reaction is considered by many to be a uniquely human phenomenon, possibly due to humans' advanced self-awareness. Some studies suggest that elephants, gorillas, and even camels may cry as well.[8]

In nearly all cultures, crying is seen as a specific act 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 strongly-experienced emotions. In many cultures, crying is associated with babies and children. Some cultures 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 cultures, it is more socially acceptable for women and children to cry than men. In some Latin regions crying among men is acceptable.[9][10][11]

Some modern therapy movements such as Re-evaluation Counseling teach that crying is beneficial to health and mental well-being, encouraging it positively.[12]

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.[13] Additionally, in medical terms, someone is said to have Crocodile tears syndrome as an uncommon consequence of recovery from Bell's palsy, where faulty regeneration of the facial nerve causes sufferers to shed tears while eating.

See also

References

  1. ^ "Are human beings the only animals that cry?" Yahoo! Answers. March 13, 2003
  2. ^ a b "eye, human."Encyclopædia Britannica from Encyclopædia Britannica 2006 Ultimate Reference Suite DVD 2009
  3. ^ Ocular Pathology Study Guide: Tear Proteins
  4. ^ Skorucak A. "The Science of Tears." ScienceIQ.com. Accessed September 29, 2006.
  5. ^ Szczesna DH, Jaronski J, Kasprzak HT, Stenevi U. "Interferometric measurements of dynamic changes of tear film." J Biomed Opt. 2006 May-Jun;11(3):34028. PMID 16822077.
  6. ^ [Crocodile tears syndrome] [Acta Otorrinolaringol Esp. 1990 May-Jun] - PubMed Result
  7. ^ * Felicia B Axelrod and Gabrielle Gold-von Simson (October 3, 2007). "Hereditary sensory and autonomic neuropathies: types II, III, and IV". Orphanet Journal of Rare Diseases 2 (39). doi:10.1186/1750-1172-2-39. PMID 17915006. PMC 2098750. http://www.ojrd.com/content/2/1/39. Retrieved 2008-11-04. 
  8. ^ Masson, Jeffrey Moussaieff, McCarthy, Susan, When Elephants Weep, Delta 1996 isbn: 978-0385314282
  9. ^ Dianne Hales (October 2005). "Big Boys Don't Cry -- and Other Myths About Men and Their Emotions (page 2 of 3)". Reader's Digest. http://www.rd.com/living-healthy/big-boys-dont-cry----and-other-myths-about-men-and-their-emotions/article18053-1.html. Retrieved 2008-07-20. 
  10. ^ Fran Metcalf (May 8, 2008). "These days it's OK for men to cry, say famous guys". The Courier Mail. http://www.news.com.au/couriermail/story/0,23739,23660131-23272,00.html. Retrieved 2008-07-20. 
  11. ^ John-Paul Flintoff (August 30, 2003). "Why we cry". The Age. http://www.theage.com.au/articles/2003/08/27/1061663846142.html. Retrieved 2008-07-20. 
  12. ^ Re-evaluation Counseling site: "The Recovery Process"
  13. ^ World Wide Words: Crocodile tears

External links


Source material

Up to date as of January 22, 2010

From Wikisource

Tears
disambiguation
This is a disambiguation page, which lists works which share the same title. If an article link referred you here, please consider editing it to point directly to the intended page.


Tears may refer to:


Simple English

Tears are a liquid made by the body to clean and lubricate the eyes.

There are three different types of tears that the body makes:

  • Basal tears: In a normal, healthy mammal, tears are regularly made to clean the cornea (front part of the eye) and to keep it clean from dust.
  • Reflex tears: This type of tear is made when something irritates the eye, say, bits of dust or the smell of an onion. These tears are made to clean the eye and wash out anything irritating it.
  • Crying tears (physic): The last category is called crying or weeping, when tears are made because of physical pain or strong emotions. Most mammals make this type of tear simply out of pain. Humans are the only known mammals to cry as an emotional reaction. Some studies suggest that elephants and gorillas may do this, too.

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