Tears
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Tears are a clear liquid secreted by the lacrimal glands (tear gland) found in the eyes of all land 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]
The functions of tears include lubricating the eyes (basal tears), removing irritants (reflex tears), and also aiding the immune 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 have symbolic significance among humans.[4]
Physiology
Chemical composition
Tears are made up of three layers: lipid, aqueous, and mucous.
Name | Contents | 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 located among the tarsal 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] 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 film
The lacrimal glands secrete 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 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.[10] The lacrimal canaliculi open into the papilla.[10] The opening of each canaliculus is the lacrimal punctum. From the punctum, tears will enter the lacrimal sac,[2] then on to the nasolacrimal duct, and finally into the nasal cavity.[10] An excess of tears, as caused by strong emotion, can cause the nose to run. Quality of vision is affected by the stability of the tear film.[11]
Types
There are three basic types of tears: basal, reflex and emotional.[3]
Category | Description |
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Basal tears | In healthy 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 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]
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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 TRP channels in the ophthalmic 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] These reflex tears attempt to wash out irritants that may have come into contact with the eye.
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Emotional tears (psychic tears) | The third category, in general, referred to as crying or weeping, is increased tearing due to strong muscarinic receptors. When these receptors are activated, the lacrimal gland is stimulated to produce tears.[12]
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Nictitating membrane
Some mammals, such as cats, camels, polar bears, seals and aardvarks, have a full translucent third eyelid called a nictitating membrane, while others have a vestigial nictitating 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]
Neurology
The
Human culture
In nearly all human cultures,
Some modern
Pathology
Bogorad's syndrome
Bogorad's syndrome, also known as "Crocodile Tears Syndrome", is an uncommon consequence of nerve regeneration subsequent to
Keratoconjunctivitis sicca (dry eye)
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
Familial dysautonomia is a genetic condition that can be associated with a lack of overflow tears (Alacrima) during emotional crying.[28]
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.[citation needed]
Pseudobulbar affect
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
References
- ^ "Definition of TEAR". www.merriam-webster.com. Retrieved 2019-08-04.
- ^ Encyclopædia Britannica 2006 Ultimate Reference Suite DVD2009
- ^ S2CID 35508652.
- ^ a b "Why do we cry? The scientific reasons behind sobbing". The Independent. 2014-09-18. Archived from the original on 2022-05-26. Retrieved 2019-08-04.
- ^ "All About Emotional Tears". American Academy of Ophthalmology. 2017-02-28. Retrieved 2019-08-04.
- PMID 7294117.
- ^ PMID 25114502.
- PMID 7294117.
- ^ "Ocular Pathology Study Guide: Tear Proteins". medrounds.org. Archived from the original on 2006-06-27.
- ^ a b c d e "Tears". Target Health Blog. July 2, 2018. Retrieved 2019-07-07.
- S2CID 24410898.
- ^ Skorucak A. "The Science of Tears." ScienceIQ.com. Accessed September 29, 2006.
- PMID 14717063.
- ^ "Nictitating Membrane - an overview | ScienceDirect Topics". www.sciencedirect.com. Retrieved 2022-10-18.
- ^ "Why do cats have an inner eyelid as well as outer ones?". Scientific American. Retrieved 2022-10-18.
- PMID 19376264.
- PMC 5384201.
- ^ .
- ^ 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 from the original on January 17, 2009. Retrieved 2008-07-20.
- ^ Fran Metcalf (8 May 2008). "These days it's OK for men to cry, say famous guys". The Courier Mail. Retrieved 3 June 2015.
- ^ John-Paul Flintoff (August 30, 2003). "Why we cry". The Age. Retrieved 2008-07-20.
- ^ "Re-evaluation Counseling".
- ^ "No Faking It, Crocodile Tears Are Real". ScienceDaily. Retrieved 2019-06-18.
- ^ PMID 2261223.
- ^ S2CID 24362510.
- ^ F. A. Bogorad (trans Austin Seckersen), "The symptom of crocodile tears", Journal of the History of Medicine and Allied Sciences, 02/1979; 34(1):74-9.
- ^ Lester Allen Russin, "Paroxysmal Lacrimation During Eating as a Sequal of Facial Palysyndrome of Crocodile Tears", JAMA. 1939;113(26):2310-2311.
- PMID 17915006.
- ^ "Pseudobulbar affect - Symptoms and causes". Mayo Clinic. Retrieved 2019-06-18.
- ^ "Pseudobulbar Affect: What Is It?". WebMD. Retrieved 2019-06-18.
- ^ "About PBA". PBA Info. 2014-03-10. Retrieved 2019-06-18.
External links
- Nasolacrimal System Anatomy at eMedicine
- Brody, Jane E. (1984-02-22). "It's O.K. to cry". The New York Times. Personal Health. p. C10.