Tears are the liquid product of a process of crying to clean and lubricate the eyes Eyes are organs that detect light, and convert it to electro-chemical impulses in neurons. The simplest photoreceptors in conscious vision connect light to movement. In higher organisms complex neural pathways exist that connect the eye, via the optic nerve to the visual cortex and other areas of the brain. Complex optical systems with resolving. 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 Crying is shedding tears as a response to an emotional state in humans. The act of crying has been defined as "a complex secretomotor phenomenon characterized by the shedding of tears from the lacrimal apparatus, without any irritation of the ocular structures". The medical term for this is to lacrimate, which also refers to non-. Strong emotions, such as sorrow It is one of four interconnected sentiments in the system of Alexander Faulkner Shand, the others being fear, anger, and joy. In this system, when an impulsive tendency towards some important object is frustrated, the resultant sentiment is sorrow or elation Happiness is a state of mind or feeling characterized by contentment, love, satisfaction, pleasure, or joy. A variety of biological, psychological, religious, and philosophical approaches have striven to define happiness and identify its sources, may lead to crying. The process of yawning may also result in lacrimation.
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 Lipids are a broad group of naturally occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins , monoglycerides, diglycerides, phospholipids, and others. The main biological functions of lipids include energy storage, as structural components of cell membranes, and as important signaling molecules |
meibomian glands (or tarsal glands) |
coats the aqueous layer; provides a hydrophobic In chemistry, hydrophobicity is the physical property of a molecule (known as a hydrophobe) that is repelled from a mass of water barrier that evaporates 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.[1] |
| Aqueous layer |
water Water is a chemical substance with the chemical formula H2O. Its molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state, water vapor or steam and other substances such as proteins (e.g. tear lipocalin, lactoferrin Lactoferrin , also known as lactotransferrin (LTF), is a globular multifunctional protein with antimicrobial activity (bacteriocide, fungicide) and is part of the innate defense, mainly at mucoses. Lactoferrin is found in milk and many mucosal secretions such as tears and saliva. Lactoferrin is also present in secondary granules of PMN and also is, lysozyme Lysozyme, also known as muramidase or N-acetylmuramide glycanhydrolase, are glycoside hydrolases, enzymes that damage bacterial cell walls by catalyzing hydrolysis of 1,4-beta-linkages between N-acetylmuramic acid and N-acetyl-D-glucosamine residues in a peptidoglycan and between N-acetyl-D-glucosamine residues in chitodextrins. Lysozyme is[2] and lacritin Lacritin is a 12.3 kDa glycoprotein encoded in humans by the LACRT gene. Lacritin is a secreted protein found in tears and saliva. Lacritin also promotes tear secretion and proliferation of some epithelial cells. Lacritin is thus a prosecretory mitogen) |
lacrimal gland The lacrimal glands are paired almond-shaped glands, one for each eye, that secrete the aqueous layer of the tear film. They are situated in the upper, outer portion of each orbit. Inflammation of the lacrimal glands is called dacryoadenitis |
promotes spreading of the tear film; promotes the control of infectious agents; promotes osmotic regulation |
| Mucous layer |
mucin Mucins are a family of high molecular weight, heavily glycosylated proteins produced by epithelial tissues in most metazoans. Mucins' key characteristic is their ability to form gels; therefore they are a key component in most gel-like secretions, serving functions from lubrication to cell signalling to forming chemical barriers. They often take |
conjunctival goblet cells Goblet cells are glandular simple columnar epithelial cells whose sole function is to secrete mucus. They use both apocrine and merocrine methods for secretion |
coats the cornea The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is approximately 43 dioptres. While the cornea contributes most of the eye';provides a hydrophilic Hydrophiles, from the Greek "water" and φιλια (philia) "bonding," refers to a physical property of a molecule that can transiently bond with water (H2O) through hydrogen bonding. This is thermodynamically favorable, and makes these molecules soluble not only in water, but also in other polar solvents. There are 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 A contact lens is a corrective, cosmetic, or therapeutic lens usually placed on the cornea of the eye. Leonardo da Vinci is credited with describing and sketching the first ideas for contact lenses in 1508, but it was more than 300 years later before contact lenses were actually fabricated and worn on the eye. Modern soft contact lenses were as the amount of oxygen Oxygen (pronounced /ˈɒksɨdʒɨn/, OK-si-jin, from the Greek roots ὀξύς (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter), is the element with atomic number 8 and represented by the symbol O. It is a member of the chalcogen group on the periodic table, and is a highly 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 Eye drops are saline-containing drops used as a vector to administer medication in the eye. Depending on the condition being treated, they may contain steroids, antihistamines, sympathomimetics, beta receptor blockers, parasympathomimetics, parasympatholytics, prostaglandins, non-steroidal anti-inflammatory drugs or topical anesthetics. 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
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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 The lacrimal lake is the pool of tears in the lower conjunctival cul-de-sac, which drains into the opening of the tear drainage system . The volume of the lacrimal lake has been estimated to be between 7 and 10 µL, and is drawn into the puncta by capillary action, then flows through the lacrimal canaliculi The lacrimal canaliculi, also known as the lacrimal canals or lacrimal ducts, are the small channels in each eyelid that commence at minute orifices, termed puncta lacrimalia, on the summits of the papillae lacrimales, seen on the margins of the lids at the lateral extremity of the lacus lacrimalis at the inner corner of the eyelids entering the lacrimal sac The lacrimal sac is the upper dilated end of the nasolacrimal duct, and is lodged in a deep groove formed by the lacrimal bone and frontal process of the maxilla. It connects the lacrimal canaliculi, which drain tears from the eye's surface, and the nasolacrimal duct, which conveys this fluid into the nasal cavity[1], then on to the nasolacrimal duct The nasolacrimal duct carries tears from the lacrimal sac into the nasal cavity. Excess tears flow through nasolacrimal duct which opens in the nose. This is the reason the nose starts to run when a person is crying or has watery eyes from an allergy, and why one can sometimes taste eye drops, and finally into the nasal cavity The nasal cavity conditions the air to be received by the other areas of the respiratory tract. Owing to the large surface area provided by the conchae, the air passing through the nasal cavity is warmed or cooled to within 1 degree of body temperature. In addition, the air is humidified, and dust and other particulate matter is removed by. An excess of tears, as with strong emotion Emotion is the complex psychophysiological experience of an individual's state of mind as interacting with biochemical and environmental influences. In humans, emotion fundamentally involves "physiological arousal, expressive behaviors, and conscious experience". Emotion is associated with mood, temperament, personality and disposition,, can thus cause the nose to run. [1]
Types
There are three very basic types of tears:
| Category |
Description |
| Basal tears |
In healthy mammalian Mammals are a class of vertebrate, air-breathing animals whose females are characterized by the possession of mammary glands while both males and females are characterized by hair and/or fur, three middle ear bones used in hearing, and a neocortex region in the brain. Some mammals have sweat glands, but most do not eyes, the cornea The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is approximately 43 dioptres. While the cornea contributes most of the eye' is continually kept wet and nourished by basal tears. They lubricate the eye, and help to keep it clear of dust Dust is a general name for solid particles with diameters less than 20 thou . Particles in the atmosphere arise from various sources such as soil dust lifted up by wind, volcanic eruptions, and pollution. Dust in homes, offices, and other human environments contains small amounts of plant pollen, human and animal hairs, textile fibers, paper. Tear fluid contains water, mucin Mucins are a family of high molecular weight, heavily glycosylated proteins produced by epithelial tissues in most metazoans. Mucins' key characteristic is their ability to form gels; therefore they are a key component in most gel-like secretions, serving functions from lubrication to cell signalling to forming chemical barriers. They often take, lipids Lipids are a broad group of naturally occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins , monoglycerides, diglycerides, phospholipids, and others. The main biological functions of lipids include energy storage, as structural components of cell membranes, and as important signaling molecules, lysozyme Lysozyme, also known as muramidase or N-acetylmuramide glycanhydrolase, are glycoside hydrolases, enzymes that damage bacterial cell walls by catalyzing hydrolysis of 1,4-beta-linkages between N-acetylmuramic acid and N-acetyl-D-glucosamine residues in a peptidoglycan and between N-acetyl-D-glucosamine residues in chitodextrins. Lysozyme is, lactoferrin Lactoferrin , also known as lactotransferrin (LTF), is a globular multifunctional protein with antimicrobial activity (bacteriocide, fungicide) and is part of the innate defense, mainly at mucoses. Lactoferrin is found in milk and many mucosal secretions such as tears and saliva. Lactoferrin is also present in secondary granules of PMN and also is, lipocalin The lipocalins are a family of proteins which transport small hydrophobic molecules such as steroids, bilins, retinoids, and lipids. They share limited regions of sequence homology and a common tertiary structure architecture. This is an eight stranded antiparallel beta-barrel with a repeated + 1 topology enclosing an internal ligand binding site, lacritin Lacritin is a 12.3 kDa glycoprotein encoded in humans by the LACRT gene. Lacritin is a secreted protein found in tears and saliva. Lacritin also promotes tear secretion and proliferation of some epithelial cells. Lacritin is thus a prosecretory mitogen, immunoglobulins Antibodies are gamma globulin proteins that are found in blood or other bodily fluids of vertebrates, and are used by the immune system to identify and neutralize foreign objects, such as bacteria and viruses. They are typically made of basic structural units—each with two large heavy chains and two small light chains—to form, for example,, glucose Glucose , a simple sugar (monosaccharide), is an important carbohydrate in biology. Cells use it as a source of energy and a metabolic intermediate. Glucose is one of the main products of photosynthesis and starts cellular respiration. Starch and cellulose are polymers derived from the dehydration of glucose. The name "glucose" comes, urea Urea or carbamide is an organic compound with the chemical formula (N
, sodium Sodium is a metallic element with a symbol Na (from Latin natrium or Arabic ناترون natrun; perhaps ultimately from Egyptian netjerj) and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals within "group 1" (formerly known as ‘group IA’). It has only one stable isotope, 23Na, and potassium Potassium is the chemical element with the symbol K (Latin: kalium, from Arabic: القَلْيَه al-qalyah "plant ashes" cf. Alkali from the same root, more commonly known in Modern Standard Arabic as بوتاسيوم ‹bwtasywm›), atomic number 19, and atomic mass 39.0983. Potassium was first isolated from potash. Elemental. Some of the substances in lacrimal fluid (such as lysozyme) fight against bacterial The bacteria ( [bækˈtɪəriə] ; singular: bacterium)[α] are a large group of single-celled, prokaryote microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals. Bacteria are ubiquitous in every habitat on Earth, growing in soil, acidic hot springs, radioactive waste, infection An infection is the detrimental colonization of a host organism by a foreign species. In an infection, the infecting organism seeks to utilize the host's resources to multiply, usually at the expense of the host. The infecting organism, or pathogen, interferes with the normal functioning of the host and can lead to chronic wounds, gangrene, loss as a part of the immune system An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own healthy cells and tissues in order to function. 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 Onion is a term used for many plants in the genus Allium. They are known by the common name "onion" but, used without qualifiers, it usually refers to Allium cepa. Allium cepa is also known as the "garden onion" or "bulb" onion. Above ground, the onion shows only a single vertical shoot; the bulb grows underground, vapors, tear gas Tear gas, formally known as a lachrymatory agent or lachrymator , is a chemical compound that stimulates the corneal nerves in the eyes to cause tearing, pain, and even blindness. Common lachrymators include OC, CS, CR, CN, nonivamide, bromoacetone, phenacyl bromide, xylyl bromide and syn-propanethial-S-oxide (from onions). Lacrymators often share or pepper spray Pepper spray, also known as OC spray , OC gas, and capsicum spray, is a lachrymatory agent (a chemical compound that irritates the eyes to cause tears, pain, and even temporary blindness) that is used in riot control, crowd control, and personal self-defense, including defense against dogs and bears 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 Crying is shedding tears as a response to an emotional state in humans. The act of crying has been defined as "a complex secretomotor phenomenon characterized by the shedding of tears from the lacrimal apparatus, without any irritation of the ocular structures". The medical term for this is to lacrimate, which also refers to non- (psychic tears) |
The third category, generally referred to as crying or weeping, is increased lacrimation due to strong emotional stress Stress is a term in psychology and biology, first coined in the 1930s, which has in more recent decades become a commonplace of popular parlance. It refers to the consequence of the failure of an organism – human or animal – to respond appropriately to emotional or physical threats, whether actual or imagined, suffering Suffering, or pain in a broad sense, is an individual's basic affective experience of unpleasantness and aversion associated with harm or threat of harm. Suffering may be qualified as physical or mental. It may come in all degrees of intensity, from mild to intolerable. Factors of duration and frequency of occurrence usually compound that of, mourning Mourning is, in the simplest sense, synonymous with grief over the death of someone. The word is also used to describe a cultural complex of behaviours in which the bereaved participate or are expected to participate. Customs vary between different cultures and evolve over time, though many core behaviors remain constant, or physical pain Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage. It is the feeling common to such experiences as stubbing a toe, burning a finger, putting iodine on a cut, and bumping the "funny bone". 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 Prolactin or Luteotropic hormone (LTH) is a peptide hormone discovered by Dr. Henry Friesen, primarily associated with lactation. In breastfeeding, the act of an infant sucking the nipple stimulates the production of prolactin, which fills the breast with milk via a process called lactogenesis, in preparation for the next feed. Oxytocin, another, 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.[3] |
A toddler producing tears due to emotional stress or pain
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[citation needed]. 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
Main article:
List of eye diseases and disorders
Quality of vision is affected by the stability of the tear film.[4]
"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.[5]
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.[6]
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[who?] to be a uniquely human phenomenon, possibly due to humans' advanced self-awareness. However, some studies suggest that elephants, gorillas, and camels may cry.[7]
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.[8][9][10]
Some modern therapy movements such as Re-evaluation Counseling teach that crying is beneficial to health and mental well-being, encouraging it positively.[11] 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.[12] 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.
In fiction a single tear can mean an intense display of emotion, ranging from sad to nostalgiac. It is most commonly portrayed as a single tear traveling down the left cheek, as all singular tears originate in the left eye.[citation needed]
See also
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