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Tears
Tears
Tears
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Human tears

Tears (tear film) are a transparent fluid secreted primarily by the lacrimal glands (tear gland) found in the eyes of all land mammals.[1] According to the mode of production, tears are classified into four types: basal, closed eye, emotional, and reflex.[2] The basal rate of tear secretion is ~0.5–2.2 µL/min,[3] and irritation can increase secretion by up to ~100-fold, reaching ~300 µL/min.[4] Tears are made up of water, electrolytes, proteins, lipids, and mucins that form layers on the surface of eyes.[5] The four types of tears differ significantly in their composition.[5]

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

Some of the functions of tears include lubricating the eyes (basal tears), removing irritants (reflex tears), and also aiding the immune system.[6] Tears also occur as a part of the body's natural pain response.[7] Emotional secretion of tears may serve a biological function by excreting stress-inducing hormones built up through times of emotional distress.[8][9] Tears have symbolic significance among humans.[7]

Physiology

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Chemical composition

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Tear film was described in 1946 by Wolff using a slit lamp to have a three-layered structure: lipid, aqueous, and mucous.[10] Tears are composed of water, salts, antibodies, and lysozymes (antibacterial enzymes). The composition of each layer determines its function. For example, the diverse lipid classes in the tear film lipid layer (TFLL) confer unique physicochemical properties that support roles such as enabling thin film formation and preventing its collapse onto the ocular surface.[11] More recently, Mazyar Yazdani at Oslo University Hospital proposed that the TFLL may also contribute to corneal oxygenation, based on its composition-driven properties.[12]

The 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 as adrenocorticotropic hormone and leucine enkephalin (a natural pain killer), which suggests that emotional tears play a biological role in balancing stress hormone levels.[13]

Name Composition Origin Functions
Lipid layer (TFLL) (with ~0.015–0.160 µm thickness) The main lipid classes in whole human meibum: 1) Nonpolar lipids including wax esters (WE, 41%), cholesteryl esters (Chl-E, 31%), cholesteryl esters of (O-acyl)-ω-hydroxy fatty acids (Ch-OAHFA, 3%) and triacylglycerols (TAG, 1%); 2) Amphiphilic lipids consisting of (O-acyl)-ω-hydroxy fatty acids (OAHFA, 4%), cholesterol (Chl, 0.5%), free fatty acids (FFA, 0.1%), phospholipids (PL, 0.1%) and ceramides in various proportions (CER, 0.1%). An unknown fraction (19.2%) with nonpolar (e.g., diacylated α,ω-diols, diacylated a,b-diols and other more complex lipids), amphiphilic and non-lipid properties (e.g., denatured proteins, salts, etc.) has also been suggested.[14][12] The main source is Meibomian glands (or tarsal glands). Other proposed sources with minor roles are the Harderian (especially in rabbits), Moll, and Zeiss glands.[12] Diverse functions such as producing a smooth optical surface to improve the refraction of light, providing lubrication for blinks and eye movements, thickening the aqueous sub-phase due to Marangoni effect, sealing the lid margins during prolonged eye closure, providing resistance to evaporation, defending against the external environment (e.g., foreign particles and microbes),[15][16] enabling the formation of a thin film, preventing its collapse,[11] and involving in the oxygenation of the cornea.[12]
Aqueous layer (with ~4 µm thickness) 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,[17] lipocalin, lactoferrin, lysozyme,[18] and lacritin) Lacrimal gland Promotes spreading of the tear film, the control of infectious agents, and osmotic regulation.
Mucous layer (with ~2.5–5 µm thickness) Mucins Conjunctival goblet cells Coats the cornea, provides a hydrophilic layer and allows for even distribution of the tear film.

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 the lids.[19] When the eyes blink, the lacrimal fluid is spread across the surface of the eye.[19] 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.[19] The lacrimal canaliculi open into the papilla.[19] 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.[19] 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.[20]

Types

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There are three basic types of tears: basal, reflex and emotional.[6]

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 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 salt content similar to blood plasma. Usually, in a 24-hour period, 0.75 to 1.1 grams (0.03 to 0.04 oz) of tears are secreted; this rate slows with age.[5]
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.[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.[5] These reflex tears attempt to wash out irritants that may have come into contact with the eye.
Emotional tears (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 humor 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 makeup 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 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. 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.[21]

Nictitating membrane

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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.[22] 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.[23] 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.[24]

A toddler producing tears due to emotional stress or pain

Neurology

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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 not. The great (superficial) petrosal nerve from cranial nerve VII provides autonomic innervation to the lacrimal gland.[25] It is responsible for the production of much of the aqueous portion of the tear film.

Human culture

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Crying boy
Queen Maria II of Portugal shedding tears and hugging a bust of her late father King Pedro IV (also Emperor of Brazil as Pedro I), 1836

In nearly all human cultures, crying is associated with tears, active tear ducts and abrupt strong respiration, due to strong emotional impetuses. Triggers of crying can vary from sadness and grief to intense anger, happiness, fear, mirth, frustration, confusion, and any form of overwhelming stimuli. Emotional tears can also be triggered by social and personal experiences, like listening to music,[26] reading social media content, sharing thoughts, and communicating.

Crying is often associated with babies and children. The infants that are unable to vocally communicate have many alternating tones in their crying, attracting the attention of the caregiver and specifically their biological mothers.[27] Blood-related mothers go through physiological changes upon exposure to the crying, with a deceleration in heart rate, followed by a quick acceleration, as well as understanding the vocalizations of the baby's crying.[27] This is a mother-specific case, as the other caregivers, like biological father or adoptive parents, are not able to decode the sound.

Some cultures[which?] consider crying to be undignified and infantile, casting aspersions on those who cry in public settings, excluding circumstances which concerns loss of a relative or a loved one.[citation needed] In most Western cultures, it is more socially acceptable for women and children to cry than men, reflecting masculine sex-role stereotypes.[28] In some[which?] Latin regions, crying among men is more acceptable.[29][30][31] There is evidence for an interpersonal function of crying as tears express a need for help and foster willingness to help in an observer.[28]

Some modern psychotherapy movements such as Re-evaluation Counseling encourage crying as beneficial to health and mental well-being.[32] 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.[33] In addition, "crocodile tears syndrome" is a colloquialism for Bogorad's syndrome, an uncommon consequence of recovery from Bell's palsy in which faulty regeneration of the facial nerve causes people to shed tears while eating.[34][35]

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 to Bell's palsy or other damage to the facial nerve. Efferent fibers from the superior salivary nucleus become improperly connected to nerve axons projecting to the lacrimal 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.[34][35] 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.[36][37]

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]

For meibomian gland dysfunction (MGD), one of the treatments is intense pulsed light (IPL). It is a therapeutic modality that was originally developed for dermatological applications and later adopted in ophthalmology.[38]

Familial dysautonomia

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Familial dysautonomia is a genetic condition that can be associated with a lack of overflow tears (alacrima) during emotional crying.[39]

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

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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.[40] Scientists believe PBA results from prefrontal cortex damage.[41] PBA often involves crying. Hence, PBA is mistakable for depression. But PBA is neurological; depression is psychological.[42] Patients with PBA do not experience typical depression symptoms like sleep disturbances or appetite loss.

See also

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Wikimedia Commons has media related to Tears.

References

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[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Tears

View on Grokipedia
from Grokipedia
Tears are the clear, saline fluid secreted by the lacrimal glands and accessory glands in the eyes, forming a thin tear film that coats the ocular surface to maintain hydration, protect against environmental threats, and facilitate clear vision. This film consists of three distinct layers: an inner layer produced by conjunctival goblet cells for ; a middle aqueous layer rich in water, electrolytes, proteins (such as and for antimicrobial activity), and metabolites; and an outer layer from meibomian glands that prevents evaporation and stabilizes the film. With a typical volume of 3-10 μL and a thickness of about 3 μm, tears are continuously produced at a rate of 1-2 μL per minute and spread across the with each blink before draining via the nasolacrimal ducts. Human tears are classified into three main types based on their production triggers and composition. Basal tears, secreted continuously, provide baseline lubrication, remove debris, deliver nutrients and oxygen to the avascular cornea, and offer innate immune defense through antimicrobial proteins like secretory IgA, lysozyme (at concentrations up to 2.5 mg/mL), and lactoferrin (1.5 mg/mL). Reflex tears are elicited by irritants such as smoke, onions, or foreign bodies via the trigeminal-parasympathetic reflex, producing a higher volume to flush the ocular surface and neutralize threats. Emotional tears, unique to humans and emerging in infants around 4-8 weeks of age, arise from strong affective states like sorrow, joy, or empathy, involving activation of the central autonomic network including the amygdala, hypothalamus, and periaqueductal gray; these tears differ biochemically, potentially containing higher levels of stress hormones like adrenocorticotropic hormone. Beyond ocular maintenance, tears play critical roles in broader and . The tear film contributes approximately 80% of the eye's refractive power for optical clarity, promotes via growth factors such as and transforming growth factor-β, and suppresses inflammation with factors like IL-1 receptor antagonist. Emotional , occurring on average 0-5 times per month in adults (more frequently in women), signals distress to elicit and support, enhances social bonding through pro-social responses, and may aid emotional regulation by releasing and oxytocin while reducing arousal. Disruptions in tear production or quality, as seen in dry eye disease, can lead to discomfort, , and increased risk, underscoring tears' essential protective functions.

Anatomy and Physiology

Tear Production Glands

The main lacrimal glands are paired exocrine structures responsible for the primary production of the aqueous component of tears. They are located in the superolateral aspect of the , within the lacrimal fossa of the , and are divided into a larger orbital lobe and a smaller palpebral lobe by the aponeurosis of the . These glands secrete a serous, watery fluid that is isotonic to plasma, primarily through acinar cells that form the bulk of the glandular . Accessory lacrimal glands, including the glands of Krause and Wolfring, supplement the main glands by providing continuous basal tear secretion. The glands of Krause, numbering about 20-40, are situated in the superior and inferior conjunctival fornices, while the glands of Wolfring are fewer and embedded along the superior tarsal plate near its nonmarginal border. These accessory structures contribute approximately 10% of the total lacrimal secretory output and secrete a watery, protein-rich fluid similar to that of the main glands. Meibomian glands, also known as tarsal glands, are sebaceous glands embedded within the tarsal plates of the upper and lower eyelids, with 20-40 glands in the upper lid and 20-30 in the lower. They secrete a lipid-rich substance called meibum, which forms the outermost oily layer of the tear film to prevent evaporation. Goblet cells are specialized unicellular glands dispersed throughout the non-keratinized of the , particularly concentrated in the fornices. These cells produce and secrete gel-forming s, such as MUC5AC, that constitute the innermost mucin layer of the tear film, aiding in its stability and adhesion to the ocular surface. The histological structure of the lacrimal glands features acinar units surrounded by myoepithelial cells, which contract to facilitate the expulsion of secretions into an interconnected ductal system. The main lacrimal glands possess 8-12 major excretory ducts that pierce the palpebral lobe and open into the superior conjunctival fornix, approximately 5 mm above the lateral tarsus. Myoepithelial cells, contractile in nature, envelop the acini and support the of fluid from secretory cells to the ducts. Innervation of the tear production glands primarily involves parasympathetic fibers from the (cranial nerve VII), which stimulate aqueous from the main and accessory lacrimal glands via the and . Sympathetic and sensory inputs from the (via the lacrimal nerve) also modulate glandular activity, though parasympathetic stimulation is the dominant driver for tear production. Together, these glands produce the basal and types of tears that assemble into the multilayered tear film.

Human tears consist primarily of , which accounts for approximately 98% of their volume and serves to hydrate the ocular surface. The electrolyte profile of tears includes sodium, , and ions at concentrations similar to those in , with typical values around 131 mM for sodium, 21 mM for , and 123 mM for , contributing to osmotic balance. Proteins represent a key component of tears, comprising about 6-11 mg/mL in total concentration. , an secreted by the lacrimal glands, constitutes 20-30% of the total tear proteins and functions by hydrolyzing bacterial cell walls. Other prominent proteins include , which sequesters iron to inhibit microbial growth; immunoglobulins such as secretory IgA, which provide adaptive immunity by neutralizing pathogens; and lipocalin, which facilitates the transport of and exhibits siderophore-binding antimicrobial activity. Lipids in tears, primarily derived from the meibomian glands, form a thin outer layer and include , wax esters, and other nonpolar molecules that prevent . A 2023 study by Yazdani proposed that the tear film lipid layer may also play a role in facilitating corneal oxygenation by allowing diffusion of atmospheric oxygen to the avascular . Mucins, particularly MUC5AC produced by conjunctival goblet cells, contribute to the mucin layer of tears, creating a gel-like structure that enhances adhesion to the ocular surface. Additional components in tears encompass glucose, , and various vitamins, such as , which are present in trace amounts and reflect systemic levels. The biochemical composition varies by tear type: basal tears maintain steady hydration with balanced electrolytes and proteins, reflex tears are enriched in factors like , and emotional tears exhibit higher overall protein concentrations, potentially including stress-related hormones. Tears maintain a neutral of approximately 7.4, ranging from 7.14 to 7.82, which supports the properties of components like and by optimizing their enzymatic activity.

Tear Film Layers and Drainage

The tear film covering the ocular surface consists of three distinct layers that together maintain ocular health by providing hydration, lubrication, and protection against environmental stressors. The innermost layer, secreted primarily by goblet cells in the and epithelial cells on the and , forms a thin gel-like foundation with a thickness of up to 0.5 μm; it facilitates adhesion of the tear film to the and stabilizes the overlying layers by reducing and promoting even spreading. The middle aqueous layer, the bulk of the tear film at approximately 3–5 μm thick, is composed of , electrolytes, proteins, and metabolites derived from the lacrimal glands; it delivers essential nutrients and oxygen to the avascular while flushing away debris and pathogens. Outermost is the layer, produced by the meibomian glands of the eyelids, which measures 15–157 nm (mean ~42 nm) in thickness and acts as a barrier to prevent excessive of the aqueous components, thereby preserving tear volume and film integrity. The stability of the tear film is crucial for preventing dry spots on the ocular surface, which can lead to discomfort and epithelial damage if the film breaks prematurely. In healthy eyes, the non-invasive tear breakup time (NIBUT) typically ranges from 4 to 19 seconds, allowing the film to remain intact between blinks. Blinking plays a pivotal role in renewing and redistributing the tear film, occurring at an average interval of 5–10 seconds during normal activities; each blink spreads the lipid layer across the aqueous surface at speeds up to 10 mm/s, reforms the mucin-aqueous interface, and mechanically removes debris while stimulating minor tear secretion to counteract thinning. Instability arises when factors disrupt layer interactions, such as inadequate lipid coverage, resulting in accelerated breakup and localized dry areas that compromise the film's protective function. Excess tears are drained from the ocular surface to prevent overflow and maintain balance, primarily through the lacrimal drainage system. Tears collect in the tear meniscus along the margins and enter the lacrimal puncta—small openings located at the medial canthi of the upper and lower s—before flowing into the canaliculi, which are short ducts that converge at the . From the , tears pass through the into the inferior nasal meatus of the , where they are either reabsorbed by the mucosa or evaporate further; this passive and blink-assisted process handles the majority of tear removal, with approximately 90% of volume reabsorbed nasally. For basal tears, the production and drainage flow rate is about 1 μL per minute per eye, ensuring steady-state hydration. accounts for 20–40% of total tear loss in normal conditions, varying with and , while the remainder is managed via drainage.

Types of Tears

Tears are classified into three primary types based on the stimuli that trigger their production and their distinct physiological roles: basal, , and emotional (also known as ) tears. These categories differ in rates, chemical compositions, and functions, with basal tears maintaining ongoing ocular , reflex tears providing rapid defense against irritants, and emotional tears responding to psychological states. Basal tears are continuously secreted at a low volume to lubricate the ocular surface, supply nutrients to the avascular and , and offer baseline protection against environmental debris and microbes. Their production rate typically ranges from 1 to 2 μL per minute, resulting in a total daily output of approximately 0.9 to 2.9 mL, which is evenly distributed across the eye via and drained through the nasolacrimal system. Compositionally, basal tears feature the highest concentrations of proteins and among tear types, including for iron-binding antimicrobial activity, lipocalin-1 for lipid transport, and for bacterial lysis, ensuring stable tear film integrity. Reflex tears are elicited by physical or chemical irritants, such as smoke, onions, or foreign bodies, activating sensory nerves to flush the ocular surface and dilute potential threats. This response is mediated primarily through the (cranial nerve V), which signals the to increase secretion dramatically, often up to 100-fold over basal levels, reaching rates of approximately 100 to 300 μL per minute. Reflex tears are enriched with antimicrobial proteins, including higher levels of , , and immunoglobulins like secretory IgA, compared to basal tears, to enhance clearance during acute exposure. Their osmolarity remains comparable to basal tears, typically around 300 to 310 mOsm/L, indicating no significant hypertonicity despite rapid production. Emotional tears arise in response to intense psychological stimuli, such as , , or , and are considered unique to humans among , distinguishing them from other vertebrates that lack this affective lacrimation. Production rates can mirror reflex tears, escalating to 100 to 300 μL per minute during episodes, though sustained may yield higher cumulative volumes over time. Unlike basal or reflex tears, emotional tears exhibit elevated protein content—up to several times higher—along with stress-related hormones such as and leucine enkephalin, potentially aiding in emotional regulation by excreting these compounds. This compositional variance contributes to their greater and slower compared to other tear types.

Nictitating Membrane

The , also known as the third eyelid, is a transparent or translucent conjunctival fold located at the medial of the eye in many vertebrates. It consists of a thin, fibrous stroma lined by on both its external (conjunctival) and internal (bulbar) surfaces, often supported by a T-shaped plate that facilitates its movement. This structure is prominently featured in birds, reptiles, and numerous mammals, including cats, dogs, and camels, where it folds into a pocket when not in use. In these animals, the primarily functions to sweep horizontally across the during , driven by the retractor bulbi oculi muscle and associated , which evenly distributes the over the ocular surface for and clearance of . This action protects the eye from environmental hazards such as , , or projectiles while maintaining clear vision, as the membrane's translucency allows transmission. Additionally, glands within the membrane, including lacrimal and Harderian glands, contribute to tear production, enhancing the precorneal tear film's immunologic and properties. In and birds, the membrane can fully cover the eye, providing robust protection during predatory strikes or high-velocity flight and aiding tear distribution in aqueous or arid environments. In humans, the equivalent structure is the vestigial plica semilunaris, a small, crescent-shaped fold of at the medial that represents a remnant of the fully developed seen in other vertebrates. Unlike its functional counterpart, the plica semilunaris does not actively sweep across the eye or significantly contribute to tear distribution, relying instead on the upper and lower eyelids for these roles; it may provide minor passive lubrication but lacks associated musculature for movement. This reduced form highlights evolutionary divergence in ocular anatomy. The 's presence in diverse species underscores its adaptive value for active lifestyles, enabling efficient tear management without compromising .

Functions

Protective and Lubricative Roles

Basal tears, produced continuously at a rate of approximately 1 μL per minute per eye, form the foundational layer responsible for the protective and lubricative functions of the tear film, ensuring ongoing ocular surface . These tears create a stable, multilayered film that coats the and , preventing and mechanical damage during everyday activities like blinking. The lubricative role of tears is primarily mediated by the mucin layer, which anchors to the corneal epithelium and reduces friction between the eyelid and ocular surface during blinks, thereby preventing corneal abrasion and maintaining smooth visual clarity. Mucins, including gel-forming types like MUC5AC secreted by conjunctival goblet cells, provide viscosity and anti-adhesive properties that facilitate shear-thinning behavior, allowing efficient spreading of the tear film without excessive resistance. This lubrication is essential for protecting the delicate epithelial cells from mechanical stress, with disruptions leading to conditions like dry eye syndrome. In terms of protection, tears serve as a mechanical and chemical barrier against environmental threats. The continuous flow of basal tears flushes irritants, debris, and pathogens from the ocular surface, aiding in the clearance of allergens and reducing infection risk. Antimicrobially, components such as hydrolyze β-1,4-glycosidic bonds in the of bacterial cell walls, particularly targeting and contributing to innate immune defense. Additionally, tear buffering systems, primarily involving , maintain ocular around 7.4 by neutralizing acids and bases, with more effective responses to acidic challenges to preserve epithelial integrity. The lipid layer, derived from meibomian glands, forms an outermost barrier that minimizes of the aqueous component, preserving corneal hydration and preventing hyperosmolarity that could damage epithelial cells. This evaporation resistance is achieved through the amphiphilic nature of lipids like and fatty acids, which create a compact interface at the air-tear boundary. Furthermore, tears deliver vital nutrients to the avascular , including dissolved oxygen from atmospheric exposure and glucose for metabolic support, ensuring epithelial viability without vascular supply. Tears also support wound healing by containing growth factors such as epidermal growth factor (EGF), which stimulates epithelial cell proliferation and migration to repair corneal abrasions. EGF, present in tear fluid at concentrations around 0.1–10 ng/mL, activates receptor signaling pathways that accelerate re-epithelialization, often in synergy with other factors like transforming growth factor-α. This reparative function underscores tears' role in maintaining long-term ocular surface homeostasis.

Emotional and Communicative Roles

Emotional tears, distinct from basal or reflex tears, play a significant role in emotional regulation by facilitating and reducing physiological stress. According to catharsis theory, allows for the release of pent-up emotional tension, leading to mood improvement in approximately 30% of episodes, particularly when the underlying issue is resolved. This process is supported by evidence that emotional activates the , promoting self-soothing through vagal rebound and potential oxytocin release, which enhances feelings of attachment and security. Furthermore, emotional tears contain higher concentrations of stress-related proteins and hormones, such as and leucine-enkephalin, compared to other tear types, suggesting a mechanism for excreting toxins accumulated during distress. In social contexts, tears serve as a potent signal of , universally eliciting and intentions to provide support across cultures. A large-scale study involving 7,007 participants from 41 countries found that visible emotional tears increase observers' prosocial intentions by a moderate (d = 0.49), mediated by empathic concern rather than personal distress, with stronger effects in higher societies. This signaling function promotes social bonding, as tears handicap vision and convey submission or need, discouraging and fostering affiliation. In infants, emotional tears emerge around 1-2 months and act as visual distress signals that amplify caregiving responses; shows that infant tearful faces activate somatosensory regions and mentalizing areas in observers' brains more intensely than adult tears, prompting protective behaviors to alleviate the child's distress. Among adults, tears facilitate by soliciting help and , as observers report greater willingness to offer emotional support or mediate disputes when tears are present, enhancing relational repair. Gender differences influence crying frequency, with women experiencing emotional 30-64 times per year compared to 6-17 times for men, a pattern observed consistently across studies. This disparity may stem from hormonal factors, including higher levels in women that promote crying proneness and lower testosterone in men that inhibits it. As non-verbal cues, tears amplify the perceived intensity of facial expressions, drawing visual attention to the eyes and cheeks while heightening attributions of and emotional depth. This enhancement engages empathy-related circuits, such as those involved in mentalizing, making tearful expressions more compelling for interpersonal understanding.

Neurology

Reflex Pathways

The reflex pathways governing involuntary tear production, known as reflex tears, involve a well-defined that responds rapidly to ocular irritants. The afferent limb of this pathway is mediated by the ophthalmic division (V1) of the (cranial nerve V), which innervates sensory receptors in the and to detect mechanical, chemical, or thermal irritants. These sensory fibers, primarily nasociliary branches, transmit signals to the , where they synapse in the principal sensory nucleus or descend to the for processing of pain and irritation. The integrates these inputs, facilitating the coordination of protective responses including lacrimation. The efferent limb is predominantly parasympathetic, originating from the superior salivatory nucleus in the , which sends preganglionic fibers via the (cranial nerve VII) and the to the pterygopalatine (sphenopalatine) . Postganglionic parasympathetic fibers then travel along the zygomatic and lacrimal nerves to innervate the , stimulating aqueous tear secretion through mechanisms. This parasympathetic activation forms the core of the reflex arc, which operates through brainstem structures in the and medulla, enabling a short latency response on the order of milliseconds to protect the ocular surface from irritants. Sympathetic modulation plays a minor role in this pathway, with fibers from the releasing norepinephrine to influence blood flow and potentially basal secretion, though it does not drive the primary response. In contrast to emotional tearing, which involves higher cortical integration, the pathway remains a peripheral, brainstem-mediated circuit focused on immediate ocular defense.

Emotional Mechanisms

Emotional tearing is primarily regulated by the , where the processes emotional stimuli such as distress or , subsequently signaling the to orchestrate an autonomic response that culminates in activation. This pathway integrates higher-order emotional appraisal with visceral motor control through the central autonomic network, distinguishing emotional tears from irritant-induced ones. The serves as a key integrator, activating parasympathetic outflow via the superior salivatory nucleus and the (cranial nerve VII), which innervates the lacrimal glands to promote tear secretion; this contrasts with the sensory-driven trigeminal pathway for tears. , released from parasympathetic nerve endings, binds to muscarinic receptors on lacrimal acinar cells to stimulate fluid secretion, forming the neurochemical basis for this response. The receives inputs from the and , integrating emotional and autonomic signals to modulate tear production. Cortical regions further modulate emotional tearing: the medial shows increased activity during , potentially involved in regulating emotional expression, while the insula, linked to via spindle neurons, contributes to the emotional resonance that may trigger tears. Endogenous opioids contribute to the emotional context by dampening distress signals, potentially providing through their anti-nociceptive effects during prolonged episodes. Functional MRI studies since 2015 have revealed activation during emotional processing of stimuli, underscoring its role in evaluating the affective salience of tear-inducing events. Gender differences in hypothalamic responses to negative emotional stimuli show greater activation in women, which may underlie observed variations in frequency and intensity.

Evolutionary Aspects

Origins in Vertebrates

In the earliest vertebrates, such as fish and amphibians, eye protection relies on simple mucous secretions rather than specialized tear glands. Aquatic environments provide constant hydration, so fish produce a thin mucus layer from goblet cells in the conjunctiva and corneal epithelium to shield the eye from pathogens, debris, and osmotic stress, without dedicated lacrimal structures. This mucous film is analogous to early forms of ocular lubrication, evolving as part of the vertebrate eye's adaptation to submersion, where water itself aids in maintaining corneal integrity. In amphibians, which bridge aquatic and terrestrial life, similar mucous secretions persist, supplemented by rudimentary orbital glands that secrete viscous fluids to prevent desiccation during brief land excursions. The transition to fully terrestrial life in tetrapods, beginning around 375 million years ago during the period, marked the emergence of more complex tear production mechanisms, including the development of lacrimal glands approximately 300 million years ago in early amniotes. These glands arose as an evolutionary innovation to lubricate and protect the exposed in dry air, producing an aqueous tear film that spreads via —a behavior that originated concurrently with the water-to-land shift. In early tetrapods like , osteological evidence suggests the presence of retractor bulbi muscles facilitating eye retraction and rudimentary wetting, setting the stage for glandular elaboration in subsequent lineages. Mammalian adaptations further refined tear production for arid environments, with enhanced lacrimal glands providing increased aqueous to combat evaporation and maintain ocular . Non-primate mammals retain a , a vestigial third that aids in distributing tears efficiently across the . Comparatively, birds and reptiles rely heavily on the for lipid-rich secretions that stabilize the tear film, a structure conserved across sauropsids and linked to their ectothermic lifestyles and diverse habitats. This glandular specialization reflects phylogenetic divergence, yet both groups share conserved parasympathetic innervation via the for reflex tear , underscoring a deep homology in neural control. Fossil evidence for tear drainage structures is inferred from skull anatomy in early synapsids, the mammalian lineage's precursors, dating back to the Late Carboniferous around 300 million years ago. In pelycosaur-grade synapsids like , the forms a for the , indicating an early apparatus for channeling secretions away from the eye to prevent overflow. Later cynodonts, such as Riograndia from the (~225 million years ago), preserve clear openings for the lacrimal duct bordered by the and lacrimal bones, suggesting functional tear drainage adapted for terrestrial vision. These features highlight the gradual refinement of the lacrimal system in synapsids amid increasing during the Permian.

Development of Psychic Tears

Psychic tears, also known as emotional or basal tears triggered by psychological stimuli, represent a uniquely in the of , emerging as visible lacrimation accompanying distress vocalizations. While the order originated approximately 60 million years ago, fostering advanced social bonding through grooming and vocal signals, the production of emotional tears appears absent in non- and most mammals. This innovation likely coincided with the development of more complex emotions in early around 2 million years ago, enhancing non-aggressive signaling of vulnerability in increasingly cooperative groups. In human infants, emotional crying begins as primarily vocal distress calls to solicit caregiving, with visible tears developing several months after birth as lacrimal glands mature, tying into attachment behaviors that ensure survival. This progression reflects an evolutionary refinement where tears amplify the signal of need without the risks of aggressive displays, promoting and group cohesion as per , which posits as an innate appeal for proximity to caregivers. Comparative studies show that while chimpanzees exhibit proto-emotional responses through intense vocalizations during separation or , they do not produce accompanying tears, relying instead on auditory and facial cues for social bonding. Similarly, dogs demonstrate oxytocin-mediated increases in tear volume during joyful reunions with owners, suggesting an emotional component, but this lacks the full multimodal integration of sobbing and tears seen in humans. display profound distress and social mourning behaviors, yet lack evidence of emotional tearing, with any ocular fluid stemming from non-emotional sources like temporal glands. The genetic underpinnings of psychic tears involve conserved mechanisms for lacrimal gland development across vertebrates, notably the FOXC1 , which regulates glandular outgrowth and branching essential for tear production. However, the specific emotional regulation of tearing is modulated by neuropeptides such as oxytocin and endogenous opioids, which facilitate attachment and stress . Psychic tears are distinguished from basal or reflex types by their higher concentrations of proteins and certain hormones, such as (ACTH) and , as well as neuropeptides like . This dual basis underscores the adaptive value of psychic tears in , enabling nuanced communication that fosters and social support without verbal .

Cultural and Social Aspects

Symbolism in Art and Literature

In religious texts, particularly the , tears serve as a profound symbol of and spiritual purification. The frequently depict tears as an offering to , as in Psalm 56:8, where they are collected in a divine "bottle," signifying divine attentiveness to human sorrow and the potential for redemption through . This imagery extends to purification, where weeping represents a cleansing of the soul from , akin to a second that enables spiritual enlightenment and union with the divine. In literature, tears amplify pathos in Shakespearean tragedies, evoking audience empathy through characters' raw vulnerability. In King Lear, Cordelia's tears during her reunion with her father, King Lear, underscore forgiveness and filial devotion, heightening the emotional stakes and contributing to the play's tragic catharsis by contrasting innocence against familial betrayal. Similarly, the Romantic era elevated crying as a cathartic release, with poets like Percy Bysshe Shelley portraying tears as a transformative emotional outlet that resolves inner turmoil and fosters renewal, as seen in the redemptive weeping in Prometheus Unbound. Visual art has long employed tears to convey profound sorrow and human frailty. During the , Michelangelo's (1498–1499) captures maternal grief through the Virgin Mary's serene yet anguished expression as she cradles Christ's body, symbolizing universal compassion and the quiet endurance of loss without explicit tears but implying an inner well of sorrow. In modern , Frida integrated tears into her self-portraits to externalize emotional devastation; in (1944), tears stream down her face amid a shattered spine and piercing nails, representing unyielding physical and psychological pain while asserting defiant resilience. In and media, shots of tears powerfully evoke viewer by mimicking facial expressions that trigger responses. Directors like in Schindler's List (1993) use such close-ups to immerse audiences in characters' grief, fostering shared emotional processing as theorized in neurocinematic studies. By the , postmodern works shifted toward ironic tears, subverting traditional ; in films like David Lynch's Blue Velvet (1986), crying becomes a detached, absurd spectacle that critiques emotional authenticity in a fragmented society. Gender tropes in literature often associate tears with femininity, reinforcing Victorian ideals of women as emotionally expressive yet fragile. In sensation novels like Wilkie Collins's The Woman in White (1859), female characters' copious weeping highlights their vulnerability and moral purity, aligning tears with domestic sentimentality and social constraints on women. Feminist literature challenges this by depicting tears as a universal human response unbound by gender, as in Virginia Woolf's Mrs. Dalloway (1925), where male and female characters alike cry to confront societal pressures, dismantling stereotypes of feminine hysteria.

Cross-Cultural Perspectives

In Western societies, emotional crying is generally more socially acceptable for women and children than for men, where it has historically been suppressed due to norms associating tears with weakness. Post-2000 studies indicate a gradual increase in male openness to crying, particularly in contexts of gender equality and emotional vulnerability, reflecting shifts toward greater acceptance in individualistic cultures like the United States and Europe. As of 2024, surveys and cultural analyses indicate growing acceptance of male crying in Western societies, with public figures and media portraying it as a sign of emotional health. In collectivist cultures such as and , tears often signify a disruption to social harmony, leading to norms that discourage public to maintain group cohesion and emotional restraint. Public displays of tears are rare, as they may be perceived as burdensome to others or indicative of personal failure in upholding collective values. Latin American and Mediterranean cultures tend to be more expressive regarding tears, particularly in mourning rituals where serves as a communal release of and respect for the deceased. For instance, in Mexican traditions like Día de los Muertos, tears accompany celebrations of the dead, blending sorrow with communal remembrance to honor familial bonds. In some Mediterranean practices, such as those in and , ritual wailing and tearing of garments historically emphasized overt emotional display during funerals. Among Indigenous groups, such as certain Native American tribes, tears play a role in spiritual practices like the Lakota (Hanblečeyapi, or " for a vision"), where during and facilitates purification and connection to guardian spirits for personal insight and healing. Research indicates higher crying frequencies in individualistic societies compared to collectivist ones. Studies on emotional regulation suggest that and exposure influence emotional expressions across cultures.

Pathology

Insufficient Tear Production

Insufficient tear production, a key feature of aqueous-deficient dry eye, arises when the lacrimal glands produce inadequate volumes of basal tears, compromising the ocular surface and leading to discomfort and potential corneal damage. Common causes include aging, which diminishes lacrimal gland function and tear secretion, typically beginning after age 50 and affecting up to 35% of adults over 65 with keratoconjunctivitis sicca (KCS). Hormonal shifts during menopause in women further reduce tear production by altering glandular activity, increasing dry eye risk in postmenopausal individuals. Refractive surgeries such as LASIK can also impair tear production through nerve damage and inflammation, with symptoms persisting in some patients for months or longer. Keratoconjunctivitis sicca (KCS), the hallmark condition of insufficient tearing, manifests as and is often autoimmune in origin, particularly in Sjögren's syndrome, where lymphocytic infiltration targets lacrimal and salivary glands. Sjögren's syndrome affects an estimated 1–72 per 10,000 people globally, with higher rates (up to 0.25% or 1 in 400) in screened populations and objective KCS evident in 55.5%–88% of systemic sclerosis cases. Symptoms include ocular irritation, burning, foreign body sensation, and due to unstable tear film and surface epithelial damage. Another notable cause is , a rare primarily affecting individuals of Ashkenazi Jewish descent, with a carrier frequency of about 1 in 32 and incidence of 1 in 3,700 in that population. This failure leads to severely reduced tear production—often described as an inability to produce tears, with studies indicating up to 90% reduction—resulting in corneal insensitivity and recurrent ulcers. Diagnosis relies on objective tests assessing tear quantity and quality. The Schirmer test measures basal tear production by wetting of a strip over 5 minutes, with values below 5 mm indicating severe aqueous deficiency and below 10 mm supporting dry eye per Dry Eye Workshop criteria. (TBUT), evaluated via fluorescein instillation, is reduced below 10 seconds in dry eye, reflecting instability from insufficient aqueous component. Tear biomarkers, such as elevated matrix metalloproteinase-9 (MMP-9) levels, serve as indicators of underlying , with point-of-care immunoassays detecting MMP-9 positivity in severe cases to guide . Treatments aim to restore ocular surface homeostasis and address inflammation. Artificial tears provide symptomatic relief by supplementing the deficient aqueous layer, used frequently in mild to moderate cases. Topical cyclosporine eye drops (0.05%) inhibit T-cell mediated inflammation in the lacrimal glands, improving tear production and symptoms in autoimmune dry eye, often combined with other modalities for enhanced efficacy. Punctal plugs occlude tear drainage ducts to conserve existing tears, increasing retention and surface wetness, particularly beneficial in moderate KCS. In May 2025, the FDA approved acoltremon (Tryptyr) ophthalmic solution 0.003%, a TRPM8 agonist that stimulates natural tear production, for treating signs and symptoms of dry eye disease. Emerging regenerative approaches, including stem cell therapies targeting lacrimal gland repair, have shown promise in post-2020 clinical trials for severe dry eye, with mesenchymal stem cells modulating inflammation and promoting tissue regeneration in preclinical and early-phase studies.

Excessive or Abnormal Tearing

Excessive or abnormal tearing, known as epiphora, occurs when tears overflow onto the cheek due to overproduction or impaired drainage, often stemming from obstructions in the lacrimal drainage system. Common causes include , which can result from aging-related narrowing, chronic infections, or , leading to poor tear outflow. In elderly populations, epiphora is particularly prevalent, with lacrimal obstruction accounting for approximately 46% of cases in clinical studies, and overall symptomatic tearing affecting a significant portion due to age-related changes in eyelid function and ocular surface health. Another form of abnormal tearing is Bogorad's syndrome, also called crocodile tears syndrome, characterized by gustatory lacrimation where eating or salivary stimulation triggers excessive tearing. This condition arises from aberrant regeneration of the following damage, such as after or trauma, resulting in miswiring between salivary and lacrimal pathways. The tearing is reflexive and unilateral, typically emerging months after the initial nerve injury. Allergic conjunctivitis can also induce excessive tearing as part of an to allergens like pollen or dust. release from mast cells in the stimulates reflex tearing to flush irritants, often accompanied by itching, redness, and swelling. This hypersecretion is a protective mechanism but becomes abnormal when persistent or severe. Diagnosis of excessive tearing involves assessing tear drainage and production. The fluorescein disappearance (FDDT) evaluates outflow by instilling fluorescein in the eye and observing clearance time; delayed disappearance (beyond 10 minutes) indicates obstruction. Tear osmolarity measurement helps identify underlying ocular surface issues; values exceeding 316 mOsm/kg suggest hyperosmolarity that may drive reflex tearing. Management depends on the underlying cause. For duct obstructions, (DCR) surgery creates a new drainage pathway from the to the , achieving success rates of 85-95% in resolving epiphora. In Bogorad's syndrome, injections into the temporarily inhibit tearing, providing relief for 3-6 months with minimal side effects. For , topical antihistamines or stabilizers reduce histamine-mediated tearing and inflammation.

Neurological Dysregulation

Neurological dysregulation of tear production and emotional expression primarily manifests in conditions like (PBA), a disorder characterized by sudden, involuntary outbursts of or laughing that are disproportionate to or incongruent with the individual's actual emotional state. PBA often arises following neurological events such as or in progressive diseases like (MS) and (ALS), where it affects approximately 38% of ALS patients according to meta-analyses of clinical studies. These episodes involve excessive tearing during fits, stemming from disrupted neural pathways that normally modulate emotional responses. The underlying causes of PBA center on damage to the frontal lobes and corticobulbar tracts, which impair the brain's inhibitory control over emotional reflexes, leading to uninhibited activation during affective outbursts. In and MS, degeneration of upper motor neurons in these pathways exacerbates the condition, while post- lesions in similar regions can trigger it acutely. Beyond PBA, other neurological disorders contribute to tear dysregulation; for instance, is associated with reduced basal tear production due to autonomic dysfunction and decreased blink rate, resulting in dry eye symptoms in up to 61% of patients. Conversely, some auras include unilateral tearing as part of autonomic features, affecting about 45% of migraineurs with ocular symptoms. Diagnosis of neurological dysregulation, particularly PBA, relies on clinical observation of recurrent, involuntary episodes and exclusion of peripheral tear disorders through history and examination, often using validated scales like the Center for Neurologic Study-Lability Scale (CNS-LS). Functional MRI (fMRI) studies reveal dysregulation, with altered connectivity in emotional processing networks in affected individuals compared to controls. Differentiation from genuine emotional crying hinges on the involuntary nature of PBA episodes, which lack corresponding internal feelings and resolve quickly without emotional residue, unlike reflexive tearing pathways in normal emotional mechanisms. Treatment focuses on symptom management; the combination of and quinidine (Nuedexta), approved by the FDA in , reduces PBA episode frequency and severity by modulating activity in randomized controlled trials. Antidepressants such as selective serotonin reuptake inhibitors (SSRIs) provide adjunctive relief in smaller studies by enhancing serotonergic inhibition of . Emerging approaches in the include techniques like audio-visual entrainment therapy, which has shown promise in reducing PBA symptoms through brainwave synchronization in preliminary trials with sustained effects at three months. For Parkinson's-related hyposecretion, address symptoms, while migraine-associated tearing typically resolves with acute abortive therapies.

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