Tears
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Tears are a clear liquid secreted by the
lacrimal gland The lacrimal glands are paired exocrine glands, one for each eye, found in most terrestrial vertebrates and some marine mammals, that secrete the aqueous layer of the tear film. In humans, they are situated in the upper lateral region of each or ...
s (tear gland) found in the eyes of all land mammals. Tears are made up of water, electrolytes, proteins, lipids, and mucins that form layers on the surface of eyes. The different types of tears—basal, reflex, and emotional—vary significantly in composition. The functions of tears include lubricating the eyes (basal tears), removing irritants (reflex tears), and also aiding the
immune system The immune system is a network of biological processes that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to parasitic worms, as well as cancer cells and objects such as wood splinte ...
. Tears also occur as a part of the body's natural pain response. Emotional secretion of tears may serve a biological function by excreting stress-inducing hormones built up through times of emotional distress. Tears have symbolic significance among humans.


Physiology


Chemical composition

Tears are made up of three layers: lipid, aqueous, and mucous. Tears are composed of
water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...
,
salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quantitie ...
s,
antibodies An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and viruses. The antibody recognizes a unique molecule of the ...
, and
lysozyme Lysozyme (EC 3.2.1.17, muramidase, ''N''-acetylmuramide glycanhydrolase; systematic name peptidoglycan ''N''-acetylmuramoylhydrolase) is an antimicrobial enzyme produced by animals that forms part of the innate immune system. It is a glycoside ...
s (antibacterial enzymes); though composition varies among different tear types. The composition of tears caused by an emotional reactions 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 Adrenocorticotropic hormone (ACTH; also adrenocorticotropin, corticotropin) is a polypeptide tropic hormone produced by and secreted by the anterior pituitary gland. It is also used as a medication and diagnostic agent. ACTH is an important c ...
and leucine enkephalin (a natural pain killer), which suggests that emotional tears play a biological role in balancing stress hormone levels.


Drainage of tear film

The
lacrimal gland The lacrimal glands are paired exocrine glands, one for each eye, found in most terrestrial vertebrates and some marine mammals, that secrete the aqueous layer of the tear film. In humans, they are situated in the upper lateral region of each or ...
s secrete lacrimal fluid, which flows through the main excretory ducts into the space between the eyeball and the 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 ''puncta lacrimalia''). The volume of the lacrimal lake has been estimated to be between 7 and 10 μL. ...
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. The lacrimal canaliculi open into the papilla. The opening of each canaliculus is the lacrimal punctum. From the punctum, tears will enter the
lacrimal sac The lacrimal sac or lachrymal 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 ey ...
, then on to the
nasolacrimal duct The nasolacrimal duct (also called the tear duct) carries tears from the lacrimal sac of the eye into the nasal cavity. The duct begins in the eye socket between the maxillary and lacrimal bones, from where it passes downwards and backwards. The ...
, and finally into the
nasal cavity The nasal cavity is a large, air-filled space above and behind the nose in the middle of the face. The nasal septum divides the cavity into two cavities, also known as fossae. Each cavity is the continuation of one of the two nostrils. The nasal c ...
. An excess of tears, as caused by strong
emotion Emotions are mental states brought on by neurophysiological changes, variously associated with thoughts, feelings, behavioral responses, and a degree of pleasure or displeasure. There is currently no scientific consensus on a definition. ...
, can cause the nose to run. Quality of vision is affected by the stability of the tear film.


Types

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


Nictitating membrane

Some mammals, such as
cat The cat (''Felis catus'') is a domestic species of small carnivorous mammal. It is the only domesticated species in the family Felidae and is commonly referred to as the domestic cat or house cat to distinguish it from the wild members of ...
s,
camel A camel (from: la, camelus and grc-gre, κάμηλος (''kamēlos'') from Hebrew or Phoenician: גָמָל ''gāmāl''.) is an even-toed ungulate in the genus ''Camelus'' that bears distinctive fatty deposits known as "humps" on its back. C ...
s, polar bears,
seals Seals may refer to: * Pinniped, a diverse group of semi-aquatic marine mammals, many of which are commonly called seals, particularly: ** Earless seal, or "true seal" ** Fur seal * Seal (emblem), a device to impress an emblem, used as a means of a ...
and
aardvark The aardvark ( ; ''Orycteropus afer'') is a medium-sized, burrowing, nocturnal mammal native to Africa. It is the only living species of the order Tubulidentata, although other prehistoric species and genera of Tubulidentata are known. Unlike ...
s, have a full translucent third eyelid called a nictitating membrane, while others have a
vestigial Vestigiality is the retention, during the process of evolution, of genetically determined structures or attributes that have lost some or all of the ancestral function in a given species. Assessment of the vestigiality must generally rely on co ...
nictitating membrane. 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. 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.


Neurology

The
trigeminal In neuroanatomy, the trigeminal nerve ( lit. ''triplet'' nerve), also known as the fifth cranial nerve, cranial nerve V, or simply CN V, is a cranial nerve responsible for sensation in the face and motor functions such as biting and chewing; ...
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. It is responsible for the production of much of the aqueous portion of the tear film.


Human culture

In nearly all human cultures,
crying Crying is the dropping of tears (or welling of tears in the eyes) in response to an emotional state, or pain. Emotions that can lead to crying include sadness, anger, and even happiness. The act of crying has been defined as "a complex secr ...
is associated with tears trickling down the cheeks and accompanied by characteristic sobbing sounds. Emotional triggers are most often
sadness Sadness is an emotional pain associated with, or characterized by, feelings of disadvantage, loss, despair, grief, helplessness, disappointment and sorrow. An individual experiencing sadness may become quiet or lethargic, and withdraw thems ...
and grief, but crying can also be triggered by
anger Anger, also known as wrath or rage, is an intense emotional state involving a strong uncomfortable and non-cooperative response to a perceived provocation, hurt or threat. A person experiencing anger will often experience physical effects, su ...
,
happiness Happiness, in the context of Mental health, mental or emotional states, is positive or Pleasure, pleasant emotions ranging from contentment to intense joy. Other forms include life satisfaction, well-being, subjective well-being, flourishin ...
,
fear Fear is an intensely unpleasant emotion in response to perceiving or recognizing a danger or threat. Fear causes physiological changes that may produce behavioral reactions such as mounting an aggressive response or fleeing the threat. Fear ...
, laughter or
humor Humour (English in the Commonwealth of Nations, Commonwealth English) or humor (American English) is the tendency of experiences to provoke laughter and provide amusement. The term derives from the humorism, humoral medicine of the ancient Gre ...
,
frustration In psychology, frustration is a common emotional response to opposition, related to anger, annoyance and disappointment. Frustration arises from the perceived resistance to the fulfillment of an individual's will or goal and is likely to in ...
,
remorse Remorse is a distressing emotion experienced by an individual who regrets actions which they have done in the past that they deem to be shameful, hurtful, or wrong. Remorse is closely allied to guilt and self-directed resentment. When a perso ...
, or other strong, intense emotions. Crying is often 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 Western cultures, it is more socially acceptable for women and children to cry than men, reflecting masculine sex-role stereotypes. In some Latin regions, crying among men is more acceptable. There is evidence for an interpersonal function of crying as tears express a need for help and foster willingness to help in an observer. Some modern psychotherapy movements such as
Re-evaluation Counseling Re-evaluation Counseling (RC) is an organization that practices a procedure – 'co-counseling' – in which people try to help each other deal with the effects of emotional hurt through catharsis (called "discharge") while also embracing a utopian ...
encourage crying as beneficial to health and mental well-being. An insincere display of grief or dishonest remorse is sometimes called
crocodile tears Crocodile tears, or superficial sympathy, is a false, insincere display of emotion such as a hypocrite crying fake tears of grief. The phrase derives from an ancient belief that crocodiles shed tears while consuming their prey, and as such is p ...
in reference to an Ancient Greek anecdote that crocodiles would pretend to weep while luring or devouring their prey. 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.


Pathology


Bogorad's syndrome

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 The facial nerve, also known as the seventh cranial nerve, cranial nerve VII, or simply CN VII, is a cranial nerve that emerges from the pons of the brainstem, controls the muscles of facial expression, and functions in the conveyance of tas ...
. Efferent fibers from the
superior salivary nucleus The salivatory nuclei are the superior salivatory nucleus, and the inferior salivatory nucleus that innervate the salivary glands. They are located in the pontine tegmentum in the brainstem. They both are examples of cranial nerve nuclei. The su ...
become improperly connected to nerve axons projecting to the lacrimal glands, causing one to shed tears (
lacrimate Crying is the dropping of tears (or welling of tears in the eyes) in response to an emotional state, or pain. Emotions that can lead to crying include sadness, anger, and even happiness. The act of crying has been defined as "a complex secreto ...
) on the side of the palsy during salivation while smelling foods or eating. It is presumed 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. 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.


Keratoconjunctivitis sicca (dry eye)

Keratoconjunctivitis sicca Dry eye syndrome (DES), also known as keratoconjunctivitis sicca (KCS), is the condition of having dry eyes. Other associated symptoms include irritation, redness, discharge, and easily fatigued eyes. Blurred vision may also occur. Symptoms range ...
, 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.


Familial dysautonomia

Familial dysautonomia Familial dysautonomia (FD), also known as Riley-Day Syndrome, is a rare, progressive, recessive genetic disorder of the autonomic nervous system that affects the development and survival of sensory, sympathetic and some parasympathetic neurons ...
is a genetic condition that can be associated with a lack of overflow tears (
Alacrima Alacrima refers to an abnormality in tear production that could mean reduced tear production or absent tear production. Because a lack of tears presents in only in a few rare disorders, it aids in diagnosis of these disorders, including Triple-A ...
) during emotional crying. Obstruction of the punctum, nasolacrimal canal, or
nasolacrimal duct The nasolacrimal duct (also called the tear duct) carries tears from the lacrimal sac of the eye into the nasal cavity. The duct begins in the eye socket between the maxillary and lacrimal bones, from where it passes downwards and backwards. The ...
can cause even normal levels of the basal tear to overflow onto the face ( Epiphora), giving the appearance of constant psychic tearing. This can have significant social consequences.


Pseudobulbar affect

Pseudobulbar affect Pseudobulbar affect (PBA), or emotional incontinence, is a type of emotional disturbance characterized by uncontrollable episodes of crying, laughing, anger or other emotional displays. PBA occurs secondary to a neurologic disorder or brain i ...
(PBA) is a condition involving episodic uncontrollable laughter or crying. PBA mostly occurs in people with neurological injuries affecting how the brain controls emotions. Scientists believe PBA results from prefrontal cortex damage. PBA often involves crying. Hence, PBA is mistakable for depression. But PBA is neurological; depression is psychological. patients with PBA do not experience typical depression symptoms like sleep disturbances or appetite loss.


See also

*
Artificial tears Artificial tears are lubricating eye drops used to relieve dryness and irritation of the ocular surface. Dry eye syndrome ( keratoconjunctivitis sicca) is a common ocular surface disorder and is characterized by disruption of the tear film and i ...
*
Dacryocystocele Dacryocystocele (Dacryocystitis) or timo cyst is a benign, bluish-gray mass in the inferomedial canthus that develops within a few days or weeks after birth. The uncommon condition forms as a result as a consequence of narrowing or obstruction of t ...
* Epiphora *
Eyelid An eyelid is a thin fold of skin that covers and protects an eye. The levator palpebrae superioris muscle retracts the eyelid, exposing the cornea to the outside, giving vision. This can be either voluntarily or involuntarily. The human eyel ...
*
Professional mourning Professional mourning or paid mourning is an occupation that originates from Egyptian, Chinese, Mediterranean and Near Eastern cultures. Professional mourners, also called moirologists and mutes, are compensated to lament or deliver a eulogy an ...
*
Sadness Sadness is an emotional pain associated with, or characterized by, feelings of disadvantage, loss, despair, grief, helplessness, disappointment and sorrow. An individual experiencing sadness may become quiet or lethargic, and withdraw thems ...
*
Harderian gland The Harderian gland is a gland found within the eye's orbit that occurs in tetrapods (reptiles, amphibians, birds and mammals) that possess a nictitating membrane. The gland can be compound tubular or compound tubuloalveolar, and the fluid it s ...


References


External links

* * {{Authority control Crying Human physiology Eye Body fluids