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

Tears are the liquid product of a process of lacrimation to clean and lubricate the eyes. The word lacrimation may also be used in a medical or literary sense to refer to crying. Strong emotions, such as sorrow or elation, may lead to crying. The process of yawning may also result in lacrimation. Although most land mammals have a lacrimation system to keep their eyes moist, humans are the only mammal generally accepted to cry emotional tears.^[1][2]

Physiology

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

1. The lipid layer contains oils secreted by the meibomian glands (or tarsal glands). The outer-most layer of the tear film coats the aqueous layer to provide a hydrophobic barrier that retards evaporation and prevents tears spilling onto the cheek.
2. The aqueous layer contains water and other substances such as proteins (e.g. tear lipocalin, lactoferrin, lysozyme^[3] and lacritin) secreted by the glands and the lacrimal gland. The aqueous layer serves to promote spreading of the tear film, control of infectious agents and osmotic regulation.
3. The mucous layer contains mucin secreted by the conjunctival goblet cells. The inner-most layer of the tear film, it coats the cornea to provide a hydrophilic layer that allows for even distribution of the tear film, as well as mucus covering of the cornea.

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

Drainage of tear film

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

Types of tears

There are three very basic types of tears:

1. Basal tears: In healthy mammalian eyes, the cornea is continually kept wet and nourished by basal tears. They lubricate the eye, and help to keep it clear of dust. Tear fluid contains water, mucin, lipids, lysozyme, lactoferrin, lipocalin, lacritin, immunoglobulins, glucose, urea, sodium, and potassium. Some of the substances in lacrimal fluid (such as lysozyme) fight against bacterial infection as a part of the immune system. Lysozyme does this by dissolving the outer coating of certain bacteria. It is a typical body fluid with a salt content similar to blood plasma. Usually, in a 24-hour period, 0.75 to 1.1 grams (0.03-0.04 ounce avoirdupois) of tears are secreted, this rate slows with age.

2. Reflex tears: The second type of tears results from irritation of the eye by foreign particles, or from the presence of irritant substances such as onion vapors, tear gas or pepper spray in the eye's environment, including the cornea, conjunctiva, or nasal mucosa. It can also occur with bright light and hot or peppery stimuli to the tongue and mouth. It is also linked with vomiting. These reflex tears attempt to wash out irritants that may have come into contact with the eye.

A child producing tears due to emotional stress or pain

3. Crying or weeping (psychic tears): The third category, generally referred to as crying or weeping, is increased lacrimation due to strong emotional stress, suffering or physical pain. This practice is not restricted to negative emotions; many people cry when extremely happy. In humans, emotional tears can be accompanied by reddening of the face and sobbing?cough-like, convulsive breathing, sometimes involving spasms of the whole upper body. Tears brought about by emotions have a different chemical make up than those for lubrication; emotional tears contain more of the protein-based hormones prolactin, adrenocorticotropic hormone, and leucine enkephalin (a natural painkiller) than basal or reflex tears. The limbic system is involved in production of basic emotional drives, such as anger, fear, etc. The limbic system, specifically the hypothalamus, also has a degree of control over the autonomic system. The parasympathetic branch of the autonomic system controls the lacrimal glands via the neurotransmitter acetylcholine through both the nicotinic and muscarinic receptors. When these receptors are activated, the lacrimal gland is stimulated to produce tears.^[4]

Neural Aspects

The trigeminal (fifth cranial) nerve bears the sensory pathway of the tear reflexes. When the trigeminal nerve is cut, tears from reflexes will stop, but not emotional tears. Likewise, application of cocaine to the surface of the eye inhibits the reflex even under exposure to strong tear gases. The motor pathway is autonomic (involuntary), and generally uses the pathway of the facial (seventh) nerve in the parasympathetic division. In parasympathetic imitators (such as acetylcholine), more tears are produced, and an anticholinergic drug like atropine, inhibits tear production. A newborn infant has insufficient development of nervous control, so s/he "cries without weeping." Lest the cornea be damaged in surgery or other failure of lacrimal function occur, it is not a serious matter, for the accessory glands are enough for general secretion. In reflex situations, copious tears are produced mainly in emergencies.

Diseases and disorders

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

"Crocodile tears syndrome" is an uncommon consequence of recovery from Bell's palsy where faulty regeneration of the facial nerve causes sufferers to shed tears while eating.^[6]

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

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

Societal aspects

Most mammals will produce tears in response to extreme pain or other stimuli, but crying as an emotional reaction is considered by many to be a uniquely human phenomenon, possibly due to humans' advanced self-awareness. Some studies suggest that elephants, gorillas, and camels may cry as well.^[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 anger and grief, but crying can also be triggered by sadness, joy, fear, laughter or humor, frustration, remorse or other strongly-experienced emotions.

In many cultures, crying is associated with babies and children. The human brain is programmed to consider the crying of a child irritating, and thus make the person desire to aid the baby to stop the crying. 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 to cry than men, however this stereotype is slowly dying.^[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 called crocodile tears, from the ancient anecdote that crocodiles would pretend to weep while luring or devouring their prey.^[12]

See also

• Epiphora
• Eyelid

References

External links

• Nasolacrimal System Anatomy - eMedicine.com

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