Tears

thumb|upright=1.15|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. According to the mode of production, tears are classified into four types: basal, closed eye, emotional, and reflex. The basal rate of tear secretion is ~0.5–2.2 μL/min, and irritation can increase secretion by up to ~100-fold, reaching ~300 μL/min. Tears are made up of water, electrolytes, proteins, lipids, and mucins that form layers on the surface of eyes. The four types of tears differ significantly in their composition. 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. 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.

{| class="wikitable"

! style="width: 10%;" |Name

! style="width: 40%;" |Composition

! style="width: 10%;" |Origin

! style="width: 40%;" |Functions

|- style="vertical-align: top;"

|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.

|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. enabling the formation of a thin film, preventing its collapse, and involving in the oxygenation of the cornea. lipocalin, lactoferrin, lysozyme, and lacritin)

|Lacrimal gland

|Promotes spreading of the tear film, the control of infectious agents, and osmotic regulation.

|- style="vertical-align: top;"

|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

The lacrimal glands 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.

Types

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

Nictitating membrane

Some mammals, such as cats, camels, polar bears, seals and aardvarks, have a full translucent third eyelid called a nictitating membrane, while others have a vestigial nictitating membrane. 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.thumb|A toddler producing tears due to emotional stress or pain|150px

Neurology

The trigeminal V<sub>1</sub> (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

thumb|Crying boy

thumb|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, 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. 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. There is evidence for an interpersonal function of crying as tears express a need for help and foster willingness to help in an observer. 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. 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. 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 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.

Keratoconjunctivitis sicca (dry eye disease)

Keratoconjunctivitis sicca, commonly known as dry eye, is a prevalent condition 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.

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.

Familial dysautonomia

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

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.

Pseudobulbar affect

Pseudobulbar affect (PBA) is a condition involving episodic uncontrollable laughter or crying. PBA mostly occurs in people with neurological injuries affecting how the brain controls emotions. 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.