Target Health Blog

Tears

July 2, 2018

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Quiz
Source:

Anatomy of lacrimation, 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

Graphic credit: Erin Silversmith, The labels in Portuguese were replaced with letters, for internationalization purposes. If a version with labels in a certain language is deemed necessary, it should be uploaded with the language suffix i.e. Tear system xx.svg where xx is the language code. User: FML uploaded to Commons as Image: Sistema lacrimal.gif and Image:Sistema lacrimal.svg., CC BY-SA 2.5, https://commons.wikimedia.org/w/index.php?curid=1045167

Tearing, lacrimation, or lachrymation (from Latin lacrima, meaning ?tear') is the secretion of tears, which often serves to clean and lubricate the 1) ___ in response to an irritation of the eyes. Tears formed through crying are associated with strong internal emotions, such as sorrow, elation, love, awe, and pleasure. Laughing or yawning may also lead to the production of tears. In humans, the tear film coating the eye, known as the precorneal film, has three distinct layers, from the most outer surface:

Lipid layer: Oils from the Meibomian glands (or tarsal glands) coats the aqueous layer, provides a hydrophobic barrier that envelops tears and prevents their spilling onto the cheek. These glands are found among the tarsal plates. Thus, the tear fluid deposits between the eye proper and oil barriers of the lids.

Aqueous layer: Electrolytes, 60 metabolites including amino 2) ___, purines and derivatives, quaternary amines (acetylcholine, glycerolphosphocholine, phosphocholine), and tricarboxylic acids (citric acid)-and other substances such as proteins (e.g., antibodies, lipocalin, lactoferrin, lysozyme, and lacritin)      The lacrimal gland promotes spreading of the tear film, the control of infectious agents and osmotic regulation.

Mucous layer: Mucins from the conjunctival goblet cells coat the cornea, provides a hydrophilic layer and allows for even distribution of the tear film. 

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 3) ___ are designed to let more oxygen through to the eye.

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 entering the lacrimal sac, then on to the nasolacrimal duct, and finally into the nasal cavity. An excess of tears, as with strong emotion, can thus cause the 4) ___ to run.

There are three very basic types of tears:

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 a layer in the outer coating, called peptidoglycan, 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 of tears are secreted; this rate slows with 5) ___. In addition, the basal tears are composed of antioxidants such as ascorbate, urate, cysteine, glutathione, and tyrosine. Ascorbate and urate constitute half of the tears.

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. 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. These reflex tears attempt to wash out irritants that may have come into contact with the eye.

Psychic tears: Crying or weeping (psychic tears) are the 3rd category. In general, these tears are referred to as crying or weeping, is increased tearing due to strong emotional stress, pleasure, anger, suffering, mourning, or physical pain.

Tears are not restricted to negative emotions, as many people cry when extremely happy such as during 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 make-up 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 production of basic emotional drives, such as anger, fear, etc. The limbic system, to be specific, the hypothalamus, also has a degree of control over the autonomic system. The parasympathetic branch of the autonomic nervous 6) ___ 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.

The trigeminal V1 (fifth cranial) nerve bears the sensory pathway of the tear reflexes. When the trigeminal nerve is cut, tears from reflexes will stop, but not emotional tears. Likewise, application of cocaine to the surface of the eye, due to its paralyzing effect on the sensory nerve endings, inhibits the reflex even under exposure to strong tear gases. The motor pathway is autonomic (involuntary), and, in general, 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 the baby “cries without weeping.“ If the lacrimal gland malfunctions or is damaged (e.g. by surgery) but does not cause any severe drying of the cornea, 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.

“Crocodile tears syndrome“, also known as Bogorad's syndrome, is an uncommon consequence of nerve regeneration subsequent to Bell's palsy or other damage to the facial nerve in which efferent fibers from the superior salivary nucleus become improperly connected to nerve axons projecting to the lacrimal glands, causing one to shed tears (lacrimate) during salivation while smelling foods or eating. It is presumed that one would also salivate while crying due to the inverse improper connection of the lacrimal nucleus to the salivary glands, but this would be less noticeable. 

Keratoconjunctivitis sicca, known as dry eye, is a very common disorder of the tear film. However, sufferers can 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 the tears are not enveloped in a hydrophobic film coat, leading to tears spilling onto the face.

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 basal tear to overflow onto the face (epiphora), giving the appearance of constant psychic tearing. This can have significant social consequences. 

In nearly all cultures, crying is associated with tears trickling down the cheeks and accompanied by characteristic sobbing sounds. Emotional triggers are most often sadness and grief; but crying can also be triggered by anger, happiness, fear, laughter or humor, frustration, remorse, or other strong, intense emotions. In many cultures, crying is associated with babies and children. Some cultures consider crying to be undignified and infantile, casting aspersions on those who cry publicly, except if it is due to the 7) ___ of a close friend or relative. In most cultures, it is more socially acceptable for women and children to cry than men. In some Latin regions, crying among men is acceptable. Some modern therapy movements such as Re-evaluation Counseling teach that crying is beneficial to health and mental well-being, encouraging it positively. 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, in medical terms, someone is said to have crocodile tears syndrome as an uncommon consequence of recovery from Bell's palsy, in which faulty regeneration of the facial nerve causes sufferers to shed tears while eating.

On a study conducted by the Weizmann Institute of Science in Rehovot, Israel, emotional tears from women have been found to reduce sexual arousal in men. Also, emotional tears are made up of a different chemical component than those evoked by eye irritants and can relay chemical messages to others. The change in sex drive could be attributed to a drop in testosterone provoked by the tear chemicals, reducing aggression. In the animal world, it has been found that some blind mole rats rub tears all over their bodies as a strategy to keep aggressive mole rats away.  Emotional tears are composed of more protein-based hormones, such as prolactin, adrenocorticotropic, and leucine enkephalin (a natural pain killer), which is suggested to be the mechanism behind the experience of crying from emotion making an individual feel better. Emotional tears, unlike basal or reflex tears, contain stress 8) ___, which the body is able to physically push out through the process of crying. Crying triggers the body to release feel-good endorphins (the same ones you get from exercise or laughing.

Crying and Survival

The parent-infant bond is central to the human condition, contributes to risks for mood and anxiety disorders, and provides the potential for resiliency and protection against the development of psychopathology. Animal models of parenting provide compelling evidence that biological mechanisms may be studied in humans. This has led to brain imaging and endocrine system studies of human parents using baby stimuli (crying) and concerted psychological and behavioral measures. Certain brain circuits and related hormonal systems, including subcortical regions for motivation (striatum, amygdala, hypothalamus and hippocampus) and cortical regions for social cognition (anterior cingulate, insula, medial frontal and orbitofrontal cortices), appear to be involved. These 9) ___ circuits work with a range of endocrine systems to manage stress and motivate appropriate parental caring behavior with a flexibility appropriate to the environment. Work in this field links evolving models of parental brain performance with resilience, risk and treatment toward mother-infant mental health. Parenting is critical for an infant's survival and development across mammalian species. Despite increased levels of stress as a result of physiological, emotional and economical demands (crying) from the baby, parents typically find themselves highly motivated to take care of their infants' needs and also find the interactions with infants rewarding. Recent human and animal brain research has used infant (crying) stimuli to examine psychological and biological aspects of maternal motivations and behaviors. Using the example of selected maternal brain and endocrine responses to the hunger cry of an infant, it has been discovered that the modulatory roles of dopamine (DA), oxytocin (OT), cortisol (CORT) and endogenous opioids (EO), all play an important role. A baby's cry triggers the 10) ___ (stress) levels (other hormonal levels are involved, as well) of the parents, which in turn, motivates them to do what they can to calm and satisfy a baby needs.

Keep in mind that our bodies aim for biobehavioral parental and infant responses that will modulate a neuroendocrine homeostasis.

Sources: nih.govpubmed.gov; Wikipedia

Fig. 1: Graphic credit: Object name is NIHMS-659808-f0001.jpg

An integrated neuroendocrine model of parenting. From Figure 1 above, left to right in time, the baby stimulus, behavioral response, brain activity and associated hormonal shifts of dopamine, endogenous opioids. No strict timeline for events is given. During baby-cry, dopamine may be the first to rise to assist with arousal, motivation and decision-making circuits, including striatum and amygdala. Oxytocin also rises relatively quickly with hypothalamus brain activity to support milk let-down and promote parenting behaviors. Cortisol (CORT) increases with a slower time course to support stress responses and prepare the mother for demanding behavior depending on the reason for baby-cry and other circumstances. Assuming a simple hunger cry, CORT will start to drop first, with oxytocin next after it has performed lactation-supporting anxiety-reducing functions. Finally, the parent-infant interaction requires regulatory cortical regions of anterior cingulate, medial frontal and orbitofrontal cortices that are modulated by opioids to mediate satisfaction and reinforce caring behavior. This may be paired with a proposed second rise in dopamine to mediate reinforcement, learning and any extended play or other dyadic interactions. Abnormalities in these neuroendocrine systems, such as in mood, anxiety or substance abuse disorders, contribute to the impaired orchestration of these parental neuroendocrine systems. DA, dopamine; EO, endogenous opioids; mPFC, medial prefrontal cortex; OFC, orbitofrontal cortex; OT, oxytocin.

ANSWERS: 1) eyes; 2) Acids; 3) lenses; 4) nose; 5) age; 6) system; 7) death; 8) hormones; 9) brain; 10) cortisol

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