Tear Secretion - A Short Review PDF

J. Lakshmi Prabha /J. Pharm. Sci. & Res. Vol. 6(3), 2014,155-157 Tear Secretion-A Short Review J. Lakshmi Prabha First...

J. Lakshmi Prabha /J. Pharm. Sci. & Res. Vol. 6(3), 2014,155-157 Tear Secretion-A Short Review J. Lakshmi Prabha First Year BDS Saveetha Dental College, Chennai, India Abstract: Tears are fluid secreted by the lacrimal glands of the eye which helps to moisten the eyes. Tears contain a wide variety of biologically active substances like mucin, electrolytes and other substances. Tears contain proteins including lysozyme, serum albumin etc. which have important functions. It also contains various chemicals, the most importantly those that are evoked by eye- irritants. Thiopropanal S-oxide which is a factor present in onions induces tear secretion by reflex mechanisms. There are three types of tear secretion namely basal, emotional and reflex. Generally females tend to cry more than men as their ability to express emotions are better. Defect in tear secretion produces various disorders like dry eye, Sjogren’s syndrome, crocodile tears etc. Keywords: tears, lacrimal, basal, emotional, reflex, dry eye, Sjogren’s syndrome INTRODUCTION: other proteins include apolipoproteins H, phospholipase A2, Tears are the fluid secreted by the eyes to nourish it by ribonuclease4, lipophilin C etc.. Tears also contain IgA, maintaining a balance of the ingredients of the eyes and IgD, IgG, IgE.Secretory IgA acts as a defence agent along moisture content of the eye. Precorneal (tear) film is spread with phagocytes and lisoymes present in the tear film. Hence across the eye and it has three layers namely, lipid layer, they protect the eye from dust and other infectious agents. aqueous layer and mucous layer. Lipid layer (secretes lipid) acts as an hydrophobic barrier and prevents the overflow of TEAR FILM: tears. Aqueous layer (contains water and tear proteins) acts as Tear film is the thin layer of tear fluid secreted continuously a physiological barrier and controls infection to the eyes. by the lacrimal glands. Tear film is important as the relation Mucous layer (secrete mucin) acts as a hydrophilic layer. In a between the air and tear film is responsible for two- thirds of day, 0.75-1.1 grams of tears is secreted which decreases with the total refractory power of the eye. Tear film covers an age. Crying is considered healthy due to certain proteins area of 1-3 square cm and has a thickness of 2.7-11micro m present in it. People cry when they are alone especially. Tear film break up time is used to check the stability of adults. Other reasons why people cry include embarrassing or the tear film in case of dry eye and other eye disorders. It is shameful situations, low level frustration or during sad, seen that lithium carbonate and sodium valproate help in frightened, angry situations, while cutting onions and in decreasing the time taken for tear film break up. Cortisol situations that emotionally affects their brain. On average and dihydroepiandrosterone are some stress biomarkers Women cry 30-64 times a year and men cry just 6-17 times in present in tear secretion. Thickness of the tear film is a year. The tears are secreted every second at the rate of 2 contributed mainly by the aqueous secretions of the eye and it microlitres per minute or 10 ounces each day and are drained contributes nearly 60% of the total value. Tear film is by the puncta which are 0.3mm wide. Thus there is overflow smooth layer formed during blinking and finally breaks up of tears during sneezing, vomiting, coughing yawning or any during evaporation. Thus the images become blurred with reflexes. The tears are secreted by the lacrimal gland situated increase in time. Irregular tear films cause visual laterally. They are spread by periodic blinking of the eyelid problems. and they are drained by the puncta, pass through the canaliculi and is drained into the lacrimal sac. Further it TYPES OF TEARING: passes through the nasolacrimal duct and causes the nose to Tear secretion is classified into three types namely, basal, be stuffy when we cry. emotional and reflex tears. Basal tears are secreted and spread continuously by the eyes. They moisten the eyes. Reflex TEAR PROTEINS: tears are those which are produced by pain, external stimulus Tear proteins are secreted by the lacrimal glands and contains or foreign bodies. Hypo secretion is caused by dry eye lactoferrins, antimicrobial molecules, lacrimal secretory IgA and hyper secretion is caused by ocular inflammation, corneal etc. Lactoferrin (lactotransferrin) is a glycoprotein present or nasal irritations, etc. Although basal tears are vital for majorly in neutrophils and secreted in small concentrations healthy eyes, reflex tears are secreted excessively than basal by exocrine glands including lacrimal glands. Lactoferrin is a tears. part of specific defence of the body in an indirect way. Lactoferrin has antimicrobial activity and is a carier protein. EMOTIONAL TEARING: Today almost 60 tear proteins are known including the well Generally women tend to cry for six minute whereas men known proteins like lacitin, proline rich proteins, lipocalin, only for two- three minutes. People tend to cry to express lipophilin, etc. Lysozyme has antibacterial activity. Lipocalin their emotions in mainly two circumstances namely, 1 (prealbumin) and lipophilin A are carrier proteins. Some requesting-help and offering-help. Previous classification 155  J. Lakshmi Prabha /J. Pharm. Sci. & Res. Vol. 6(3), 2014,155-157 classifies emotional tearing as positive and negative. Thus etc. which causes dry eye. It is seen with increase in requesting-help includes only negative emotions but age and is predominantly seen in adult women especially emotional-help includes both positive as well as negative after menopause. Use of humidifier, artificial tear drops, emotions. Turkish face cloth, moist chamber spectacles, tear replacement therapy and maintaining good lid hygiene helps REFLEX TEARS DUE TO ONIONS: in the management of dry eye. Tears are produced when certain chemicals are released while cutting an onion. These include methionine, cysteine, syn- SJOGREN’S SYNDROME: propanethial-s-oxide. When metionine and cysteine are It is an auto immune disease affecting cells of lachrymal released into the air, the enzyme lacrymatory factor gland and finally leads to xerostomia and synthetase allinases convertes sulfoxides into lacrymatory keratoconjunctivitis. It is prevalent among women than men compounds like sulfuric acid, allyl sulphide which causes tear and occurs mostly after menopause. There are two types: secretion. It can be reduced by keeping the onion in Primary sjogren syndrome (occurs on its own) and secondary refrigerator (reduces enzyme activity), using without cutting sjogren syndrome(occurs in association with other the root part frequency of cutting onions using running water autoimmune disorders). Dryness of body parts is the major or fan while cutting, etc. cause which includes dryness of mouth, vagina, bronchi, trachea and skin. Other symptoms include muscle and joint TEARS AND ITS EFFECT ON AGING: pain, tiredness and swelling of glands. During aging all physiological process tend to decrease and this attributes to decrease in tear secretion. Sometimes there CONCLUSION: is overflow of tears due to inefficient drainage function of the Thus tears are useful because they have important proteins, gland and blocks in the lachrymal ducts. Epiphora is a immunoglobulins including its role in defence of the eye. symptom which results from deficiency in the drainage of Tears also have antimicrobial activity due to lactoferrin tear film. There are two types of epiphora: chronic and acute present in it. Tears wash away the unwanted substances conditions. Dry eye is caused in old age due to less tear present in the eye. Crying helps in expressing our emotions secretion. and also to overcome sorrow. Tear film helps to maintain visual acuity and contains major proteins. Abnormalities of PHYSIOLOGY OF TEAR SECRETION: tear secretion which cause various diseases like those The sensory nerve for tear reflexes is the fifth cranial nerve- mentioned above are of significant importance and should be the trigeminal nerve. If this nerve is cut reflex tears alone are treated with care. Therefore these studies show that tear affected. When the corneal surface is exposed to cocaine, secretion is an important biological process. reflex tears are stopped even on exposure to strong gases which cause tear secretion. The tear secretion is regulated REFERENCE: physiologically by cholinergic fibres of the parasympathetic 1) Immunological and antimicrobial molecules in human tears: a review and preliminary report, R Schnetlera, WDH Gillanb and G Koorsena*, The nervous system, sympathetic stimulation to adrenal gland, South African optometrist 2012 71(3) 123-321 certain peptides and humoral factors. 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