Tears: more than just water
Types of tears
The emotional connection
Hormones and tears
Tears as a window to health
The evolutionary perspective
Conclusion
References
Further reading
Tears have a wide range of functions that are critical to ocular health and emotional regulation. Each type of tear has a slightly different biochemical structure and content specialized to their specific needs.
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Tears: more than just water
Water is a large component of tears, although other materials also make up the fluid. Such materials include hormones, neurotransmitters, and more.
Most substances found in tears are proteins. Tear lipocalin, lactoferrin, and lysozyme are all particularly noteworthy. For example, lysozyme has a higher concentration in tears than in any other bodily fluid, making up approximately 20-30% of the protein in basal and reflexive tears.
The tear film
Tears may bring to mind crying or extreme bouts of emotion, but the eye is also continuously covered by tear fluid. This is known as the tear film found on the ocular surface.
The tear film comprises three layers of fluid that account for the protection, comfort, mechanical functioning, and epithelial health of the eye. These functions are achieved through the proteins and biochemicals in each layer.
The three layers are the inner mucin, middle aqueous, and outer lipid. Both systemic regulation and the lacrimal glands produce and regulate the chemical makeup of the tear film.
The middle aqueous layer of the tear film protects the eye from its external environment using biochemical substances. These are largely antimicrobial or bacteriolytic agents, such as the immunoglobulins IgA, IgG, and IgE, anti-proteinase lysozyme, lactoferrin, transferrin, ceruloplasmin, and glycoprotein.
Types of tears
Basal, reflexive, and emotional are the three types of tears. Each type has its own functions and chemical makeup. While all types of tears have similar biochemical compositions, the volume of each substance tends to vary between types.
Basal tears
Basal tears are produced in the accessory lacrimal glands found beneath the eyelids. Its main function is to protect and provide nutrients to the cornea continually and is referred to as “basic” tear production. Typically, basal tears are produced at 2μL per minute.
Basal tears predominantly consist of proteins such as lysozyme and lactoferrin, like the tear film, in addition to the electrolytes sodium and potassium. The quantity of lipids and protein secreted is the highest in basal tears of the three types of tears.
Reflexive tears
Reflexive tears occur in response to foreign particles invading the internal ocular environment. Transient Receptor Potential (TRP) channels in the ophthalmic nerve (V1), which is found in the trigeminal nerve (CN V), are triggered in response to irritation. This leads to the release of these tears.
Reflexive tears were first identified in the ocular systems of Drosophila, a type of fruit fly, and at least 30 subtypes of TRP channels have been identified since. They are non-selective cation channels in all forms of ocular tissue. The channels are mostly permeable to positively charged ions like sodium, calcium, and magnesium.
The biochemical makeup of reflexive tears is similar to basal tears, although there is a comparably higher level of tear lipocalin. The main lacrimal gland, found in the lacrimal fossa, produces this protein via acinar cells. Tear lipocalin makes up around a quarter of the protein in reflexive tears.
Emotional tears
Emotional tears contain a higher concentration of hormones, neuropeptides, and neurotransmitters when compared to reflexive or basal tears. This is thought to be because emotional tears are the waste product of emotional responses.
Acetylcholine is a neurotransmitter found in emotional tears, an ester of acetic acid and choline. Catecholamine neurotransmitters, such as adrenaline and noradrenaline, also make up the contents of emotional tears. These neurotransmitters then have a reciprocal impact on the nervous system.
Why do we cry? The three types of tears - Alex Gendler
The emotional connection
Some biochemical materials are thought to be found in emotional tears due to the role the autonomic nervous system (ANS) holds in crying. When releasing emotional tears, the parasympathetic nervous system (PNS) is activated. The PNS, along with the sympathetic nervous system (SNS), is a subsystem of the ANS that regulates activity like the fight-or-flight response.
ANS regulation is achieved through several chemicals: adrenaline, noradrenaline, and acetylcholine. They have an excitatory effect on PNS activity, which relaxes the body. The heart rate slows, blood pressure decreases as the body leaves the fight-or-flight state, and adrenaline and noradrenaline decrease.
Emotional tears are often released as a part of this process, leaving an end product containing these neurotransmitters as waste.
Hormones and tears
Hormones are particularly concentrated in emotional tears. For instance, adrenaline and noradrenaline both act as hormones as well as neurotransmitters.
The hormones found in tears are the waste product of activity in the nervous system. The nervous system is responsible for many emotional responses, particularly regarding stress and excitement.
Adrenocorticotropic hormone (ACTH) and prolactin are both protein-based hormones that are found in emotional tears, while leu-enkephalin is an amino acid neuropeptide that is also present.
Tears as a window to health
The eyes are an entrance for viral or infection transmission. As such, the organ and its functioning can also indicate health.
Lysozyme and lactoferrin found in basal tears are thought to be of particular clinical significance as bioindicators of immune health. This is because the proteins help to block infectious materials from entering the eye.
The volume of each protein in the basal tears varies individually and may be a biomarker of immune system functioning.
The evolutionary perspective
Charles Darwin infamously stated that emotional tears hold no “purpose.” However, emotional tears are now thought to have an evolutionary basis. This is because they act as social releasers for infants, which may be a component of human attachment.
Crying is also likened to a distress call, akin to other animals. However, these theories do not always fully account for the biochemical processes that lead to the release of emotional tears.
Conclusion
Tears act as an emotional outlet and as a component of eye health. Like other body parts, different types of tears are specialized for different functions.
The emotional basis of tears relates to the ANS. Emotional tears, therefore, have the highest levels of hormones and neurotransmitters compared to other types of tears.
Basal and reflexive tears, alongside the tear film, are important to ocular health. These types of tears protect the eye from infection and foreign bodies via proteins and immunoglobulins.
Analyzing the biochemical concentrations of all tears can provide researchers and healthcare professionals with an insight into both the physical health and emotional well-being of individuals. As such, they also give a strong basis for psychobiological research.
References
- Chang, A.Y. & Purt, B. (2022). Biochemistry, Tear Film. https://www.ncbi.nlm.nih.gov/books/NBK572136/#article-133108.s8
- Kopacz, D., Niezgoda, Ł., Fudalej, E., Nowak, A. & Maciejewicz, P. (2021). Tear Film – Physiology and Disturbances in Various Diseases and Disorders. Ocular Surface Diseases - Some Current Date on Tear Film Problem and Keratoconic Diagnosis. doi: 10.5772/intechopen.94142
- McDermott A. M. (2013). Antimicrobial compounds in tears. Experimental eye research, 117, 53–61. https://doi.org/10.1016/j.exer.2013.07.014
- Patel, J., Levin, A. & Patel, B. C. (2022). Epiphora. https://pubmed.ncbi.nlm.nih.gov/32491381/
- Spiriev, T. Y., Chowdhury, T. & Schaller, B. J. (2015). The trigeminal nerve: Anatomical pathways. Trigeminocardiac reflex trigger points. https://www.sciencedirect.com/science/article/pii/B9780128004210000023
- Bylsma, L. M., Gračanin, A. & Vingerhoets, A. J. J. M. (2019). The neurobiology of human crying. https://doi.org/10.1007/s10286-018-0526-y
- Hanstock, H. G., Edwards, J. P., & Walsh, N. P. (2019). Tear lactoferrin and lysozyme as clinically relevant biomarkers of mucosal immune competence. https://www.frontiersin.org/articles/10.3389/fimmu.2019.01178/full
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