What Are Tears?
Tears are made of salt water, as you will know if you have ever licked a finger after wiping them away. They happen when the gland which provides moisture to keep the eyeball rotating properly produces more water than usual. The eye cannot drain all this water as it does normally, so a flood occurs.
The gland which makes tears is called the lachrymal gland (Lacryma is the Latin word for tear). It is about the size of an almond nut and is situated above the eye. It opens on to the surface of the eyeball by six or more little ducts. Every time we blink the water is spread over the eye. Any extra water is collected in two little canals at the inner corner of the eye where the upper and lower eyelids join, and carried away to the lachrymal sac near the nose.
The tear gland makes more water than usual when stimulated by a really pungent smell, like onions, or household ammonia, or by a situation of great emotional stress, whether happy, sad or frightening. The lachrymal sac then becomes too full and water enters the nose. The canals overflow and this overflow manifests itself as a stream of liquid.
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 (0.03–0.04 ounce avoirdupois) of tears is secreted; this rate slows with age. 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, 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.
Crying or weeping (psychic tears): The third category, in general, referred to as crying or weeping, is increased tearing due to strong emotional stress, pleasure, anger, suffering, mourning, or physical pain. This practice is not restricted to negative emotions; many people cry when extremely happy such as during times of intense humour 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 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.