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This question already has an answer here:

I was thinking like whenever when we feel emotional or whenever we got hurt we got tears from our eyes but why is that happen ? Why our eyes got wet ?

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marked as duplicate by WYSIWYG, Amory, March Ho, Chris, Oreotrephes Jun 8 '15 at 2:45

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ the other answer does not discuss specific nuclei or larger structures within the brain $\endgroup$ – Jasand Pruski Aug 9 '16 at 2:44
  • $\begingroup$ The other question is only about emotions. $\endgroup$ – Rodrigo Aug 9 '16 at 4:28
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The lacrimal gland, the major source for tear production, is situated just above the eye. It sends it's secretions to the eye via the lacrimal duct. Tears secreted collect in the conjunctiva of the upper lid. There is also a nasolacrimal duct which drains the tears from the eye into the nasal cavity. Hence the sniffles when we cry.

How is it controlled? The lacrimal nucleus in the brain is the major source for innervating the lacrimal gland, is a subnucleus of the superior salivary nucleus in the tegmentum of the pons (part of the brain stem). The lacrimal nucleus is connected to the lacrimal gland by the facial nerve (more specifically the greater petrosal nerve). This is parasympathetic (meaning it is part of the autonomic nervous sytem) and the neurotransmitter transmitting the signal from nerve to gland is acetylcholine (the receptor is muscarinic subtype of the cholinergic... though strangely wikipedia says nicotinic subtype is involved as well, but maybe this is simply poorly worded and is just refering to the ganglia stopover between pre/post ganglionic parasympathetic neurons), this information helps explain how some drugs affect lacrimation (tearing up).

Taken from source (1) [I do not know how reliable this website is]: How is the lacrimal nucleus told to start lacrimation? The lacrimal nucleus receives projections from the autonomic nervous system as well as from various brain structures, including the frontal lobe, globus pallidus, thalamus, and hypothalamus. The projections from the frontal lobe are thought to be important in human psychic lacrimation (crying because of emotions).

What I know: Another brain area called the limbic system is involved in production of basic emotional drives, such as anger, fear, etc. The limbic system [in the case of sympathetic nervous system it would be the hypothalamus specifically, but this is irrelevant here since parasympathetic nerves cause lacrimation] also has a degree of control over the autonomic system. As I under stand it, the frontal lobe usually inhibits the limbic system.

Again from source (1): While humans across the world produce psychic tears in response to both positive and negative emotions, and while similar emotions have been identified in other animal species, no reports exist of other animals showing psychic lacrimation.

according to wiki (3): Compared to tearing for physical reasons, tears for emotional reasons have s different chemical composition. They 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.

sources:

(1) http://carta.anthropogeny.org/moca/topics/lacrimation-tearing

(2) http://en.wikipedia.org/wiki/Lacrimal_gland#Innervation

(3) http://en.wikipedia.org/wiki/Tears

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  • $\begingroup$ you are explaining in great detail the process of How, but not the Why. $\endgroup$ – Darwin PC Jun 8 '15 at 2:22
  • $\begingroup$ good catch... not only do I have no idea on "the why", but some of the info in my answer way be inacurrate for "the how"... I'm hoping someone posts a better answer and then the credit is given to them... $\endgroup$ – Jasand Pruski Jun 9 '15 at 2:45
  • $\begingroup$ don't Dogs and Cows shed tears? I thought they do $\endgroup$ – asgs Aug 8 '16 at 18:39
  • $\begingroup$ maybe, but i only know about humans $\endgroup$ – Jasand Pruski Aug 9 '16 at 2:38
  • $\begingroup$ Without having anything but intuition, I would tend to think that it is a communicative signal, i.e. laughter the other way round. It helps individuals in social groups to infer the state of mind of other group members. $\endgroup$ – AlexDeLarge Aug 9 '16 at 13:46

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