Optics / Protective mechanisms / Tear production & crying
Tear production and crying
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the tear film that covers the suface of the eye has 3 layers:
- lipid secreted by oil glands in the eyelids
- aqueous-based solution from lacrimal gland (contains lysozyme and other enzymes that provide protection against infection)
- mucous from the conjunctiva
- the composition of the tear layer varies depending upon the stimulus and with age
- emotional tears contain more hormones, such as prolactin, ACTH and enkephalin
- basal tear production decreases with age
- tear flow occurs via evaporation and drainage through the nasolacrimal ducts into the nasal cavity
- parasympathetic stimulation produces epiphora (overflow of tears) by:
- increasing tear production by the lacrimal gland
- decreasing outflow by facilitating closure of the lacrimal duct passage
- epiphora can be induced by:
- stimulation of the cornea (cranial nerve V) which produces reflex tears
- strong emotional responses (mediated by the limbic system, especially the hypothalamus) which produce psychic tears (crying or weeping)
- strong parasympathetic stimulation is accompanied by other symptoms, like reddening of the face and convulsive breathing
- parasympathetic stimulation produces epiphora (overflow of tears) by: