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How It Works: Crying

How It Works: Crying

By Mike Davis

Whilst the jury may still be out on whether humans are really the only species to cry in response to emotional triggers, there is little doubt that the act of weeping is perceived as an essentially human action. I cry, therefore I am. But just how does it work and why is it, mostly, beyond our control?

Tears are produced by our lachrymal glands, one of which is located behind each upper eyelid, just above the outer corner of our eyes.

These glands secrete lachrymal fluid which flows along ducts and into the space between the eyeball and eyelids. When we blink, the fluid is spread across the surface of the eye. Excess fluid falls from our eyes and rolls down our cheeks as tears. Some of the fluid often enters our nasal cavity, causing the runny nose we often associate with snotty hay-fever episodes.

We cry for any number of reasons. Some purely physiological, some emotional, and some a complex mixture of both.
Click image to enlarge.

‘Basal’ tears are those that we produce continually to keep our eyes moist and lubricated. We generate some five to ten ounces of this liquid a day and without it, blinking would be very painful. Basal tear fluid contains water, nutrients and anti-bacterial components such as glucose, mucin (sticky stuff), lysozyme (antibacterial), lactoferrin (antibiotic), lipocalin (protein transporter), urea, potassium and sodium. The sodium is primarily responsible for that salty taste you experience when kissing the tears from the cheek of a loved one.

‘Reflex’ tears are produced to cleanse our eyes of irritants. An evolved response mechanism, they result from, for example, the invisible irritants released during onion-slicing, or being poked in the eye by a helpful colleague who is attempting to remove a small object that has resisted the previous efforts of the reflex tears themselves. Sensory nerves in our cornea send signals to the brain stem; our brain stems (a regulator of involuntary processes like breathing) send a hormonal signal to our lachrymal glands causing the tear response to kick in.

Click image to enlarge
Finally, there are ‘Emotional’ tears. These have a very different chemical make-up to the other types. The hormones prolactin, adrenocorticotropic hormone (ACTH), and leucine enkephalin (a natural painkiller) have been detected, all of which are produced by our body when under stress. Research suggests that emotion-induced crying is therefore, at least in part, an excretory process which removes stress-related toxins. This echoes Aristotle’s theory that crying cleanses the mind, reducing distress by releasing emotion. It’s true that we certainly do tend to feel better after a good cry.

Emotional tears are also produced by nerves in our eyes interacting with our brains. Those same corneal nerves responsible for the reflex tears also have connections that reach deeper into the brain, to the cerebrum. This area of the brain deals with complex emotional events such as perception, judgment, thought, imagination and decision-making. When crying-related emotions (many of which will be quite unique to you, of course) are registered here, the cerebrum causes hormones to be released which travel to the glands behind your eyes and produce tears. You cry.

Different emotional stimuli provoke tears in different people. Some causes are obvious and fairly universal – loss of a loved one, etc. British psychologist, John Siaboda, wrote that the most tear-inducing musical passage of all time was the start of the third movement in Rachmaninoff’s Second Symphony. For me, it is the first note of Led Zeppelin’s, ‘Stairway to Heaven’, played by pretty much any amateur guitarist – but for very different reasons.

It is a truth, by the way, that women cry more than men. On average, more than sixty times a year compared to some seventeen times for men.

All mammals, as far as we know, produce tears, but none, other than humans, have ever been proven to weep due to their emotions. Charles Darwin, however, was told by a keeper of the Indian elephants at London Zoo that the elephants would sometimes cry from sorrow.

So what about tears of joy? Explanations vary. Some researchers think that crying at weddings, in response to hysterically-funny jokes or happy endings might actually reflect unhappy feelings: the ‘loss’ of the child or friend to their new spouse, the release of tension (stress) when the movie reveals that all is well for the characters with whom you had sympathised. It may simply be that intense emotions, whether ‘negative’ or ‘positive’, are physiologicaly as stressful as each other and thus provoke a similar reaction.

But I’ll leave you with a different thought; what’s the difference between a banjo and an onion? No-one cries when you cut up a banjo.

More stories from the Null:

How It Works: Dark Matter
  How It Works: Brain Freeze
How Marijuana Works
  How It Works: Electric Eel
Title image: Jyn Meyer
Eye diagram adapted from: Erin Silversmith/W

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07 Jul 2011
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