As Tears Go By

A look at animal and human tears

Children cry easily and even after a minor bump or hurt will shed tears. Adults may feel pain, cry and scream when hurt, but unlike children will not shed tears. An adult’s tears are associated with emotions or may be caused by some disorder, an eye infection or irritation, but not pain. And animals? They, too, have lacrimal (= tear) glands and can have watery eyes as the result of an infection or as part of a physiological control to remove excess salt from the body, but apparently not in connection with an injury. Sea turtles and a few other reptiles remove excess salt not only via their kidneys but with the help of their orbital eye glands as “white tears of saline” that drip out of their eyes.

Although human tears are not white, but watery, transparent and very slightly sticky because of mucins in them, they too contain salt  – as do, in fact, the tears of all land vertebrates that may not ‘shed tears’ but use the lacrimal fluid to lubricate their eyes and keep the cornea moist. Chemically tears are mostly water (ca. 98%); and apart from salts the lacrimal fluid contains a cocktail of amino acids and proteins, antibacterial enzymes and minute quantities of stress hormones. A tear’s chemical composition depends on the cause of its shedding and varies on whether the tear’s function is to wash out dust from the eye, to fight off irritants such as fumes (smoke or onions come to mind), to lubricate the eye’s surface, and as a response to physical pain and emotional upheaval. The autonomic nervous system through its parasympathetic branch governs the production and release of tears from the lacrimal glands, which are located in the upper region of the eye’s orbit. The tears are stored in the lacrimal sac near the nasal corner of the eye; from there the fluid via lacrimal canaliculi is released into the eye upon a signal from the parasympathetic nerve’s acetylcholine transmitter. In healthy individuals, there is a constant release of minute quantities that are distributed with each eye blink across the cornea, but of greater amounts if required. Excessive fluid is drained through the nasolacrimal duct and causes the ‘sniffle’ during weeping.

Basal tears are continually-produced via the 5th cranial nerve’s innervation to keep the eye’s cornea moist and to prevent bacterial infections. In humans, about 0.75-1.1 ml of the liquid is produced each day. Reflex tears are produced when the eye is irritated, and through their copious amount and high water content function to remove the irritation from the eye. Psychic, also known as ‘emotional’  tears, occur in response to strong feelings, which could be sadness, but also joy, stress and  physical pain. Because these tears contain such natural painkillers like leucine-enkephalin and prolactin, it may explain the role of the parasympathetic nervous system and that “a good cry can feel relieving”.  But it does not explain why men shed tears less often than women, a fact that is often explained with the traditional roles men and women are expected to play in life (the advice “boys don’t cry” is a case in point).

The fourth reason for tears is related to diseases and the release of tears accompanying other activities (e.g. yawning). Although elephants have been described as shedding emotional tears, crocodile tears are not an expression of emotional distress, but the result of compression of a nerve that controls the jaw muscles during feeding. In humans suffering from Bogorad syndrome “crocodile tears” also accompany swallowing. Reference to tears can generate resolve (Churchill’s famous “Blood, Sweat and Tears” comes to mind); tears evoke empathy: children know that (and actors train to shed tears at will) and tears appear in poems and songs (the record “Tears on my Pillow” is in my collection) and who wouldn’t remember Marianne Faithful’s beautiful song “As Tears Go By” or Eric Clapton’s touching “Tears in Heaven” (which I heard it for the first time in Chile in 1993). I actually heard of people who shed tears when listening to it.

© Dr V.B. Meyer-Rochow and, 2022. Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and with appropriate and specific direction to the original content.


I Hope It’s Never Happened Reading “Bioforthebiobuff”

At high school we had a history teacher by the name of Dr. L., who had spent 11 years in a Soviet prisoner-of-war camp before being released in 1955. He used to put the history book on the classroom’s desk, positioned himself comfortably on a chair near the side of the classroom and asked some of the best readers in class to take turns to read from the book. That’s how his lesson went. Although he can perhaps be forgiven for killing our interest in history by this behavior of his, his antics were also a cause of hilarity, especially when we noticed the regularity of his yawns and could predict when another big yawn of his would appear (silently counting: 14, 15, 16, 17, “yawn”!). But what made him yawn so much? Boredom, lack of sleep, or something else? And why is it so ‘contagious’? Mirror neurons perhaps?

The common view has always been that yawning was related to a lack of oxygen, a build-up of carbon dioxide and a room that was too warm and stuffy. Consequently, a call to open the window and let in ‘fresh air’, could often be heard in situations where people were seen to yawn frequently and appear sleepy. Yet, numerous studies have shown that lack of oxygen and carbon dioxide increases are by themselves not a cause of yawns. The situation is complex and although the amount of yawning appears to be correlated with boredom and sleepiness, it must leave us puzzled to notice that even after a good night’s sleep we wake up and then more often than not yawn upon awakening. Why yawn at that time? And cooling the brain in the morning or at other times by gaping wide: does it make sense? The idea that yawning is a component of thermoregulation has not yet achieved the acceptance it hoped to get.

If we examine objectively what happens during a yawn, we notice that it involves a wide open mouth and a long and deep inspiration of several seconds, sometimes accompanied by some soft vocalization during expiration. It is an involuntary behaviour that can be triggered by thinking and reading about yawning and/or seeing someone yawn. Yawning is communicative and is generally coupled with inactivity, lethargy and sluggishness (sometimes worry as well). To suppress the yawns can be difficult, especially when hindered to move as in boring meetings, lectures, and waiting rooms. And this actually gives us a clue: our bodies need us to stretch occasionally, to shake our arms and legs, to release tension.

The realization that yawning is a stretch response has been gaining attention ever since it was observed that when hemiplegic individuals that not normally can move their arms do move them when they pandiculate with an associated yawn. Yawning when pandiculating, i.e. stretching and thereby contracting and relaxing muscles, reduces muscular tension, is resetting and restoring the control over muscles, something that is critical for posture and movement and something that yoga instructors constantly emphasize. Obviously, the fact that the slow expiration following a yawn is associated with a sympathetic activation marked by an increase in blood pressure, suggests that at the start of the yawn it is associated with a sympathetic suppression that favours a parasympathetic dominance. This might also explain the observation of a paraplegic’s involuntary movement of its toes during a yawn.

Yawning must have ancient roots in the animal kingdom, for it can be observed in almost any animal group and is not even restricted to vertebrates alone as this delightful recording of a yawning leech shows here . Lizards, frogs, toads and even fish can be seen to yawn and all of them are ectothermic (often referred to as ‘cold-blooded’). As such, they would not be expected to use the yawning response to cool their brains as has been suggested for mammals, but could find yawning useful in connection with stretching and therefore the restoration of muscle control. Yawning:  a kind of physical exercise without having to get up? I think that that is a distinct possibility.

© Dr V.B. Meyer-Rochow and, 2021. Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and with appropriate and specific direction to the original content.

biology zoology blog benno meyer rochow heart flies

Hearts of Flies and Humans

Not so terribly different after all

One of the standard laboratory exercises for students in physiology I had to supervise in New Zealand, but never enjoyed much ‘cause I like frogs, involved live hearts of frogs. Sure, it’s interesting for students to see how the isolated heart keeps beating all by itself and to prove that the heart muscles of the atria and ventricles operate independently and have their own intrinsic rhythms. The students had to apply the so-called Stannius ligatures between sinus venosus and the atria and the atria and the ventricles. These ligatures disrupt the coordinated contractions from sinus venosus via atria to the ventricles and slow down the heart’s beat of the latter, but do not eliminate it. The experiment served to demonstrate the ‘myogenic origin’ of the vertebrate and therefore of course also the human heartbeat: to switch on the heartbeat, a nerve input was not required (although the vagus nerve can slow it down but not stop it while sympathetic nerves contacting the sino-atrial node can increase the heartbeat by adrenaline).

In the lectures I would then say that insects don’t operate with myogenic, i.e. self-beating hearts but have neurogenic hearts, in other words hearts that according to text book wisdom, beat only when a nerve impulse causes them to contract. It’s all wrong according to careful studies by the jovial and imposing, famous Czech academician Karel Sláma. Insects possess an open blood system without arteries, veins and capillaries and their blood does not carry oxygen around to the various tissue, because insects “breathe” with air-filled tracheae and tracheoles. But insects do have a tubular heart on the dorsal side of their body. It beats and propels the colourless blood mostly forward towards the head via systolic contractions of 4-7 Hz in the fruitfly, but up to 10 Hz in the hoverfly Episyrphus balteatus, in which systolic contractions reach propagations of 32.2 mm/s. Occasional switchovers from a forward-directed heartbeat to a retrograde beat, in which the heart reverses the direction that it propels the blood is common in insects. Young fruitfly larvae, however, only exhibit a unidirectional forward systolic contraction. But what is the evidence that Dr Sláma advances to show that the hearts of insects and those of humans aren’t all that different?

First of all, the primordial formation of insect and human heart is orchestrated by similar sets of genes. There is also an electrophysiological analogy with regard to the onset of depolarization of the systolic contraction at the apex of the heart. There is the conical compact muscular chamber of the insect heart at the abdominal base that is almost like the ventricle in the human heart. Most convincing is the demonstration of the purely myogenic nature of the insect heart, when the neuromuscular system of waxmoth larvae was paralysed by a venom obtained from a parasitic wasp and injected into the larvae. The larvae then remained perfectly motionless, unable to move any body muscle for 3-4 weeks. Despite their immobility, their heart continued to beat like clockwork and the heart muscle contractions were fully preserved without any nerve input. The contractions in the insect heart muscle were determined by a terminal pacemaker nodus in insects, analogous to the atrioventricular nodi in humans. Is that the end of the assumed categoric dichotomy between vertebrate myogenic and insect neurogenic hearts? Maybe not quite, as there are some insect species in which heartbeats are under considerable neural control. But the similarity goes even further, for when Dr Sláma tested the actions of drugs like digitoxin and nitrates on the insect hearts he examined, he found responses that resembled those that could also be observed in human hearts. Might the results serve one day as a convenient and inexpensive way, avoiding the use of dogs and other large animals, for testing cardiologically active chemicals? Dr Slama (and I, too) hope so.

© Dr V.B. Meyer-Rochow and, 2021.
Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and with appropriate and specific direction to the original content.