Itch and Scratch

A positive feedback with a negative outcome

Who remembers or better who wouldn’t remember Billy Wilder’s film “The Seven Year Itch” starring Marilyn Monroe or have memories of movies featuring gangsters with an itchy trigger finger? But those itches are not what I want to cover in this blog. The kinds of itches I mean can be hilarious and funny to watch when it involves animals, e.g. “Does a bear itch in the woods?” and “Watching a Grizzly Bear Scratching against a tree at Knight Inlet BC“. It seems that scratching oneself is something universal, for you can even see fish engaging in what must be a response to an itchy stimulus and I have even seen newts seemingly trying to remove an itchy old skin by deliberately squeezing themselves through dense plant growth.  While watching animals solve their itchy problems may be amusing, when the itch involves us, ourselves, it’s much less of a laughing matter.

Once seen as a category of pain, researchers now consider itch to involve a unique sense with specific receptor points and neurons involved. Although progress has been made in recent years to understand the physiology of itch a little better, the phenomenon is difficult to study. Animal models are only partially helpful and animal protection societies will not permit certain itch-inducing experiments to be done while ethics committees watch that observations on humans won’t deviate from their ethical guidelines.  What has become clear, nevertheless, is that different stimuli elicit responses in different nerve fibres. Delta fibres, for example, are thicker and conducting information faster than so-called 1 µm thick C-fibres, of which one type is involved in conducting pain responses and another in responding to itch stimuli at speeds of 1 µm/sec. But the causes of itch vary and an itch caused by the irritating hairs of the pods of a tropical plant known as Velvet Bean Vine ( Mucuna pruriens ) involve the faster Delta fibres and is mediated by a cysteine protease and not a histamine, which is a response to insect bites and causes swelling and itching. An anti-histamine may reduce that kind of itch, but not the type caused by the Velvet Bean. Other identified itch mediators are, e.g. serotonin 5-HT, substance P, cytonkines from white blood cells, dust mite allergens, etc.

Four or five categories of pruritus (the scientific name of ‘itch’) are distinguished. The skin-derived itch stems from damage to the skin, be it from an insect bite or chemical exposure. The psychogenic pruritus can be elicited by seeing creepy crawlies or even thinking of them or by nervousness. The neuropathic pruiritus is due to a pathologic change facilitating conduction from skin receptor to the brain, while the neurogenic itch is the result of signals from the central nervous system to an itchy region reducing inhibition from the central nervous system and can accompany diabetes, kidney malfunction, herpes zoster or multiple sklerosis, etc. The final category would be that of the atopic pruritus, and as the name suggests the causes are difficult to identify as skin-derived components may “team up” with neurogenic causes. What all the various origins of itch have in common, however, is that they lead the sufferer to scratch himself or herself to relieve the irritating itchy sensation. Unfortunately, the scratching triggers a positive feedback loop, which means one action leads to more (that’s why it is termed “positive”). In other words, you scratch and instead of reducing the itch, the latter increases; you scratch more and that further increases the itching and so it goes on and on until you are too exhausted to continue to scratch.

The body of a human adult is covered by approx. 1.8-2.2 m2 of skin. An itch makes you want to touch the itchy place, but a painful area is something you want to avoid. Another difference is that exposure to heat will usually aggravate the pain, but heat can lead to an easing of the itch, just like antihistamines and an exposure to UV-radiation can. Obviously, all mammals experience an itch sometimes and then respond with scratching; birds do, and even fish display responses to itchy stimuli.  And insects? The rubbing and ‘washing’ that one can see flies and other insects engage in, isn’t that a kind of ‘scratching’, a response to an ‘itch’? Now that’s something for which we really need to scratch our heads, I think.

© 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.

Fluorescent Animals

 Something special for photography buffs

I have mentioned fluorescence a few times in my blogs, have written an item about GFP (the Green Fluorescent Protein that won O. Shimomura, R.Tsien and M. Chalfie in 2008 the Nobel prize in Chemistry) and explained how a scientist had erroneously described a bioluminescent cockroach from South America that turned out a misinterpretation, because the insect was not bioluminescent (= creating its own light), but fluorescent  – just like it has recently been reported from the platypus, some other mammals and even the hawksbill sea turtle (namely emitting visible light only when illuminated with light of shorter wavelengths like, for instance, ultraviolet  radiation).

To distinguish the two types of responses in the natural environment is actually quite simple: when struck by a beam of light, say by UV radiation, and the animal “glows”, it will only glow as long as it is illuminated by the light source. The moment the UV-source is taken away or switched off, the fluorescence will also disappear. Bioluminescence, however, whether induced as a response to illumination or, more commonly, a physical disturbance or due to an exposure to a chemical, the light emitted by the aroused animal will linger on for a while. Therefore, fluorescence and bioluminescence are two quite different phenomena based on totally different chemicals and reactions. For the fluorescence so-called fluorophore chemicals need to be housed in the fluorescent tissue, but the bioluminescence requires a substrate known as luciferin and an enzyme known as luciferase and oxygen. Different luminescent animals may have chemically different luciferins and luciferases.

What causes some animals to fluoresce when in the beam of a UV-light are various chemicals like fluoresceine that gives off green light and similar molecules that emit other colours. Fluorescein is a widely used fluorophore substance used to label antibodies with, so that certain intracellular components like, for instance, actin filaments stand out in bright green when illuminated under a fluorescence microscope with UV-light. I used this kind of microscope and technique in conjunction with a study on chromatophores of the skin (and therefore the body) of fish.

Why some animals should be fluorescent is really a bit of a mystery. I was amazed at the extraordinarily bright fluorescence that came from newly hatched millipedes of a Paraspilobolus species. Incidentally a species that is not only fluorescent, but known to be also truly bioluminescent when forcefully touched or physically attacked. However, what (if any) function the fluorescence observed in these millipedes which spend most of their lives under loose layers of soil or hidden amongst leaf litter could possibly have, is not yet clear. All that is known is that fluorescence in millipedes, generally, is not a rare phenomenon. With spiders and scorpions, both having wonderfully luminescent species, it is similar: nobody so far has come up with a convincing explanation why so many species of them are fluorescent.

For a recently discovered truly fluorescent tree frog from Argentina, Carlos Taboada et al. suggested that it would help the nocturnally active frogs to recognize each other, as the eyes of these frogs may be more sensitive to longer than shorter wavelengths. Fluorescence, as mentioned above, emits light of longer wavelengths (e.g. in the green) than the UV-or short wavelength blue lights that might have been falling on the frog in its natural environment. However, the eyes of the frog and their spectral sensitivity have not been measured yet and secondly, the amount of UV-light during the day under the foliage of trees is not exactly high and at night it is absent. For creatures in the sea, some of which also fluorescent, the same problem of little or no UV would occur. The frog’s fluorescence, therefore, may have other functions, perhaps aimed at predators making the frogs less visible or advertising their distastefulness. What is, however, totally true is that one can take fantastically beautiful photos of fluorescing animals: see this article and gallery.

© 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.