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 http://www.bioforthebiobuff.wordpress.com, 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 http://www.bioforthebiobuff.wordpress.com with appropriate and specific direction to the original content.