The heart beats thanks to special muscle fibers, which are collected in several nodes and bundles and all together form the conducting system of the heart. These atypical muscle fibers generate excitement that spreads to the entire heart muscle. The conducting system sets the rhythm and sequence of contractions of the atria and ventricles, and everything begins here with the sinoatrial node located at the entrance of the superior vena cava into the right atrium. The cells of the sinoatrial node are called pacemakers, because it depends on them whether the heart begins to beat faster or slower.
But the heart beats in all living beings that have it, and, therefore, pacemakers-pacemakers should be in other hearts, no matter how simple they are. The cardiovascular system first appears in invertebrates, and in most of them the movement of blood is provided by a more or less rhythmic pulsation of the vessel walls. The cells that set the rhythm of this pulsation are disordered. The pacemaker of the fish heart looks like a shaped ring that surrounds the bases of the cusps of the sinoatrial valve, which separates the atrium from the venous sinus.
Until recently, a large group of animals remained among the vertebrates, about which it was almost not clear at all where the rhythm is born in their heart – these are reptiles. Only recently, the staff of Moscow State University, together with colleagues from Aarhus University, managed to find out the location and features of the work of the pacemaker of the heart in snakes. Researchers studied tiger pythons; in article in Journal of Experimental Biology they say that their pacemaker looks like a fairly compact structure at the base of the right sinoatrial valve cusp. Under certain conditions, for example, under the influence of the neurotransmitters acetylcholine and norepinephrine, other parts of the heart muscle near the same sinoatrial valve begin to set the rhythm.
In general, the pacemaker of the python heart in its organization occupies an intermediate position between the sinoatrial node of mammals and the circular pacemaker of fish hearts. This confirms the hypothesis that the pacemakers of the heart evolved from diffusely located pacemaker cells to an anatomically shaped compact structure.
Most likely, in other snakes, the heart pacemaker is arranged in the same way as in pythons, but questions still remain about the rest of the reptiles. Of particular interest here are turtles and crocodiles (recall that if most reptiles have a three-chambered heart, and the ventricle has an incomplete septum, then crocodiles have a four-chambered ventricle – their ventricle is completely divided into two parts, although they have a hole in the septum that allows partly mix venous and arterial blood). Knowing the evolution of organs – in this case, the heart – in different groups of animals, one can better understand the structure of the same organs in humans and, in the future, better understand how to cope with their diseases.
This work was supported by the Russian Science Foundation.
Based on materials from the press service of Moscow State University
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