Dual
excitation of the sinoatrial node ensures proper functioning of the
"potassium-sodium pump."
We
often talk about the influence of the parasympathetic or sympathetic nervous
system on heart function. But how do you imagine this happening if information
in the nervous system is transmitted by electrical impulses? It turns out that
we say one thing and mean another – isn't it the nervous electrical impulse
that enters the heart?
We
posed a question to artificial intelligence: Are electrical impulses from the
nervous system conducted to the heart?
Answer:
No, impulses from the central nervous system (brain) are not conducted to the
heart. The heart has its own internal conduction system, consisting of the
sinoatrial node and other structures, which generates and distributes
electrical impulses that cause its contractions. However, the nervous system
influences the heart by changing the heart rate: sympathetic nerves accelerate
the heart rate, while parasympathetic nerves slow it down.
We
emphasized the word "affects"—that's the discrepancy. Of course,
"electrical impulses from the nervous system are conducted to the
heart," and we'll explain how this happens! Since the advent of
electrocardiography, many have assumed that electrical impulses must enter the
heart. But these specialists were immediately brought down to earth, explaining
that, like a supraventricular extrasystole, which interrupts and discharges the
sinoatrial node, a nervous electrical impulse will also interrupt the sinoatrial
node—and this is logical, but incorrect! The scientific community has long
debated whether the sinoatrial node is stimulated by chemical or electrical
stimulation. However, it is hypothetically believed that the sinoatrial node
can generate electrical impulses through neurochemical influences. We are
confident in the neuroelectrical theory! The point is that the electrical
impulses in the cardiac conduction pathways are more powerful in millivolts, so
they can interfere with each other, and after reaching the myocardium, they
become less powerful, like a reentrant wave. The nervous system also has weak
electrical impulses, and they enter the sinoatrial node. Note Figure 8: it
shows phase 4 of diastole of the sinoatrial node pacemaker cell; nothing is
happening there. Furthermore, the teachers explained to us that ion channels
open, and large influxes of Na+ and K+ rush into the pacemaker cell. This
raises two questions: what stimulus causes the pacemaker cell's ion channels to
open, and how do the influxes of Na+ and K+ rush in if the pacemaker cell is
completely relaxed in phase 4 and all channels have been opened? It's like a
person who's overfed and relaxed; try to force anything else into them, but it
won't work! So, for the pacemaker cell to admit Na+ and K+ currents, the first
neuroelectrical impulse must pass through it, stimulating a mild excitation
with spasm of the sinoatrial node and electrical activation of Na+ and K+ (the
pre-depolarization phase). During this phase, the pacemaker cell contracts
(empties), rapidly leaking cytoplasm and then begins to rapidly recover,
drawing in electrified Na+ and K+ currents. This will be the second (main)
excitation of the pacemaker cell (phase 0 of the action potential), but this
time, it will be more powerful, generating an electrical impulse in the
sinoatrial node. Thus, an additional "pre-depolarization" phase
should appear in Figure 9. Once again (for better understanding), the first
neuroelectric impulse should contract the sinoatrial node and pacemaker cells,
electrify the flows of Na+ and K+, and the second, powerful electrical impulse
is formed inside and it does not contract anything, it expands the membrane of
the pacemaker cells with an electrical explosion and throws out discharged
flows of Na+ and K+ into the intercellular space. P.S.
to this section. Before writing this section, we long struggled with the
question of what triggers the sinoatrial node to begin functioning at the
beginning of the fourth week of embryonic life. Currently, it is believed that
this occurs spontaneously. As soon as we wrote this section, everything fell
into place! It turns out that at the end of the first week after conception,
the embryonic membrane attaches to the uterus, and the woman's brain receives
information that she is pregnant. If there are no genetic defects, then at this
stage, the correct electrical impulses begin to flow to the uterus via the
autonomic nervous system, similar to those sent to one's own heart (many
pregnant women feel "uterine pulsation" and tell their doctor). At
the beginning of the fourth week (the embryo measures 1.5 to 3 mm), when the
tubular heart of the embryo is formed, it is the maternal electrical impulses
that "ignite the electric fire in the embryonic heart"! Next, up
until the 11th week of pregnancy, the mother will teach her unborn baby how the
heart should function properly. Then, the fetal membrane will harden (become
covered with connective tissue, which blocks electrical impulses from reaching
the fetus) and become the placenta. By this stage, the fetus should have
learned its "maternal lessons" and begun to control the heart using
its nervous system. These two stages are very dangerous; if there are genetic
defects in the mother or fetus, dangerous pauses in the tiny creature's heart
can occur (80% of all miscarriages occur in the first trimester of pregnancy).
Of course, not all miscarriages are due to this cause; quite a lot.